Merge topic 'custom-string'

2d68b2c593 String: Add str_if_stable() as a const alternative to str()
a0841b59bd String: Add support for a ""_s string literal syntax
9d5fe8e96a String: Add 'borrow' member to construct borrowing instances
80802a002c String: Add support for concatenation by operator+
ff69763ca0 String: Add a custom string type
410a3e4b22 Add support for using C++17 string_view or a fallback

Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: Daniel Pfeifer <daniel@pfeifer-mail.de>
Acked-by: Pavel Solodovnikov <hellyeahdominate@gmail.com>
Merge-request: !2578

6 files changed, 1531 insertions, 0 deletions

diff --git a/Source/CMakeLists.txt b/Source/CMakeLists.txt
index e86cc2f..91c45bc 100644
--- a/Source/CMakeLists.txt
+++ b/Source/CMakeLists.txt
@@ -383,6 +383,9 @@ set(SRCS
   cmXMLWriter.h
   cmake.cxx
   cmake.h
+  cm_string_view.cxx
+  cm_string_view.hxx
+  cm_static_string_view.hxx
 
   cmCommand.cxx
   cmCommand.h
@@ -574,6 +577,8 @@ set(SRCS
   cmSiteNameCommand.h
   cmSourceGroupCommand.cxx
   cmSourceGroupCommand.h
+  cmString.cxx
+  cmString.hxx
   cmStringReplaceHelper.cxx
   cmStringCommand.cxx
   cmStringCommand.h
diff --git a/Source/cmString.cxx b/Source/cmString.cxx
new file mode 100644
index 0000000..2a0c125
--- /dev/null
+++ b/Source/cmString.cxx
@@ -0,0 +1,152 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+#define _SCL_SECURE_NO_WARNINGS
+
+#include "cmString.hxx"
+
+#include <memory>
+#include <ostream>
+#include <stdexcept>
+#include <string>
+#include <type_traits>
+
+namespace cm {
+
+static std::string const empty_string_;
+
+void String::internally_mutate_to_stable_string()
+{
+  // We assume that only one thread mutates this instance at
+  // a time even if we point to a shared string buffer refernced
+  // by other threads.
+  *this = String(data(), size());
+}
+
+bool String::is_stable() const
+{
+  return str_if_stable() != nullptr;
+}
+
+void String::stabilize()
+{
+  if (is_stable()) {
+    return;
+  }
+  this->internally_mutate_to_stable_string();
+}
+
+std::string const* String::str_if_stable() const
+{
+  if (!data()) {
+    // We view no string.
+    // This is stable for the lifetime of our current value.
+    return &empty_string_;
+  }
+
+  if (string_ && data() == string_->data() && size() == string_->size()) {
+    // We view an entire string.
+    // This is stable for the lifetime of our current value.
+    return string_.get();
+  }
+
+  return nullptr;
+}
+
+std::string const& String::str()
+{
+  if (std::string const* s = str_if_stable()) {
+    return *s;
+  }
+  // Mutate to hold a std::string that is stable for the lifetime
+  // of our current value.
+  this->internally_mutate_to_stable_string();
+  return *string_;
+}
+
+const char* String::c_str()
+{
+  const char* c = data();
+  if (c == nullptr) {
+    return c;
+  }
+
+  // We always point into a null-terminated string so it is safe to
+  // access one past the end.  If it is a null byte then we can use
+  // the pointer directly.
+  if (c[size()] == '\0') {
+    return c;
+  }
+
+  // Mutate to hold a std::string so we can get a null terminator.
+  this->internally_mutate_to_stable_string();
+  c = string_->c_str();
+  return c;
+}
+
+String& String::insert(size_type index, size_type count, char ch)
+{
+  std::string s;
+  s.reserve(size() + count);
+  s.assign(data(), size());
+  s.insert(index, count, ch);
+  return *this = std::move(s);
+}
+
+String& String::erase(size_type index, size_type count)
+{
+  if (index > size()) {
+    throw std::out_of_range("Index out of range in String::erase");
+  }
+  size_type const rcount = std::min(count, size() - index);
+  size_type const rindex = index + rcount;
+  std::string s;
+  s.reserve(size() - rcount);
+  s.assign(data(), index);
+  s.append(data() + rindex, size() - rindex);
+  return *this = std::move(s);
+}
+
+String String::substr(size_type pos, size_type count) const
+{
+  if (pos > size()) {
+    throw std::out_of_range("Index out of range in String::substr");
+  }
+  return String(*this, pos, count);
+}
+
+String::String(std::string&& s, Private)
+  : string_(std::make_shared<std::string>(std::move(s)))
+  , view_(string_->data(), string_->size())
+{
+}
+
+String::size_type String::copy(char* dest, size_type count,
+                               size_type pos) const
+{
+  return view_.copy(dest, count, pos);
+}
+
+std::ostream& operator<<(std::ostream& os, String const& s)
+{
+  return os.write(s.data(), s.size());
+}
+
+std::string& operator+=(std::string& self, String const& s)
+{
+  return self += s.view();
+}
+
+String IntoString<char*>::into_string(const char* s)
+{
+  if (!s) {
+    return String();
+  }
+  return std::string(s);
+}
+
+string_view AsStringView<String>::view(String const& s)
+{
+  return s.view();
+}
+
+} // namespace cm
diff --git a/Source/cmString.hxx b/Source/cmString.hxx
new file mode 100644
index 0000000..1623a43
--- /dev/null
+++ b/Source/cmString.hxx
@@ -0,0 +1,815 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+#ifndef cmString_hxx
+#define cmString_hxx
+
+#include "cmConfigure.h" // IWYU pragma: keep
+
+#include "cm_static_string_view.hxx"
+#include "cm_string_view.hxx"
+
+#include <algorithm>
+#include <functional>
+#include <initializer_list>
+#include <memory>
+#include <ostream>
+#include <string>
+#include <type_traits>
+
+namespace cm {
+
+class String;
+
+/**
+ * Trait to convert type T into a String.
+ * Implementations must derive from 'std::true_type'
+ * and define an 'into_string' member that accepts
+ * type T (by value or reference) and returns one of:
+ *
+ * - 'std::string' to construct an owned instance.
+ * - 'cm::string_view' to construct a borrowed or null instances.
+ *   The buffer from which the view is borrowed must outlive
+ *   all copies of the resulting String, e.g. static storage.
+ * - 'cm::String' for already-constructed instances.
+ */
+template <typename T>
+struct IntoString : std::false_type
+{
+};
+
+template <typename T>
+struct IntoString<T&> : IntoString<T>
+{
+};
+
+template <typename T>
+struct IntoString<T const> : IntoString<T>
+{
+};
+
+template <typename T>
+struct IntoString<T const*> : IntoString<T*>
+{
+};
+
+template <typename T, std::string::size_type N>
+struct IntoString<T const[N]> : IntoString<T[N]>
+{
+};
+
+template <>
+struct IntoString<char*> : std::true_type
+{
+  static String into_string(const char* s);
+};
+
+template <>
+struct IntoString<std::nullptr_t> : std::true_type
+{
+  static string_view into_string(std::nullptr_t) { return string_view(); }
+};
+
+template <std::string::size_type N>
+struct IntoString<char[N]> : std::true_type
+{
+  static std::string into_string(char const (&s)[N])
+  {
+    return std::string(s, N - 1);
+  }
+};
+
+template <>
+struct IntoString<std::string> : std::true_type
+{
+  static std::string into_string(std::string s) { return s; }
+};
+
+template <>
+struct IntoString<string_view> : std::true_type
+{
+  static std::string into_string(string_view s) { return std::string(s); }
+};
+
+template <>
+struct IntoString<static_string_view> : std::true_type
+{
+  static string_view into_string(static_string_view s) { return s; }
+};
+
+template <>
+struct IntoString<char> : std::true_type
+{
+  static std::string into_string(char const& c) { return std::string(1, c); }
+};
+
+/**
+ * Trait to convert type T into a 'cm::string_view'.
+ * Implementations must derive from 'std::true_type' and
+ * define a 'view' member that accepts type T (by reference)
+ * and returns a 'cm::string_view'.
+ */
+template <typename T>
+struct AsStringView : std::false_type
+{
+};
+
+template <typename T>
+struct AsStringView<T&> : AsStringView<T>
+{
+};
+
+template <typename T>
+struct AsStringView<T const> : AsStringView<T>
+{
+};
+
+template <typename T>
+struct AsStringView<T const*> : AsStringView<T*>
+{
+};
+
+template <typename T, std::string::size_type N>
+struct AsStringView<T const[N]> : AsStringView<T[N]>
+{
+};
+
+template <>
+struct AsStringView<char*> : std::true_type
+{
+  static string_view view(const char* s) { return s; }
+};
+
+template <std::string::size_type N>
+struct AsStringView<char[N]> : std::true_type
+{
+  static string_view view(char const (&s)[N]) { return string_view(s, N - 1); }
+};
+
+template <>
+struct AsStringView<std::string> : std::true_type
+{
+  static string_view view(std::string const& s) { return s; }
+};
+
+template <>
+struct AsStringView<char> : std::true_type
+{
+  static string_view view(const char& s) { return string_view(&s, 1); }
+};
+
+template <>
+struct AsStringView<string_view> : std::true_type
+{
+  static string_view view(string_view const& s) { return s; }
+};
+
+template <>
+struct AsStringView<static_string_view> : std::true_type
+{
+  static string_view view(static_string_view const& s) { return s; }
+};
+
+template <>
+struct AsStringView<String> : std::true_type
+{
+  static string_view view(String const& s);
+};
+
+/**
+ * \class String
+ *
+ * A custom string type that holds a view of a string buffer
+ * and optionally shares ownership of the buffer.  Instances
+ * may have one of the following states:
+ *
+ * - null: views and owns nothing.
+ *   Conversion to 'bool' is 'false'.
+ *   'data()' and 'c_str()' return nullptr.
+ *   'size()' returns 0.
+ *   'str()' returns an empty string.
+ *
+ * - borrowed: views a string but does not own it.  This is used
+ *   to bind to static storage (e.g. string literals) or for
+ *   temporary instances that do not outlive the borrowed buffer.
+ *   Copies and substrings still borrow the original buffer.
+ *   Mutation allocates a new internal string and converts to
+ *   the 'owned' state.
+ *   Conversion to 'bool' is 'true'.
+ *   'c_str()' may internally mutate to the 'owned' state.
+ *   'str()' internally mutates to the 'owned' state.
+ *
+ * - owned: views an immutable 'std::string' instance owned internally.
+ *   Copies and substrings share ownership of the internal string.
+ *   Mutation allocates a new internal string.
+ *   Conversion to 'bool' is 'true'.
+ */
+class String
+{
+  enum class Private
+  {
+  };
+
+public:
+  using traits_type = std::string::traits_type;
+  using value_type = string_view::value_type;
+  using pointer = string_view::pointer;
+  using const_pointer = string_view::const_pointer;
+  using reference = string_view::reference;
+  using const_reference = string_view::const_reference;
+  using const_iterator = string_view::const_iterator;
+  using iterator = string_view::const_iterator;
+  using const_reverse_iterator = string_view::const_reverse_iterator;
+  using reverse_iterator = string_view::const_reverse_iterator;
+  using difference_type = string_view::difference_type;
+  using size_type = string_view::size_type;
+
+  static size_type const npos = string_view::npos;
+
+  /** Construct a null string.  */
+  String() = default;
+
+  /** Construct from any type implementing the IntoString trait.  */
+  template <typename T,
+            typename = typename std::enable_if<IntoString<T>::value>::type>
+  String(T&& s)
+    : String(IntoString<T>::into_string(std::forward<T>(s)), Private())
+  {
+  }
+
+  /** Construct via std::string initializer list constructor.  */
+  String(std::initializer_list<char> il)
+    : String(std::string(il))
+  {
+  }
+
+  /** Construct by copying the specified buffer.  */
+  String(const char* d, size_type s)
+    : String(std::string(d, s))
+  {
+  }
+
+  /** Construct by copying from input iterator range.  */
+  template <typename InputIterator>
+  String(InputIterator first, InputIterator last)
+    : String(std::string(first, last))
+  {
+  }
+
+  /** Construct a string with 'n' copies of character 'c'.  */
+  String(size_type n, char c)
+    : String(std::string(n, c))
+  {
+  }
+
+  /** Construct from a substring of another String instance.
+      This shares ownership of the other string's buffer
+      but views only a substring.  */
+  String(String const& s, size_type pos, size_type count = npos)
+    : string_(s.string_)
+    , view_(s.data() + pos, std::min(count, s.size() - pos))
+  {
+  }
+
+  /** Construct by moving from another String instance.
+      The other instance is left as a null string.  */
+  String(String&& s) noexcept
+    : string_(std::move(s.string_))
+    , view_(s.view_)
+  {
+    s.view_ = string_view();
+  }
+
+  /** Construct by copying from another String instance.
+      This shares ownership of the other string's buffer.  */
+  String(String const&) noexcept = default;
+
+  ~String() = default;
+
+  /** Construct by borrowing an externally-owned buffer.  The buffer
+      must outlive the returned instance and all copies of it.  */
+  static String borrow(string_view v) { return String(v, Private()); }
+
+  /** Assign by moving from another String instance.
+      The other instance is left as a null string.  */
+  String& operator=(String&& s) noexcept
+  {
+    string_ = std::move(s.string_);
+    view_ = s.view_;
+    s.view_ = string_view();
+    return *this;
+  }
+
+  /** Assign by copying from another String instance.
+      This shares ownership of the other string's buffer.  */
+  String& operator=(String const&) noexcept = default;
+
+  /** Assign from any type implementing the IntoString trait.  */
+  template <typename T>
+  typename // NOLINT(*)
+    std::enable_if<IntoString<T>::value, String&>::type
+    operator=(T&& s)
+  {
+    *this = String(std::forward<T>(s));
+    return *this;
+  }
+
+  /** Assign via std::string initializer list constructor.  */
+  String& operator=(std::initializer_list<char> il)
+  {
+    *this = String(il);
+    return *this;
+  }
+
+  /** Return true if the instance is not a null string.  */
+  explicit operator bool() const noexcept { return data() != nullptr; }
+
+  /** Return a view of the string.  */
+  string_view view() const noexcept { return view_; }
+
+  /** Return true if the instance is an empty stringn or null string.  */
+  bool empty() const noexcept { return view_.empty(); }
+
+  /** Return a pointer to the start of the string.  */
+  const char* data() const noexcept { return view_.data(); }
+
+  /** Return the length of the string in bytes.  */
+  size_type size() const noexcept { return view_.size(); }
+  size_type length() const noexcept { return view_.length(); }
+
+  /** Return the character at the given position.
+      No bounds checking is performed.  */
+  char operator[](size_type pos) const noexcept { return view_[pos]; }
+
+  /** Return the character at the given position.
+      If the position is out of bounds, throws std::out_of_range.  */
+  char at(size_type pos) const { return view_.at(pos); }
+
+  char front() const noexcept { return view_.front(); }
+
+  char back() const noexcept { return view_.back(); }
+
+  /** Return true if this instance is stable and otherwise false.
+      An instance is stable if it is in the 'null' state or if it is
+      an 'owned' state not produced by substring operations, or
+      after a call to 'stabilize()' or 'str()'.  */
+  bool is_stable() const;
+
+  /** If 'is_stable()' does not return true, mutate so it does.  */
+  void stabilize();
+
+  /** Get a pointer to a normal std::string if 'is_stable()' returns
+      true and otherwise nullptr.  The pointer is valid until this
+      instance is mutated or destroyed.  */
+  std::string const* str_if_stable() const;
+
+  /** Get a refernce to a normal std::string.  The reference
+      is valid until this instance is mutated or destroyed.  */
+  std::string const& str();
+
+  /** Get a pointer to a C-style null-terminated string
+      containing the same value as this instance.  The pointer
+      is valid until this instance is mutated, destroyed,
+      or str() is called.  */
+  const char* c_str();
+
+  const_iterator begin() const noexcept { return view_.begin(); }
+  const_iterator end() const noexcept { return view_.end(); }
+  const_iterator cbegin() const noexcept { return begin(); }
+  const_iterator cend() const noexcept { return end(); }
+
+  const_reverse_iterator rbegin() const noexcept { return view_.rbegin(); }
+  const_reverse_iterator rend() const noexcept { return view_.rend(); }
+  const_reverse_iterator crbegin() const noexcept { return rbegin(); }
+  const_reverse_iterator crend() const noexcept { return rend(); }
+
+  /** Append to the string using any type that implements the
+      AsStringView trait.  */
+  template <typename T>
+  typename std::enable_if<AsStringView<T>::value, String&>::type operator+=(
+    T&& s)
+  {
+    string_view v = AsStringView<T>::view(std::forward<T>(s));
+    std::string r;
+    r.reserve(size() + v.size());
+    r.assign(data(), size());
+    r.append(v.data(), v.size());
+    return *this = std::move(r);
+  }
+
+  /** Assign to an empty string.  */
+  void clear() { *this = ""_s; }
+
+  /** Insert 'count' copies of 'ch' at position 'index'.  */
+  String& insert(size_type index, size_type count, char ch);
+
+  /** Erase 'count' characters starting at position 'index'.  */
+  String& erase(size_type index = 0, size_type count = npos);
+
+  void push_back(char ch)
+  {
+    std::string s;
+    s.reserve(size() + 1);
+    s.assign(data(), size());
+    s.push_back(ch);
+    *this = std::move(s);
+  }
+
+  void pop_back() { *this = String(*this, 0, size() - 1); }
+
+  template <typename T>
+  typename std::enable_if<AsStringView<T>::value, String&>::type replace(
+    size_type pos, size_type count, T&& s)
+  {
+    const_iterator first = begin() + pos;
+    const_iterator last = first + count;
+    return replace(first, last, std::forward<T>(s));
+  }
+
+  template <typename InputIterator>
+  String& replace(const_iterator first, const_iterator last,
+                  InputIterator first2, InputIterator last2)
+  {
+    std::string out;
+    out.append(view_.begin(), first);
+    out.append(first2, last2);
+    out.append(last, view_.end());
+    return *this = std::move(out);
+  }
+
+  template <typename T>
+  typename std::enable_if<AsStringView<T>::value, String&>::type replace(
+    const_iterator first, const_iterator last, T&& s)
+  {
+    string_view v = AsStringView<T>::view(std::forward<T>(s));
+    std::string out;
+    out.reserve((first - view_.begin()) + v.size() + (view_.end() - last));
+    out.append(view_.begin(), first);
+    out.append(v.data(), v.size());
+    out.append(last, view_.end());
+    return *this = std::move(out);
+  }
+
+  template <typename T>
+  typename std::enable_if<AsStringView<T>::value, String&>::type replace(
+    size_type pos, size_type count, T&& s, size_type pos2,
+    size_type count2 = npos)
+  {
+    string_view v = AsStringView<T>::view(std::forward<T>(s));
+    v = v.substr(pos2, count2);
+    return replace(pos, count, v);
+  }
+
+  String& replace(size_type pos, size_type count, size_type count2, char ch)
+  {
+    const_iterator first = begin() + pos;
+    const_iterator last = first + count;
+    return replace(first, last, count2, ch);
+  }
+
+  String& replace(const_iterator first, const_iterator last, size_type count2,
+                  char ch)
+  {
+    std::string out;
+    out.reserve((first - view_.begin()) + count2 + (view_.end() - last));
+    out.append(view_.begin(), first);
+    out.append(count2, ch);
+    out.append(last, view_.end());
+    return *this = std::move(out);
+  }
+
+  size_type copy(char* dest, size_type count, size_type pos = 0) const;
+
+  void resize(size_type count) { resize(count, char()); }
+
+  void resize(size_type count, char ch)
+  {
+    std::string s;
+    s.reserve(count);
+    if (count <= size()) {
+      s.assign(data(), count);
+    } else {
+      s.assign(data(), size());
+      s.resize(count, ch);
+    }
+    *this = std::move(s);
+  }
+
+  void swap(String& other)
+  {
+    std::swap(string_, other.string_);
+    std::swap(view_, other.view_);
+  }
+
+  /** Return a substring starting at position 'pos' and
+      consisting of at most 'count' characters.  */
+  String substr(size_type pos = 0, size_type count = npos) const;
+
+  template <typename T>
+  typename std::enable_if<AsStringView<T>::value, int>::type compare(
+    T&& s) const
+  {
+    return view_.compare(AsStringView<T>::view(std::forward<T>(s)));
+  }
+
+  int compare(size_type pos1, size_type count1, string_view v) const
+  {
+    return view_.compare(pos1, count1, v);
+  }
+
+  int compare(size_type pos1, size_type count1, string_view v, size_type pos2,
+              size_type count2) const
+  {
+    return view_.compare(pos1, count1, v, pos2, count2);
+  }
+
+  int compare(size_type pos1, size_type count1, const char* s) const
+  {
+    return view_.compare(pos1, count1, s);
+  }
+
+  int compare(size_type pos1, size_type count1, const char* s,
+              size_type count2) const
+  {
+    return view_.compare(pos1, count1, s, count2);
+  }
+
+  template <typename T>
+  typename std::enable_if<AsStringView<T>::value, size_type>::type find(
+    T&& s, size_type pos = 0) const
+  {
+    string_view v = AsStringView<T>::view(std::forward<T>(s));
+    return view_.find(v, pos);
+  }
+
+  size_type find(const char* s, size_type pos, size_type count) const
+  {
+    return view_.find(s, pos, count);
+  }
+
+  template <typename T>
+  typename std::enable_if<AsStringView<T>::value, size_type>::type rfind(
+    T&& s, size_type pos = npos) const
+  {
+    string_view v = AsStringView<T>::view(std::forward<T>(s));
+    return view_.rfind(v, pos);
+  }
+
+  size_type rfind(const char* s, size_type pos, size_type count) const
+  {
+    return view_.rfind(s, pos, count);
+  }
+
+  template <typename T>
+  typename std::enable_if<AsStringView<T>::value, size_type>::type
+  find_first_of(T&& s, size_type pos = 0) const
+  {
+    string_view v = AsStringView<T>::view(std::forward<T>(s));
+    return view_.find_first_of(v, pos);
+  }
+
+  size_type find_first_of(const char* s, size_type pos, size_type count) const
+  {
+    return view_.find_first_of(s, pos, count);
+  }
+
+  template <typename T>
+  typename std::enable_if<AsStringView<T>::value, size_type>::type
+  find_first_not_of(T&& s, size_type pos = 0) const
+  {
+    string_view v = AsStringView<T>::view(std::forward<T>(s));
+    return view_.find_first_not_of(v, pos);
+  }
+
+  size_type find_first_not_of(const char* s, size_type pos,
+                              size_type count) const
+  {
+    return view_.find_first_not_of(s, pos, count);
+  }
+
+  template <typename T>
+  typename std::enable_if<AsStringView<T>::value, size_type>::type
+  find_last_of(T&& s, size_type pos = npos) const
+  {
+    string_view v = AsStringView<T>::view(std::forward<T>(s));
+    return view_.find_last_of(v, pos);
+  }
+
+  size_type find_last_of(const char* s, size_type pos, size_type count) const
+  {
+    return view_.find_last_of(s, pos, count);
+  }
+
+  template <typename T>
+  typename std::enable_if<AsStringView<T>::value, size_type>::type
+  find_last_not_of(T&& s, size_type pos = npos) const
+  {
+    string_view v = AsStringView<T>::view(std::forward<T>(s));
+    return view_.find_last_not_of(v, pos);
+  }
+
+  size_type find_last_not_of(const char* s, size_type pos,
+                             size_type count) const
+  {
+    return view_.find_last_not_of(s, pos, count);
+  }
+
+private:
+  // Internal constructor to move from existing String.
+  String(String&& s, Private) noexcept
+    : String(std::move(s))
+  {
+  }
+
+  // Internal constructor for dynamically allocated string.
+  String(std::string&& s, Private);
+
+  // Internal constructor for view of statically allocated string.
+  String(string_view v, Private)
+    : string_()
+    , view_(v)
+  {
+  }
+
+  void internally_mutate_to_stable_string();
+
+  std::shared_ptr<std::string const> string_;
+  string_view view_;
+};
+
+template <typename L, typename R>
+typename std::enable_if<AsStringView<L>::value && AsStringView<R>::value,
+                        bool>::type
+operator==(L&& l, R&& r)
+{
+  return (AsStringView<L>::view(std::forward<L>(l)) ==
+          AsStringView<R>::view(std::forward<R>(r)));
+}
+
+template <typename L, typename R>
+typename std::enable_if<AsStringView<L>::value && AsStringView<R>::value,
+                        bool>::type
+operator!=(L&& l, R&& r)
+{
+  return (AsStringView<L>::view(std::forward<L>(l)) !=
+          AsStringView<R>::view(std::forward<R>(r)));
+}
+
+template <typename L, typename R>
+typename std::enable_if<AsStringView<L>::value && AsStringView<R>::value,
+                        bool>::type
+operator<(L&& l, R&& r)
+{
+  return (AsStringView<L>::view(std::forward<L>(l)) <
+          AsStringView<R>::view(std::forward<R>(r)));
+}
+
+template <typename L, typename R>
+typename std::enable_if<AsStringView<L>::value && AsStringView<R>::value,
+                        bool>::type
+operator<=(L&& l, R&& r)
+{
+  return (AsStringView<L>::view(std::forward<L>(l)) <=
+          AsStringView<R>::view(std::forward<R>(r)));
+}
+
+template <typename L, typename R>
+typename std::enable_if<AsStringView<L>::value && AsStringView<R>::value,
+                        bool>::type
+operator>(L&& l, R&& r)
+{
+  return (AsStringView<L>::view(std::forward<L>(l)) >
+          AsStringView<R>::view(std::forward<R>(r)));
+}
+
+template <typename L, typename R>
+typename std::enable_if<AsStringView<L>::value && AsStringView<R>::value,
+                        bool>::type
+operator>=(L&& l, R&& r)
+{
+  return (AsStringView<L>::view(std::forward<L>(l)) >=
+          AsStringView<R>::view(std::forward<R>(r)));
+}
+
+std::ostream& operator<<(std::ostream& os, String const& s);
+std::string& operator+=(std::string& self, String const& s);
+
+template <typename L, typename R>
+struct StringOpPlus
+{
+  L l;
+  R r;
+#if defined(__SUNPRO_CC)
+  StringOpPlus(L in_l, R in_r)
+    : l(in_l)
+    , r(in_r)
+  {
+  }
+#endif
+  operator std::string() const;
+  std::string::size_type size() const { return l.size() + r.size(); }
+};
+
+template <typename T>
+struct StringAdd
+{
+  static const bool value = AsStringView<T>::value;
+  typedef string_view temp_type;
+  template <typename S>
+  static temp_type temp(S&& s)
+  {
+    return AsStringView<T>::view(std::forward<S>(s));
+  }
+};
+
+template <typename L, typename R>
+struct StringAdd<StringOpPlus<L, R>> : std::true_type
+{
+  typedef StringOpPlus<L, R> const& temp_type;
+  static temp_type temp(temp_type s) { return s; }
+};
+
+template <typename L, typename R>
+StringOpPlus<L, R>::operator std::string() const
+{
+  std::string s;
+  s.reserve(size());
+  s += *this;
+  return s;
+}
+
+template <typename L, typename R>
+std::string& operator+=(std::string& s, StringOpPlus<L, R> const& a)
+{
+  s.reserve(s.size() + a.size());
+  s += a.l;
+  s += a.r;
+  return s;
+}
+
+template <typename L, typename R>
+String& operator+=(String& s, StringOpPlus<L, R> const& a)
+{
+  std::string r;
+  r.reserve(s.size() + a.size());
+  r.assign(s.data(), s.size());
+  r += a.l;
+  r += a.r;
+  s = std::move(r);
+  return s;
+}
+
+template <typename L, typename R>
+std::ostream& operator<<(std::ostream& os, StringOpPlus<L, R> const& a)
+{
+  return os << a.l << a.r;
+}
+
+template <typename L, typename R>
+struct IntoString<StringOpPlus<L, R>> : std::true_type
+{
+  static std::string into_string(StringOpPlus<L, R> const& a) { return a; }
+};
+
+template <typename L, typename R>
+typename std::enable_if<StringAdd<L>::value && StringAdd<R>::value,
+                        StringOpPlus<typename StringAdd<L>::temp_type,
+                                     typename StringAdd<R>::temp_type>>::type
+operator+(L&& l, R&& r)
+{
+  return { StringAdd<L>::temp(std::forward<L>(l)),
+           StringAdd<R>::temp(std::forward<R>(r)) };
+}
+
+template <typename LL, typename LR, typename R>
+typename std::enable_if<AsStringView<R>::value, bool>::type operator==(
+  StringOpPlus<LL, LR> const& l, R&& r)
+{
+  return std::string(l) == AsStringView<R>::view(std::forward<R>(r));
+}
+
+template <typename L, typename RL, typename RR>
+typename std::enable_if<AsStringView<L>::value, bool>::type operator==(
+  L&& l, StringOpPlus<RL, RR> const& r)
+{
+  return AsStringView<L>::view(std::forward<L>(l)) == std::string(r);
+}
+
+} // namespace cm
+
+namespace std {
+
+template <>
+struct hash<cm::String>
+{
+  typedef cm::String argument_type;
+  typedef size_t result_type;
+
+  result_type operator()(argument_type const& s) const noexcept
+  {
+    result_type const h(std::hash<cm::string_view>{}(s.view()));
+    return h;
+  }
+};
+}
+
+#endif
diff --git a/Source/cm_static_string_view.hxx b/Source/cm_static_string_view.hxx
new file mode 100644
index 0000000..1bef0c6
--- /dev/null
+++ b/Source/cm_static_string_view.hxx
@@ -0,0 +1,41 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+#ifndef cm_static_string_view_hxx
+#define cm_static_string_view_hxx
+
+#include "cmConfigure.h" // IWYU pragma: keep
+
+#include "cm_string_view.hxx"
+
+#include <cstddef>
+
+namespace cm {
+
+/** A string_view that only binds to static storage.
+ *
+ * This is used together with the `""_s` user-defined literal operator
+ * to construct a type-safe abstraction of a string_view that only views
+ * statically allocated strings.  These strings are const and available
+ * for the entire lifetime of the program.
+ */
+class static_string_view : public string_view
+{
+  static_string_view(string_view v)
+    : string_view(v)
+  {
+  }
+
+  friend static_string_view operator"" _s(const char* data, size_t size);
+};
+
+/** Create a static_string_view using `""_s` literal syntax.  */
+inline static_string_view operator"" _s(const char* data, size_t size)
+{
+  return string_view(data, size);
+}
+
+} // namespace cm
+
+using cm::operator"" _s;
+
+#endif
diff --git a/Source/cm_string_view.cxx b/Source/cm_string_view.cxx
new file mode 100644
index 0000000..61fa80e
--- /dev/null
+++ b/Source/cm_string_view.cxx
@@ -0,0 +1,301 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+
+#include "cm_string_view.hxx"
+
+#ifndef CMake_HAVE_CXX_STRING_VIEW
+
+#  include "cm_kwiml.h"
+
+#  include <algorithm>
+#  include <ostream>
+#  include <stdexcept>
+
+namespace cm {
+
+string_view::const_reference string_view::at(size_type pos) const
+{
+  if (pos >= size_) {
+    throw std::out_of_range("Index out of range in string_view::at");
+  }
+  return data_[pos];
+}
+
+string_view::size_type string_view::copy(char* dest, size_type count,
+                                         size_type pos) const
+{
+  if (pos > size_) {
+    throw std::out_of_range("Index out of range in string_view::copy");
+  }
+  size_type const rcount = std::min(count, size_ - pos);
+  traits_type::copy(dest, data_ + pos, rcount);
+  return rcount;
+}
+
+string_view string_view::substr(size_type pos, size_type count) const
+{
+  if (pos > size_) {
+    throw std::out_of_range("Index out of range in string_view::substr");
+  }
+  size_type const rcount = std::min(count, size_ - pos);
+  return string_view(data_ + pos, rcount);
+}
+
+int string_view::compare(string_view v) const noexcept
+{
+  size_type const rlen = std::min(size_, v.size_);
+  int c = traits_type::compare(data_, v.data_, rlen);
+  if (c == 0) {
+    if (size_ < v.size_) {
+      c = -1;
+    } else if (size_ > v.size_) {
+      c = 1;
+    }
+  }
+  return c;
+}
+
+int string_view::compare(size_type pos1, size_type count1, string_view v) const
+{
+  return substr(pos1, count1).compare(v);
+}
+
+int string_view::compare(size_type pos1, size_type count1, string_view v,
+                         size_type pos2, size_type count2) const
+{
+  return substr(pos1, count1).compare(v.substr(pos2, count2));
+}
+
+int string_view::compare(const char* s) const
+{
+  return compare(string_view(s));
+}
+
+int string_view::compare(size_type pos1, size_type count1, const char* s) const
+{
+  return substr(pos1, count1).compare(string_view(s));
+}
+
+int string_view::compare(size_type pos1, size_type count1, const char* s,
+                         size_type count2) const
+{
+  return substr(pos1, count1).compare(string_view(s, count2));
+}
+
+string_view::size_type string_view::find(string_view v, size_type pos) const
+  noexcept
+{
+  for (; pos + v.size_ <= size_; ++pos) {
+    if (std::char_traits<char>::compare(data_ + pos, v.data_, v.size_) == 0) {
+      return pos;
+    }
+  }
+  return npos;
+}
+
+string_view::size_type string_view::find(char c, size_type pos) const noexcept
+{
+  return find(string_view(&c, 1), pos);
+}
+
+string_view::size_type string_view::find(const char* s, size_type pos,
+                                         size_type count) const
+{
+  return find(string_view(s, count), pos);
+}
+
+string_view::size_type string_view::find(const char* s, size_type pos) const
+{
+  return find(string_view(s), pos);
+}
+
+string_view::size_type string_view::rfind(string_view v, size_type pos) const
+  noexcept
+{
+  if (size_ >= v.size_) {
+    for (pos = std::min(pos, size_ - v.size_) + 1; pos > 0;) {
+      --pos;
+      if (std::char_traits<char>::compare(data_ + pos, v.data_, v.size_) ==
+          0) {
+        return pos;
+      }
+    }
+  }
+  return npos;
+}
+
+string_view::size_type string_view::rfind(char c, size_type pos) const noexcept
+{
+  return rfind(string_view(&c, 1), pos);
+}
+
+string_view::size_type string_view::rfind(const char* s, size_type pos,
+                                          size_type count) const
+{
+  return rfind(string_view(s, count), pos);
+}
+
+string_view::size_type string_view::rfind(const char* s, size_type pos) const
+{
+  return rfind(string_view(s), pos);
+}
+
+string_view::size_type string_view::find_first_of(string_view v,
+                                                  size_type pos) const noexcept
+{
+  for (; pos < size_; ++pos) {
+    if (traits_type::find(v.data_, v.size_, data_[pos])) {
+      return pos;
+    }
+  }
+  return npos;
+}
+
+string_view::size_type string_view::find_first_of(char c, size_type pos) const
+  noexcept
+{
+  return find_first_of(string_view(&c, 1), pos);
+}
+
+string_view::size_type string_view::find_first_of(const char* s, size_type pos,
+                                                  size_type count) const
+{
+  return find_first_of(string_view(s, count), pos);
+}
+
+string_view::size_type string_view::find_first_of(const char* s,
+                                                  size_type pos) const
+{
+  return find_first_of(string_view(s), pos);
+}
+
+string_view::size_type string_view::find_last_of(string_view v,
+                                                 size_type pos) const noexcept
+{
+  if (size_ > 0) {
+    for (pos = std::min(pos, size_ - 1) + 1; pos > 0;) {
+      --pos;
+      if (traits_type::find(v.data_, v.size_, data_[pos])) {
+        return pos;
+      }
+    }
+  }
+  return npos;
+}
+
+string_view::size_type string_view::find_last_of(char c, size_type pos) const
+  noexcept
+{
+  return find_last_of(string_view(&c, 1), pos);
+}
+
+string_view::size_type string_view::find_last_of(const char* s, size_type pos,
+                                                 size_type count) const
+{
+  return find_last_of(string_view(s, count), pos);
+}
+
+string_view::size_type string_view::find_last_of(const char* s,
+                                                 size_type pos) const
+{
+  return find_last_of(string_view(s), pos);
+}
+
+string_view::size_type string_view::find_first_not_of(string_view v,
+                                                      size_type pos) const
+  noexcept
+{
+  for (; pos < size_; ++pos) {
+    if (!traits_type::find(v.data_, v.size_, data_[pos])) {
+      return pos;
+    }
+  }
+  return npos;
+}
+
+string_view::size_type string_view::find_first_not_of(char c,
+                                                      size_type pos) const
+  noexcept
+{
+  return find_first_not_of(string_view(&c, 1), pos);
+}
+
+string_view::size_type string_view::find_first_not_of(const char* s,
+                                                      size_type pos,
+                                                      size_type count) const
+{
+  return find_first_not_of(string_view(s, count), pos);
+}
+
+string_view::size_type string_view::find_first_not_of(const char* s,
+                                                      size_type pos) const
+{
+  return find_first_not_of(string_view(s), pos);
+}
+
+string_view::size_type string_view::find_last_not_of(string_view v,
+                                                     size_type pos) const
+  noexcept
+{
+  if (size_ > 0) {
+    for (pos = std::min(pos, size_ - 1) + 1; pos > 0;) {
+      --pos;
+      if (!traits_type::find(v.data_, v.size_, data_[pos])) {
+        return pos;
+      }
+    }
+  }
+  return npos;
+}
+
+string_view::size_type string_view::find_last_not_of(char c,
+                                                     size_type pos) const
+  noexcept
+{
+  return find_last_not_of(string_view(&c, 1), pos);
+}
+
+string_view::size_type string_view::find_last_not_of(const char* s,
+                                                     size_type pos,
+                                                     size_type count) const
+{
+  return find_last_not_of(string_view(s, count), pos);
+}
+
+string_view::size_type string_view::find_last_not_of(const char* s,
+                                                     size_type pos) const
+{
+  return find_last_not_of(string_view(s), pos);
+}
+
+std::ostream& operator<<(std::ostream& o, string_view v)
+{
+  return o.write(v.data(), v.size());
+}
+
+std::string& operator+=(std::string& s, string_view v)
+{
+  s.append(v.data(), v.size());
+  return s;
+}
+}
+
+std::hash<cm::string_view>::result_type std::hash<cm::string_view>::operator()(
+  argument_type const& s) const noexcept
+{
+  // FNV-1a hash.
+  static KWIML_INT_uint64_t const fnv_offset_basis = 0xcbf29ce484222325;
+  static KWIML_INT_uint64_t const fnv_prime = 0x100000001b3;
+  KWIML_INT_uint64_t h = fnv_offset_basis;
+  for (char const& c : s) {
+    h = h ^ KWIML_INT_uint64_t(KWIML_INT_uint8_t(c));
+    h = h * fnv_prime;
+  }
+  return result_type(h);
+}
+#else
+// Avoid empty translation unit.
+void cm_string_view_cxx()
+{
+}
+#endif
diff --git a/Source/cm_string_view.hxx b/Source/cm_string_view.hxx
new file mode 100644
index 0000000..d368ed8
--- /dev/null
+++ b/Source/cm_string_view.hxx
@@ -0,0 +1,217 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+#ifndef cm_string_view_hxx
+#define cm_string_view_hxx
+
+#include "cmConfigure.h" // IWYU pragma: keep
+
+#if __cplusplus >= 201703L || defined(_MSVC_LANG) && _MSVC_LANG >= 201703L
+#  define CMake_HAVE_CXX_STRING_VIEW
+#endif
+
+#ifdef CMake_HAVE_CXX_STRING_VIEW
+#  include <string_view>
+namespace cm {
+using std::string_view;
+}
+#else
+#  include <cstddef>
+#  include <functional>
+#  include <iosfwd>
+#  include <iterator>
+#  include <string>
+
+namespace cm {
+
+class string_view
+{
+public:
+  using traits_type = std::string::traits_type;
+  using value_type = char;
+  using pointer = char*;
+  using const_pointer = const char*;
+  using reference = char&;
+  using const_reference = char const&;
+  using const_iterator = const char*;
+  using iterator = const_iterator;
+  using const_reverse_iterator = std::reverse_iterator<const_iterator>;
+  using reverse_iterator = const_reverse_iterator;
+  using size_type = std::string::size_type;
+  using difference_type = std::string::difference_type;
+
+  static size_type const npos = static_cast<size_type>(-1);
+
+  string_view() noexcept = default;
+  string_view(string_view const&) noexcept = default;
+
+  string_view(const char* s, size_t count) noexcept
+    : data_(s)
+    , size_(count)
+  {
+  }
+
+  string_view(const char* s) noexcept
+    : data_(s)
+    , size_(traits_type::length(s))
+  {
+  }
+
+  // C++17 does not define this constructor.  Instead it defines
+  // a conversion operator on std::string to create a string_view.
+  // Since this implementation is used in C++11, std::string does
+  // not have that conversion.
+  string_view(std::string const& s) noexcept
+    : data_(s.data())
+    , size_(s.size())
+  {
+  }
+
+  // C++17 does not define this conversion.  Instead it defines
+  // a constructor on std::string that can take a string_view.
+  // Since this implementation is used in C++11, std::string does
+  // not have that constructor.
+  explicit operator std::string() const { return std::string(data_, size_); }
+
+  string_view& operator=(string_view const&) = default;
+
+  const_iterator begin() const noexcept { return data_; }
+  const_iterator end() const noexcept { return data_ + size_; }
+  const_iterator cbegin() const noexcept { return begin(); }
+  const_iterator cend() const noexcept { return end(); }
+
+  const_reverse_iterator rbegin() const noexcept
+  {
+    return const_reverse_iterator(end());
+  }
+  const_reverse_iterator rend() const noexcept
+  {
+    return const_reverse_iterator(begin());
+  }
+  const_reverse_iterator crbegin() const noexcept { return rbegin(); }
+  const_reverse_iterator crend() const noexcept { return rend(); }
+
+  const_reference operator[](size_type pos) const noexcept
+  {
+    return data_[pos];
+  }
+  const_reference at(size_type pos) const;
+  const_reference front() const noexcept { return data_[0]; }
+  const_reference back() const noexcept { return data_[size_ - 1]; }
+  const_pointer data() const noexcept { return data_; }
+
+  size_type size() const noexcept { return size_; }
+  size_type length() const noexcept { return size_; }
+  size_type max_size() const noexcept { return npos - 1; }
+  bool empty() const noexcept { return size_ == 0; }
+
+  void remove_prefix(size_type n) noexcept
+  {
+    data_ += n;
+    size_ -= n;
+  }
+  void remove_suffix(size_type n) noexcept { size_ -= n; }
+  void swap(string_view& v) noexcept
+  {
+    string_view tmp = v;
+    v = *this;
+    *this = tmp;
+  }
+
+  size_type copy(char* dest, size_type count, size_type pos = 0) const;
+  string_view substr(size_type pos = 0, size_type count = npos) const;
+
+  int compare(string_view v) const noexcept;
+  int compare(size_type pos1, size_type count1, string_view v) const;
+  int compare(size_type pos1, size_type count1, string_view v, size_type pos2,
+              size_type count2) const;
+  int compare(const char* s) const;
+  int compare(size_type pos1, size_type count1, const char* s) const;
+  int compare(size_type pos1, size_type count1, const char* s,
+              size_type count2) const;
+
+  size_type find(string_view v, size_type pos = 0) const noexcept;
+  size_type find(char c, size_type pos = 0) const noexcept;
+  size_type find(const char* s, size_type pos, size_type count) const;
+  size_type find(const char* s, size_type pos = 0) const;
+
+  size_type rfind(string_view v, size_type pos = npos) const noexcept;
+  size_type rfind(char c, size_type pos = npos) const noexcept;
+  size_type rfind(const char* s, size_type pos, size_type count) const;
+  size_type rfind(const char* s, size_type pos = npos) const;
+
+  size_type find_first_of(string_view v, size_type pos = 0) const noexcept;
+  size_type find_first_of(char c, size_type pos = 0) const noexcept;
+  size_type find_first_of(const char* s, size_type pos, size_type count) const;
+  size_type find_first_of(const char* s, size_type pos = 0) const;
+
+  size_type find_last_of(string_view v, size_type pos = npos) const noexcept;
+  size_type find_last_of(char c, size_type pos = npos) const noexcept;
+  size_type find_last_of(const char* s, size_type pos, size_type count) const;
+  size_type find_last_of(const char* s, size_type pos = npos) const;
+
+  size_type find_first_not_of(string_view v, size_type pos = 0) const noexcept;
+  size_type find_first_not_of(char c, size_type pos = 0) const noexcept;
+  size_type find_first_not_of(const char* s, size_type pos,
+                              size_type count) const;
+  size_type find_first_not_of(const char* s, size_type pos = 0) const;
+
+  size_type find_last_not_of(string_view v, size_type pos = npos) const
+    noexcept;
+  size_type find_last_not_of(char c, size_type pos = npos) const noexcept;
+  size_type find_last_not_of(const char* s, size_type pos,
+                             size_type count) const;
+  size_type find_last_not_of(const char* s, size_type pos = npos) const;
+
+private:
+  const char* data_ = nullptr;
+  size_type size_ = 0;
+};
+
+std::ostream& operator<<(std::ostream& o, string_view v);
+
+std::string& operator+=(std::string& s, string_view v);
+
+inline bool operator==(string_view l, string_view r) noexcept
+{
+  return l.compare(r) == 0;
+}
+
+inline bool operator!=(string_view l, string_view r) noexcept
+{
+  return l.compare(r) != 0;
+}
+
+inline bool operator<(string_view l, string_view r) noexcept
+{
+  return l.compare(r) < 0;
+}
+
+inline bool operator<=(string_view l, string_view r) noexcept
+{
+  return l.compare(r) <= 0;
+}
+
+inline bool operator>(string_view l, string_view r) noexcept
+{
+  return l.compare(r) > 0;
+}
+
+inline bool operator>=(string_view l, string_view r) noexcept
+{
+  return l.compare(r) >= 0;
+}
+}
+
+namespace std {
+
+template <>
+struct hash<cm::string_view>
+{
+  typedef cm::string_view argument_type;
+  typedef size_t result_type;
+  result_type operator()(argument_type const& s) const noexcept;
+};
+}
+
+#endif
+#endif