Add support for using C++17 string_view or a fallback

Define a `cm::string_view` type implemented via C++17 `std::string_view`
when available.  Provide a fallback implementation for C++11 and C++14
compilers.

The fallback implementation was written by reading documentation of the
standard spec.  We have no dedicated tests for it, but it will be
covered by tests of its clients later.

Source/CMakeLists.txt

@@ -375,6 +375,8 @@ set(SRCS
   cmXMLWriter.h
   cmake.cxx
   cmake.h
+  cm_string_view.cxx
+  cm_string_view.hxx
 
   cmCommand.cxx
   cmCommand.h

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

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

bootstrap

@@ -430,6 +430,7 @@ CMAKE_CXX_SOURCES="\
   cmake  \
   cmakemain \
   cmcmd  \
+  cm_string_view \
 "
 
 if ${cmake_system_mingw}; then