make detail::basic_memory_buffer constexpr with C++20

include/fmt/core.h

@@ -100,6 +100,13 @@
 #  define FMT_CONSTEXPR_DECL
 #endif
 
+#if __cplusplus >= 202002L || \
+    (__cplusplus >= 201709L && FMT_GCC_VERSION >= 1002)
+#  define FMT_CONSTEXPR20 constexpr
+#else
+#  define FMT_CONSTEXPR20
+#endif
+
 // Check if constexpr std::char_traits<>::compare,length is supported.
 #if defined(__GLIBCXX__)
 #  if __cplusplus >= 201703L && defined(_GLIBCXX_RELEASE) && \
@@ -773,22 +780,22 @@ template <typename T> class buffer {
   FMT_MSC_WARNING(suppress : 26495)
   buffer(size_t sz) FMT_NOEXCEPT : size_(sz), capacity_(sz) {}
 
-  buffer(T* p = nullptr, size_t sz = 0, size_t cap = 0) FMT_NOEXCEPT
-      : ptr_(p),
-        size_(sz),
-        capacity_(cap) {}
+  FMT_CONSTEXPR20 buffer(T* p = nullptr, size_t sz = 0,
+                         size_t cap = 0) FMT_NOEXCEPT : ptr_(p),
+                                                        size_(sz),
+                                                        capacity_(cap) {}
 
-  ~buffer() = default;
+  FMT_CONSTEXPR20 ~buffer() = default;
   buffer(buffer&&) = default;
 
   /** Sets the buffer data and capacity. */
-  void set(T* buf_data, size_t buf_capacity) FMT_NOEXCEPT {
+  FMT_CONSTEXPR void set(T* buf_data, size_t buf_capacity) FMT_NOEXCEPT {
     ptr_ = buf_data;
     capacity_ = buf_capacity;
   }
 
   /** Increases the buffer capacity to hold at least *capacity* elements. */
-  virtual void grow(size_t capacity) = 0;
+  virtual FMT_CONSTEXPR20 void grow(size_t capacity) = 0;
 
  public:
   using value_type = T;
@@ -804,23 +811,23 @@ template <typename T> class buffer {
   auto end() const FMT_NOEXCEPT -> const T* { return ptr_ + size_; }
 
   /** Returns the size of this buffer. */
-  auto size() const FMT_NOEXCEPT -> size_t { return size_; }
+  constexpr auto size() const FMT_NOEXCEPT -> size_t { return size_; }
 
   /** Returns the capacity of this buffer. */
-  auto capacity() const FMT_NOEXCEPT -> size_t { return capacity_; }
+  constexpr auto capacity() const FMT_NOEXCEPT -> size_t { return capacity_; }
 
   /** Returns a pointer to the buffer data. */
-  auto data() FMT_NOEXCEPT -> T* { return ptr_; }
+  FMT_CONSTEXPR auto data() FMT_NOEXCEPT -> T* { return ptr_; }
 
   /** Returns a pointer to the buffer data. */
-  auto data() const FMT_NOEXCEPT -> const T* { return ptr_; }
+  FMT_CONSTEXPR auto data() const FMT_NOEXCEPT -> const T* { return ptr_; }
 
   /** Clears this buffer. */
   void clear() { size_ = 0; }
 
   // Tries resizing the buffer to contain *count* elements. If T is a POD type
   // the new elements may not be initialized.
-  void try_resize(size_t count) {
+  FMT_CONSTEXPR20 void try_resize(size_t count) {
     try_reserve(count);
     size_ = count <= capacity_ ? count : capacity_;
   }
@@ -829,11 +836,11 @@ template <typename T> class buffer {
   // capacity by a smaller amount than requested but guarantees there is space
   // for at least one additional element either by increasing the capacity or by
   // flushing the buffer if it is full.
-  void try_reserve(size_t new_capacity) {
+  FMT_CONSTEXPR20 void try_reserve(size_t new_capacity) {
     if (new_capacity > capacity_) grow(new_capacity);
   }
 
-  void push_back(const T& value) {
+  FMT_CONSTEXPR20 void push_back(const T& value) {
     try_reserve(size_ + 1);
     ptr_[size_++] = value;
   }
@@ -841,8 +848,11 @@ template <typename T> class buffer {
   /** Appends data to the end of the buffer. */
   template <typename U> void append(const U* begin, const U* end);
 
-  template <typename I> auto operator[](I index) -> T& { return ptr_[index]; }
-  template <typename I> auto operator[](I index) const -> const T& {
+  template <typename I> FMT_CONSTEXPR auto operator[](I index) -> T& {
+    return ptr_[index];
+  }
+  template <typename I>
+  FMT_CONSTEXPR auto operator[](I index) const -> const T& {
     return ptr_[index];
   }
 };

include/fmt/format.h

@@ -249,14 +249,6 @@ FMT_END_NAMESPACE
 
 FMT_BEGIN_NAMESPACE
 namespace detail {
-
-#if __cplusplus >= 202002L || \
-    (__cplusplus >= 201709L && FMT_GCC_VERSION >= 1002)
-#  define FMT_CONSTEXPR20 constexpr
-#else
-#  define FMT_CONSTEXPR20
-#endif
-
 // An equivalent of `*reinterpret_cast<Dest*>(&source)` that doesn't have
 // undefined behavior (e.g. due to type aliasing).
 // Example: uint64_t d = bit_cast<uint64_t>(2.718);
@@ -644,7 +636,7 @@ class basic_memory_buffer final : public detail::buffer<T> {
   Allocator alloc_;
 
   // Deallocate memory allocated by the buffer.
-  void deallocate() {
+  FMT_CONSTEXPR20 void deallocate() {
     T* data = this->data();
     if (data != store_) alloc_.deallocate(data, this->capacity());
   }
@@ -656,22 +648,31 @@ class basic_memory_buffer final : public detail::buffer<T> {
   using value_type = T;
   using const_reference = const T&;
 
-  explicit basic_memory_buffer(const Allocator& alloc = Allocator())
+  FMT_CONSTEXPR20 explicit basic_memory_buffer(
+      const Allocator& alloc = Allocator())
       : alloc_(alloc) {
     this->set(store_, SIZE);
+    if (detail::is_constant_evaluated()) {
+      detail::fill_n(store_, SIZE, T{});
+    }
   }
-  ~basic_memory_buffer() { deallocate(); }
+  FMT_CONSTEXPR20 ~basic_memory_buffer() { deallocate(); }
 
  private:
   // Move data from other to this buffer.
-  void move(basic_memory_buffer& other) {
+  FMT_CONSTEXPR20 void move(basic_memory_buffer& other) {
     alloc_ = std::move(other.alloc_);
     T* data = other.data();
     size_t size = other.size(), capacity = other.capacity();
     if (data == other.store_) {
       this->set(store_, capacity);
-      std::uninitialized_copy(other.store_, other.store_ + size,
-                              detail::make_checked(store_, capacity));
+      if (detail::is_constant_evaluated()) {
+        detail::copy_str<T>(other.store_, other.store_ + size,
+                            detail::make_checked(store_, capacity));
+      } else {
+        std::uninitialized_copy(other.store_, other.store_ + size,
+                                detail::make_checked(store_, capacity));
+      }
     } else {
       this->set(data, capacity);
       // Set pointer to the inline array so that delete is not called
@@ -688,7 +689,10 @@ class basic_memory_buffer final : public detail::buffer<T> {
     of the other object to it.
     \endrst
    */
-  basic_memory_buffer(basic_memory_buffer&& other) FMT_NOEXCEPT { move(other); }
+  FMT_CONSTEXPR20 basic_memory_buffer(basic_memory_buffer&& other)
+      FMT_NOEXCEPT {
+    move(other);
+  }
 
   /**
     \rst
@@ -710,7 +714,7 @@ class basic_memory_buffer final : public detail::buffer<T> {
     Resizes the buffer to contain *count* elements. If T is a POD type new
     elements may not be initialized.
    */
-  void resize(size_t count) { this->try_resize(count); }
+  FMT_CONSTEXPR20 void resize(size_t count) { this->try_resize(count); }
 
   /** Increases the buffer capacity to *new_capacity*. */
   void reserve(size_t new_capacity) { this->try_reserve(new_capacity); }