Add a fixed buffer

include/fmt/core.h

@@ -629,16 +629,18 @@ template <typename T> class buffer {
   T* ptr_;
   size_t size_;
   size_t capacity_;
+  bool fixed_;
 
  protected:
   // Don't initialize ptr_ since it is not accessed to save a few cycles.
   FMT_SUPPRESS_MSC_WARNING(26495)
-  buffer(size_t sz) FMT_NOEXCEPT : size_(sz), capacity_(sz) {}
+  buffer(size_t sz) FMT_NOEXCEPT : size_(sz), capacity_(sz), fixed_(false) {}
 
-  buffer(T* p = nullptr, size_t sz = 0, size_t cap = 0) FMT_NOEXCEPT
-      : ptr_(p),
-        size_(sz),
-        capacity_(cap) {}
+  buffer(T* p = nullptr, size_t sz = 0, size_t cap = 0,
+         bool fixed = false) FMT_NOEXCEPT : ptr_(p),
+                                            size_(sz),
+                                            capacity_(cap),
+                                            fixed_(fixed) {}
 
   /** Sets the buffer data and capacity. */
   void set(T* buf_data, size_t buf_capacity) FMT_NOEXCEPT {
@@ -705,6 +707,12 @@ template <typename T> class buffer {
   }
 };
 
+// A fixed capacity buffer.
+template <typename T> class fixed_buffer : buffer<T> {
+ public:
+  fixed_buffer(T* data, size_t capacity) : buffer<T>(data, 0, capacity, true) {}
+};
+
 // A container-backed buffer.
 template <typename Container>
 class container_buffer : public buffer<typename Container::value_type> {

include/fmt/format.h

@@ -372,8 +372,10 @@ reserve(std::back_insert_iterator<Container> it, size_t n) {
 }
 
 template <typename T>
-inline checked_ptr<T> reserve(buffer_appender<T> it, size_t n) {
-  return reserve(std::back_insert_iterator<buffer<T>>(it), n);
+inline buffer_appender<T> reserve(buffer_appender<T> it, size_t n) {
+  buffer<T>& buf = get_container(it);
+  buf.reserve(buf.size() + n);
+  return it;
 }
 
 template <typename Iterator> inline Iterator& reserve(Iterator& it, size_t) {