chromium/base/strings/strcat_internal.h

// Copyright 2020 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifdef UNSAFE_BUFFERS_BUILD
// TODO(crbug.com/40284755): Remove this and spanify to fix the errors.
#pragma allow_unsafe_buffers
#endif

#ifndef BASE_STRINGS_STRCAT_INTERNAL_H_
#define BASE_STRINGS_STRCAT_INTERNAL_H_

#include <concepts>
#include <string>

#include "base/containers/span.h"

namespace base {

namespace internal {

// Optimized version of `std::basic_string::resize()` that skips zero
// initialization of appended characters. Reading from the newly allocated
// characters results in undefined behavior if they are not explicitly
// initialized afterwards. Currently proposed for standardization as
// std::basic_string::resize_and_overwrite: https://wg21.link/P1072R6
template <typename CharT>
  requires requires(std::basic_string<CharT>& str, size_t total_size) { … }

// Fallback to regular std::basic_string::resize() if invoking
// __resize_default_init is ill-formed.
template <typename CharT>
void Resize(std::basic_string<CharT>& str, size_t total_size) { … }

// Appends `pieces` to `dest`. Instead of simply calling `dest.append()`
// `pieces.size()` times, this method first resizes `dest` to be of the desired
// size, and then appends each piece via `std::char_traits::copy`. This achieves
// two goals:
// 1) Allocating the desired size all at once avoids other allocations that
//    could happen if intermediate allocations did not reserve enough capacity.
// 2) Invoking std::char_traits::copy instead of std::basic_string::append
//    avoids having to write the terminating '\0' character n times.
template <typename CharT, typename StringT>
void StrAppendT(std::basic_string<CharT>& dest, span<const StringT> pieces) { … }

template <typename StringT>
auto StrCatT(span<const StringT> pieces) { … }

}  // namespace internal

}  // namespace base

#endif  // BASE_STRINGS_STRCAT_INTERNAL_H_