chromium/v8/src/base/string-format.h

// Copyright 2022 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef V8_BASE_STRING_FORMAT_H_
#define V8_BASE_STRING_FORMAT_H_

#include <array>
#include <limits>
#include <string_view>
#include <tuple>

#include "src/base/logging.h"
#include "src/base/platform/platform.h"

namespace v8::base {

// Implementation detail, do not use outside this header. The public interface
// is below.
namespace impl {

template <const std::string_view&... strs>
struct JoinedStringViews { … };

template <typename T>
struct FormattedStringPart { … };

template <>
struct FormattedStringPart<int> { … };

template <>
struct FormattedStringPart<size_t> { … };

FormattedStringPart<char[N]>;

template <const std::string_view& kFormat, int kMaxLen, typename... Parts>
std::array<char, kMaxLen> PrintFormattedStringToArray(Parts... parts) { … }

}  // namespace impl

// `FormattedString` allows to format strings with statically known number and
// type of constituents.
// The class stores all values that should be printed, and generates the final
// string via `SNPrintF` into a `std::array`, without any dynamic memory
// allocation. The format string is computed statically.
// This makes this class not only very performant, but also suitable for
// situations where we do not want to perform any memory allocation (like for
// reporting OOM or fatal errors).
//
// Use like this:
//   auto message = FormattedString{} << "Cannot allocate " << size << " bytes";
//   V8::FatalProcessOutOfMemory(nullptr, message.PrintToArray().data());
//
// This code is compiled into the equivalent of
//   std::array<char, 34> message_arr;
//   int chars = SNPrintF(message_arr.data(), 34, "%s%d%s", "Cannot allocate ",
//                        size, " bytes");
//   CHECK(chars >= 0 && chars < 34);
//   V8::FatalProcessOutOfMemory(nullptr, message_arr.data());
template <typename... Ts>
class FormattedString { … };

// Add an explicit deduction guide for empty template parameters (fixes
// clang's -Wctad-maybe-unsupported warning). Non-empty formatted strings
// explicitly declare template parameters anyway.
<deduction guide for FormattedString>();

}  // namespace v8::base

#endif  // V8_BASE_STRING_FORMAT_H_