当我自己实现 initializer_list 时遇到问题时，我发现了这篇文章，并发现编写自己的版本非常有用，该版本可以在 msvc、gcc、avr-gcc 和 clang 上编译。这个问题可能很老，但我想分享我的解决方案，希望其他人觉得它有用：
#include <stddef.h>
namespace std
{
#if defined(__GNUC__)
// Copyright (C) 2008-2020 Free Software Foundation, Inc.
// Copyright (C) 2020 Daniel Rossinsky <danielrossinsky@gmail.com>
//
// This file is part of GCC.
//
// GCC is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation; either version 3, or (at your option)
// any later version.
//
// GCC is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
//
// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.
// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
// <http://www.gnu.org/licenses/>.
template<typename T>
class initializer_list
{
public:
using value_type = T;
using reference = const T&;
using const_reference = const T&;
using size_type = size_t;
using iterator = const T*;
using const_iterator = const T*;
private:
iterator m_array;
size_type m_len;
// The compiler can call a private constructor.
constexpr initializer_list(const_iterator itr, size_type st)
: m_array(itr), m_len(st) { }
public:
constexpr initializer_list() noexcept : m_array(0), m_len(0) { }
// Number of elements.
constexpr size_type size() const noexcept { return m_len; }
// First element.
constexpr const_iterator begin() const noexcept { return m_array; }
// One past the last element.
constexpr const_iterator end() const noexcept { return begin() + size(); }
};
#elif defined(__clang__)
// Copyright (c) 2019 Chandler Carruth <https://github.com/chandlerc>
// Copyright (c) 2018 Louis Dionne <https://github.com/ldionne>
// Copyright (c) 2017 Eric <https://github.com/EricWF>
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// ---- LLVM Exceptions to the Apache 2.0 License ----
//
// As an exception, if, as a result of your compiling your source code, portions
// of this Software are embedded into an Object form of such source code, you
// may redistribute such embedded portions in such Object form without complying
// with the conditions of Sections 4(a), 4(b) and 4(d) of the License.
//
// In addition, if you combine or link compiled forms of this Software with
// software that is licensed under the GPLv2 ("Combined Software") and if a
// court of competent jurisdiction determines that the patent provision (Section
// 3), the indemnity provision (Section 9) or other Section of the License
// conflicts with the conditions of the GPLv2, you may retroactively and
// prospectively choose to deem waived or otherwise exclude such Section(s) of
// the License, but only in their entirety and only with respect to the Combined
// Software.
template<typename T>
class initializer_list
{
private:
const T* m_first;
const T* m_last;
public:
using value_type = T;
using reference = const T&;
using const_reference = const T&;
using size_type = size_t;
using iterator = const T*;
using const_iterator = const T*;
initializer_list() noexcept : m_first(nullptr), m_last(nullptr) {}
// Number of elements.
size_t size() const noexcept { return m_last - m_first; }
// First element.
const T* begin() const noexcept { return m_first; }
// One past the last element.
const T* end() const noexcept { return m_last; }
};
#elif defined(_MSC_VER)
// Copyright (c) Microsoft Corporation.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// ---- LLVM Exceptions to the Apache 2.0 License ----
//
// As an exception, if, as a result of your compiling your source code, portions
// of this Software are embedded into an Object form of such source code, you
// may redistribute such embedded portions in such Object form without complying
// with the conditions of Sections 4(a), 4(b) and 4(d) of the License.
//
// In addition, if you combine or link compiled forms of this Software with
// software that is licensed under the GPLv2 ("Combined Software") and if a
// court of competent jurisdiction determines that the patent provision (Section
// 3), the indemnity provision (Section 9) or other Section of the License
// conflicts with the conditions of the GPLv2, you may retroactively and
// prospectively choose to deem waived or otherwise exclude such Section(s) of
// the License, but only in their entirety and only with respect to the Combined
// Software.
template<typename T>
class initializer_list
{
public:
using value_type = T;
using reference = const T&;
using const_reference = const T&;
using size_type = size_t;
using iterator = const T*;
using const_iterator = const T*;
constexpr initializer_list() noexcept : m_first(nullptr), m_last(nullptr) {}
constexpr initializer_list(const T* first, const T* last) noexcept
: m_first(first), m_last(last) {}
// First element.
constexpr const T* begin() const noexcept { return m_first; }
// One past the last element.
constexpr const T* end() const noexcept { return m_last; }
// Number of elements.
constexpr size_t size() const noexcept
{
return static_cast<size_t>(m_last - m_first);
}
private:
const T* m_first;
const T* m_last;
};
#else
#error "Initializer_list is not supported for this compiler"
#endif
template<typename T>
constexpr const T* begin(initializer_list<T> il) noexcept
{
return il.begin();
}
template<typename T>
constexpr const T* end(initializer_list<T> il) noexcept
{
return il.end();
}
}
笔记：
我在实现的正上方添加了版权声明以及每个实现的许可序言，以表扬代码应归功于谁并促进公平使用。
伊纳尔：
正确包含的唯一版权来自 gcc，msvc 版本缺少一年，clang (llvm) 应该将版权分配给我手动完成的贡献者。但是，我不是专家，可能错过/误解了我被允许共享代码的方式。如果是这样，请告诉我，我会立即应用更改。
此外，据我了解，Apache-2.0 仅在一个方向上与 GPLv3 兼容。你可以在 GPLv3 下转授权我的整个代码，但不能在 Apache-2.0 下
工作证明：
我用来证明我的版本有效的示例取自learncpp：
- Arduino 1.8.9
- x86 msvc v19.28
- x86-64 铿锵声（主干）
- x86-64 gcc（主干）