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dfe1c933b3
- const README: rewrite the space-saving claim per issue #5 follow-ups (constant-expression const vs #define immediates; which const qualify as constant expressions) - const README: fix the class-section diagnosis per issue #338 (overload resolution picks the const overload; the restriction only surfaces once that overload is removed) - sizeof README: virtual-inheritance result is compiler-dependent (GCC merges vbptr+vfptr -> 16, MSVC keeps them separate -> 32) #246 - static README: new section on internal linkage / access restriction with anonymous-namespace comparison #142 - 10-day practice: full problem list table + fixed day links #136 - new design_pattern/singleton/singleton_default.hpp: reusable thread-safe CRTP singleton template, compile+concurrency tested #303
117 lines
2.4 KiB
Markdown
117 lines
2.4 KiB
Markdown
> 来自 [C++那些事网站](https://light-city.github.io/stories_things/modern-cpp/cpp17/structured-bindings/) 的文章，由站点同步脚本导出。
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# C++17 结构化绑定
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在聊结构化绑定之前，有几个面试问题，看看你会不会？
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- 如何使用结构化绑定访问自定义类的私有成员？
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- 如何使用结构化绑定修改自定义类的成员呢？
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这几个题目估计没几个人能答上来，题目与答案网上也都没有，基于这些问题，我们引出本文的内容：如何为自定义类型支持结构化绑定，里面有哪些坑？
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**注：懒人版，答案与代码已放星球。**
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为了让本文的内容更加丝滑，从示例出发。
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## 1.自定义类型
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如果给定一个类：
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```cpp
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struct Foo {
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Foo(int64_t id, std::string data) : id_(id), data_(std::move(data)) {}
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void Print() { std::cout << "id = " << id_ << ", data = " << data_ << std::endl; }
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private:
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int64_t id_;
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std::string data_;
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};
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```
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要让下面这个代码运行起来：
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```cpp
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int main() {
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Foo f{12, "hello world"};
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const auto& [id, data] = f;
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std::cout << "id = " << id << ", data = " << data << std::endl;
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f.Print();
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}
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```
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如果直接运行会编译：
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```cpp
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cannot decompose inaccessible member 'Foo::id_' of 'const Foo'
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42 | const auto& [id, data] = f;
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```
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那么怎么做呢？
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## 2.支持结构化绑定
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这个问题我们使用两个步骤：
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1.特化 `std::tuple_size` 和 `std::tuple_element`
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```cpp
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namespace std {
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template <>
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struct tuple_size<Foo> : std::integral_constant<std::size_t, 2> {};
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template <>
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struct tuple_element<0, Foo> {
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using type = int64_t;
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};
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template <>
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struct tuple_element<1, Foo> {
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using type = std::string;
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};
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}
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```
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2.为上面的结构体添加友元get函数，然后实现即可：
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```cpp
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template <std::size_t Index>
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decltype(auto) get(const Foo& f);
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template <>
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decltype(auto) get<0>(const Foo& f) {
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return f.id_;
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}
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template <>
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decltype(auto) get<1>(const Foo& f) {
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return f.data_;
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}
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```
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此时便可以正常的工作了，得到输出：
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```cpp
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id = 12, data = hello world
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id = 12, data = hello world
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```
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最后，抛出几个问题：
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- 上面提供的是只读访问，如果在访问的时候修改const auto &会发生什么？
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例如：
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```cpp
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auto& [id, data] = f;
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```
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- 如何支持对类成员的修改访问？
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即，让这个语句能够正常运行
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```cpp
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auto& [id, data] = f;
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```
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好了，本节完，欢迎留言区交流。 |