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- 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
45 lines
1.7 KiB
Markdown
45 lines
1.7 KiB
Markdown
> 来自 [C++那些事网站](https://light-city.github.io/stories_things/cpp-guides/interesting-cpp-code-snippet/) 的文章，由站点同步脚本导出。
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# 一段有趣的 C++ 代码
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## 0.导语
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目标：如何获取参数lambda或者functor的参数个数呢？
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本节将从模版 + `::*` + decay + decltype来实现！
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## 1.实现
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最终效果：
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```cpp
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auto c1 = [](int x, double y) { return x * y; };
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std::cout << argument_count<decltype(c1)>::value << std::endl;
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MyClass f;
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std::cout << argument_count<decltype(f)>::value << std::endl;
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```
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输出：2
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我们思考一下这个问题，需要直到这几个点：
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- 如何定位到functor与lambda的位置？
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- 如何获取到这个函数签名的参数数量？
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对于第一个问题，我们可以使用`::*`便可以指向成员声明，对于functor与lambda都是`&std::decay<F>::type::operator())`。简单来说都是获取operator()这个声明的地址。
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这里使用decay来移除cv限定符，以上述MyClass为例，到这里先通过`std::decay<F>::type`获取到MyClass类型，随后获取对应operator()的地址，然后传递给argument_count_impl。
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```cpp
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template <typename F>
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using argument_count = decltype(argument_count_impl(&std::decay<F>::type::operator()));
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```
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argument_count_impl的实现也比较简单，使用R (F::*)(A...)签名获取到对应的函数，这里使用了可变参数模版，`::*`，sizeof是用来获取参数数量，integral_constant则表示这个编译器常量类型为int，值为sizeof...(A)。
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```cpp
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template <typename F, typename R, typename... A>
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static std::integral_constant<int, sizeof...(A)> argument_count_impl(R (F::*)(A...));
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```
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基于此，便实现了获取Lambda or Functor参数个数的功能~ |