C ++扩展功能？ [英] C++ Extension functions?

问题描述

例如在C＃中你可以这样做：

  public static uint SwapEndian（this uint value）
 {
 var tmp = BitConverter.GetBytes（value）; 
 Array.Reverse（tmp）; 
 return BitConverter.ToUInt32（tmp，0）; 
} 
 
 someuint.SwapEndian（）; 
  

在C ++中有什么类似的东西吗？

这不是C ++的做事方式。在C ++中，您可以使用命名空间，免费功能和 Koenig查询来扩展类的行为：

 命名空间foo 
 {
 struct bar {...} 
 
 void act_on_bar（const bar& b）{...}; 
} 
 
 ... 
 
 foo :: bar b; 
 act_on_bar（b）; //不需要因为Koenig查找而限定
  

我通常认为扩展方法有害。如果你给一个类附加了太多的行为，你可能无法捕获类存在的原因。此外（像部分类），他们倾向于使与类非相关的代码非本地。

对于你的问题，在C ++中你只需：

  template< typename T& 
 T swap_endian（T x）
 {
 union {T value; char bytes [sizeof（T）]; } u; 
 u.value = x; 
 
 for（size_t i = 0; i  swap（u.bytes [i]，u.bytes [sizeof（T）-i -  1]）; 
 
 return u.value; 
} 
  

用法：

  swap_endian< std :: uint32_t>（42）; 
  

或者，如果可以推导出类型：

  std :: uint64_t x = 42; 
 std :: uint64_t y = swap_endian（x）; 
  

Are there extensions for C++ like there are in C#?

For example in C# you can do:

public static uint SwapEndian(this uint value)
{
    var tmp = BitConverter.GetBytes(value);
    Array.Reverse(tmp);
    return BitConverter.ToUInt32(tmp, 0);
}

someuint.SwapEndian();

Is there anything like that in C++?

解决方案

Extension methods (and also "static classes") exist in C#/Java languages solely because the designers decided that (the Java way of) OOP is The One True Way and that everything must be a method from a class:

This is not C++ way of doing things. In C++ you have namespaces, free functions and Koenig lookup to extend the behavior of a class:

namespace foo
{
    struct bar { ... };

    void act_on_bar(const bar& b) { ... };
}

...

foo::bar b;
act_on_bar(b); // No need to qualify because of Koenig lookup

I usually consider extension methods harmful. If you attach too much behavior to a class, you are proabably failing to capture the reason why the class exists. Also (like "partial classes"), they tend to make the code related to a class non local. Which is bad.

As to your problem, in C++ you simply do:

template <typename T>
T swap_endian(T x)
{
    union { T value; char bytes[sizeof(T)]; } u;
    u.value = x;

    for (size_t i = 0; i < sizeof(T)/2; i++) 
        swap(u.bytes[i], u.bytes[sizeof(T) - i - 1]);

    return u.value;
}

Usage:

swap_endian<std::uint32_t>(42);

or, if the type can be deduced:

std::uint64_t x = 42;
std::uint64_t y = swap_endian(x);

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