为什么C ++ 0x的lambda需要“可变”默认情况下，按值捕获的关键字？ [英] Why does C++0x's lambda require "mutable" keyword for capture-by-value, by default?

问题描述

简短示例：

  #include< iostream& 
 
 int main（）
 {
 int n; 
 [&]（）{n = 10;}（）; // OK 
 [=]（）mutable {n = 20;}（）; // OK 
 // [=]（）{n = 10;}（）; //错误：无法在不可变的lambda中修改旁边值捕获
 std :: cout<< n<< \\\
; //10
} 
  

问题：为什么我们需要 mutable 关键字？它与传统的参数传递给命名函数非常不同。后面有什么理由？

我的印象是，捕获值的整个点是允许用户更改临时 - 否则我的

任何启示？

我使用MSVC2010的方式。AFAIK这应该是标准的）

解决方案

它需要 mutable 因为默认情况下，一个函数对象应该在每次调用时产生相同的结果。这是面向对象函数和使用全局变量的函数之间的区别。

Short example:

#include <iostream>

int main()
{
    int n;
    [&](){n = 10;}();             // OK
    [=]() mutable {n = 20;}();    // OK
    // [=](){n = 10;}();          // Error: a by-value capture cannot be modified in a non-mutable lambda
    std::cout << n << "\n";       // "10"
}

The question: Why do we need the mutable keyword? It's quite different from traditional parameter passing to named functions. What's the rationale behind?

I was under the impression that the whole point of capture-by-value is to allow the user to change the temporary -- otherwise I'm almost always better off using capture-by-reference, aren't I?

Any enlightenments?

(I'm using MSVC2010 by the way. AFAIK this should be standard)

解决方案

It requires mutable because by default, a function object should produce the same result every time it's called. This is the difference between an object orientated function and a function using a global variable, effectively.

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