为什么 C++11 的 lambda 需要“可变"?默认情况下，按值捕获的关键字?

简短示例:

#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 << "
";       // "10"
}

问题:为什么我们需要 mutable 关键字?它与传统的参数传递到命名函数有很大不同.背后的原理是什么?

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?

我的印象是，按值捕获的全部意义在于允许用户更改临时值――否则我几乎总是最好使用按引用捕获，不是吗?

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?

有什么启示吗?

(顺便说一下，我正在使用 MSVC2010.AFAIK 这应该是标准的)

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

推荐答案

它需要 mutable 因为默认情况下，函数对象每次被调用时都应该产生相同的结果.这就是面向对象的函数和有效地使用全局变量的函数之间的区别.

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.