在我的印象中，按值捕获的全部意义在于允许用户更改临时对象——否则我几乎总是使用按引用捕获更好，不是吗?

问题是，它是“几乎”吗?一个常见的用例似乎是返回或传递lambdas:

void registerCallback(std::function<void()> f) { /* ... */ }

void doSomething() {
  std::string name = receiveName();
  registerCallback([name]{ /* do something with name */ });
}

我认为mutable不是almost的例子。我认为“按值捕获”就像“允许我在捕获的实体死亡后使用它的值”，而不是“允许我更改它的副本”。但这或许是有争议的。

现在有一个建议可以减轻lambda声明中对mutable的需求:n3424

You need to think what is the closure type of your Lambda function. Every time you declare a Lambda expression, the compiler creates a closure type, which is nothing less than an unnamed class declaration with attributes (environment where the Lambda expression where declared) and the function call ::operator() implemented. When you capture a variable using copy-by-value, the compiler will create a new const attribute in the closure type, so you can't change it inside the Lambda expression because it is a "read-only" attribute, that's the reason they call it a "closure", because in some way, you are closing your Lambda expression by copying the variables from upper scope into the Lambda scope. When you use the keyword mutable, the captured entity will became a non-const attribute of your closure type. This is what causes the changes done in the mutable variable captured by value, to not be propagated to upper scope, but keep inside the stateful Lambda. Always try to imagine the resulting closure type of your Lambda expression, that helped me a lot, and I hope it can help you too.

它需要mutable，因为默认情况下，函数对象每次调用都应该产生相同的结果。这就是面向对象的函数和使用全局变量的函数之间的区别。

为了扩展Puppy的回答，lambda函数被设计为纯函数。这意味着给定唯一输入集的每次调用总是返回相同的输出。让我们将input定义为调用lambda时所有参数加上所有捕获变量的集合。

在纯函数中，输出完全依赖于输入，而不依赖于某些内部状态。因此，任何lambda函数，如果是纯的，不需要改变其状态，因此是不可变的。

When a lambda captures by reference, writing on captured variables is a strain on the concept of pure function, because all a pure function should do is return an output, though the lambda does not certainly mutate because the writing happens to external variables. Even in this case a correct usage implies that if the lambda is called with the same input again, the output will be the same everytime, despite these side effects on by-ref variables. Such side effects are just ways to return some additional input (e.g. update a counter) and could be reformulated into a pure function, for example returning a tuple instead of a single value.

你的代码几乎相当于:

#include <iostream>

class unnamed1
{
    int& n;
public:
    unnamed1(int& N) : n(N) {}

    /* OK. Your this is const but you don't modify the "n" reference,
    but the value pointed by it. You wouldn't be able to modify a reference
    anyway even if your operator() was mutable. When you assign a reference
    it will always point to the same var.
    */
    void operator()() const {n = 10;}
};

class unnamed2
{
    int n;
public:
    unnamed2(int N) : n(N) {}

    /* OK. Your this pointer is not const (since your operator() is "mutable" instead of const).
    So you can modify the "n" member. */
    void operator()() {n = 20;}
};

class unnamed3
{
    int n;
public:
    unnamed3(int N) : n(N) {}

    /* BAD. Your this is const so you can't modify the "n" member. */
    void operator()() const {n = 10;}
};

int main()
{
    int n;
    unnamed1 u1(n); u1();    // OK
    unnamed2 u2(n); u2();    // OK
    //unnamed3 u3(n); u3();  // Error
    std::cout << n << "\n";  // "10"
}

因此，您可以将lambdas视为生成一个带有operator()的类，该类默认为const，除非您说它是可变的。

您还可以将[]中捕获的所有变量(显式或隐式)视为该类的成员:[=]对象的副本或[&]对象的引用。它们在声明lambda时被初始化，就像有一个隐藏的构造函数一样。