C++ – Why does C++11’s lambda require “mutable” keyword for capture-by-value, by default

c++c++11lambda

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)

Best Solution

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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