Why does C++11's lambda require "mutable" keyword for capture-by-value, by default?
c++, c++11, lambda
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.
Problem
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)