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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)
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The mutable keyword is used so that the body of the lambda expression can modify its copies of the external variables x and y , which the lambda expression captures by value. Because the lambda expression captures the original variables x and y by value, their values remain 1 after the lambda executes.
Lambdas always capture objects, and they can do so by value or by reference.
Mutable Data Members (C++)This keyword can only be applied to non-static and non-const data members of a class. If a data member is declared mutable , then it is legal to assign a value to this data member from a const member function.
C++11 introduces lambdas allow you to write an inline, anonymous functor to replace the struct f . For small simple examples this can be cleaner to read (it keeps everything in one place) and potentially simpler to maintain, for example in the simplest form: void func3(std::vector<int>& v) { std::for_each(v.
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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