As the lambda function works just like a class with member variables, it can also mutate them. In the following example, the lambda mutates the threshold variable every time it is invoked.
In order to allow the lambda to mutate its members, we need to specify mutable when declaring the lambda. The mutable modifier on a lambda function works like the inverse for a const modifier for a regular class member function; in contrast to a class member function, a lambda function is const by default, and therefore a mutating lambda must be explicitly specified:
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Capture by value |
Capture by reference |
auto func() { auto v = 7; auto lambda = [v]() mutable { std::cout << v << " "; ++v; }; assert(v == 7); lambda(); ... |
