How to increment a variable from lambda-functor's body?

Question

I tried to increment a local variable from lambda expression:

#include <iostream>

template<typename T>
T foo(T t){
    T temp{};
    [temp]() -> void { 
        temp++; 
    }();
    return temp;
}

int main()
{
    std::cout<< foo(10) << std::endl;
}

DEMO

But got the following error:

main.cpp: In instantiation of 'foo(T)::<lambda()> [with T = int]':

main.cpp:6:6:   required from 'struct foo(T) [with T = int]::<lambda()>'

main.cpp:8:6:   required from 'T foo(T) [with T = int]'

main.cpp:14:23:   required from here

main.cpp:7:13: error: increment of read-only variable 'temp'

         temp++; 

             ^

Is there some workaround about that in c++11/14?

Answer 1

temp cannot be modified when it's captured by copy in a non-mutable lambda.

You could capture temp by reference:

template<typename T>
T foo(T t){
    T temp{};
    [&temp]() -> void { 
        temp++; 
    }();
    return temp;
}

Answer 2

If you want to capture a variable by value and modify it, you need to mark the lambda mutable:

[temp]() mutable -> void {
//       ^^^^^^^ 
    temp++; 
}();

This allows the body to modify the parameters captured by value, and to call their non-const member functions