What is the purpose of specifying captured variable in lambda expression?

Question

I have this code:

int i = 0; [&i](){i++;}(); 

But I can omit i and just use:

[&](){i++;}(); 

What is the purpose of specifying &i? (and similarly =var and =). Is it affecting compile time or runtime performance?

Answer 1

&i means only i is captured as reference, while & means all variables which are used in the lambda, are captured from the scope.

int i = 0, j = 10;  [&i]{ i++; j++;}(); //error: j is not captured by reference  [&]{ i++; j++;}(); //ok: both variables are captured by reference 

The &i approach is provided by the language to limit the amount of captured variables. C++ provides you full control on the captured variables, as to which specific variables to capture, how to capture (by value, or reference).

Same reasoning with i and =:

int i = 0, j = 10;  [i]{ i++; j++;}(); //error: j is not captured (only i is captured by value)  [=]{ i++; j++;}(); //ok: both variables are captured by value  //more [&i, j]{ i++; j++;}(); //ok: captured i by reference, j by value 

Hope that helps.

Answer 2

Sometimes you may want to capture different variables in different ways:

std::vector<int> v { 7, 8, 9, 10, 11 }; int n = 3;  //Capture v by reference and n by value: auto getNth = [&v, n](){ return v[n]; };  //Behavior: n = 9999; getNth(); //returns 10 - changing n didn't affect the lambda v[3] = 42; getNth(); //returns 42 - changing v did affect the lambda 

This is why the more-detailed syntax is available.