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c++c++11lambda

Passing by constant reference in the lambda capture list

I'm building a lambda function that requires access to a fair number of variables in the context.

const double defaultAmount = [&]{
    /*ToDo*/
}();

I'd rather not use [=] in the list as I don't want lots of value copies to be made.

I'm concerned about program stability if I use [&] since I don't want the lambda to modify the capture set.

Can I pass by const reference? [const &] doesn't work.

Perhaps a good compiler optimises out value copies, so [=] is preferable.

Solution

  • You can create and capture const references explicitly:

    int x = 42;
const int& rx = x;
auto l = [&rx]() {
    x = 5; // error: 'x' is not captured
    rx = 5; // error: assignment of read-only reference 'rx'
};