Question

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What is a lambda expression in C++11?

I found this expression in C++ (one of the most exciting features of C++11):

int i = ([](int j) { return 5 + j; })(6);

Why I get the 11? Please explain this expression.

Answer 1

[](int j) { return 5 + j; } is a lambda that takes an int as an argument and calls it j. It adds 5 to this argument and returns it. The (6) after the expression invokes the lambda immediately, so you’re adding 6 and 5 together.

It’s roughly equivalent to this code:

int fn(int j) {
    return 5 + j;
}

int i = fn(6);

Except, of course, that it does not create a named function. A smart compiler will probably inline the lambda and do constant folding, resulting in a simple reduction to int i = 11;.