lambda function doesn't show the right result

Question

I wonder why this lambda function doesn't return the right answer:

int main()
{
    int a = 3, b = 7;
    [&]() ->void {(&a == &b) ? a : (a ^= b, b ^= a, a ^= b); };
    std::cout << a << " " << b;
    return 0;
}

It shows me 3 7 instead of 7 3

user_3309790 · Answer

You didn't call the lambda. You just declared an unnamed function object which is discarded immediately. You can call it as

[&]() ->void {(&a == &b) ? a : (a ^= b, b ^= a, a ^= b); } ();
//                                                         ^^

Then the function body will be evaluated and do what you expect.

user_1178207 · Answer

Defining a lambda does not implicitely call it.

You have to call it:

auto f = [&]() ->void {(&a == &b) ? a : (a ^= b, b ^= a, a ^= b); };
f();