Question

If a function returns an int, can it be assigned by an int value? I don’t see it makes too much sense to assign a value to a function.

int f() {}

f() = 1;

I noticed that, if the function returns a reference to an int, it is ok. Is it restricted only to int? how about other types? or any other rules?

int& f() {}

f() = 1;

Answer 1

The first function returns an integer by-value, which is an r-value. You can’t assign to an r-value in general. The second f() returns a reference to an integer, which is a l-value – so you can assign to it.

int a = 4, b = 5;

int& f() {return a;}

...
f() = 6;
// a is 6 now

Note: you don’t assign a value to the function, you just assign to its return value.
Be careful with the following:

int& f() { int a = 4; return a; }

You’re returning a reference to a temporary, which is no longer valid after the function returns. Accessing the reference invokes undefined behaviour.