What does “int& foo()” mean in C++?

Question

While reading this explanation on lvalues and rvalues, these lines of code stuck out to me:

int& foo();
foo() = 42; // OK, foo() is an lvalue

Answer 1

int & foo(); means that foo() returns a reference to a variable.

Consider this code:

#include 
int k = 0;

int &foo()
{
    return k;
}

int main(int argc,char **argv)
{
    k = 4;
    foo() = 5;
    std::cout << "k=" << k << "\n";
    return 0;
}

This code prints:

$ ./a.out k=5

Because foo() returns a reference to the global variable k.

In your revised code, you are casting the returned value to a reference, which is then invalid.