Question

According to ” How to get around the warning "rvalue used as lvalue"? “, Visual Studio will merely warn on code such as this:

int bar() {
   return 3;
}

void foo(int* ptr) {

}

int main() {
   foo(&bar());
}

In C++ it is not allowed to take the address of a temporary (or, at least, of an object referred to by an rvalue expression?), and I thought that this was because temporaries are not guaranteed to even have storage.

But then, although diagnostics may be presented in any form the compiler chooses, I’d still have expected MSVS to error rather than warn in such a case.

So, are temporaries guaranteed to have storage? And if so, why is the above code disallowed in the first place?

Answer 1

You’re right in saying that “temporaries are not guaranteed to even have storage”, in the sense that the temporary may not be stored in addressable memory. In fact, very often functions compiled for RISC architectures (e.g. ARM) will return values in general use registers and would expect inputs in those registers as well.

MSVS, producing code for x86 architectures, may always produce functions that return their values on the stack. Therefore they’re stored in addressable memory and have a valid address.