Detect (meaningless) assignment to temporary object

Question

Is there any compiler option that allows you to get a warning when you try to assign to temporary object?

Example:

struct S {
    S op() { return S(); }
};

int main() {
    S s;
    s.op() = s; // assign to temporary. Wants to warn here.
}

I know that you can declare the return type of op as const to prevent such a situation, but now I'm interested in is the compiler options only.

You can use any popular modern compiler.

How do you know (in general) that this is meaningless? The assignment may have side effects: `Database db; db.SinkValue() = 5;` — Kerrek SB, Oct 16 '15 at 10:10
@KerrekSB Suppose I just want to get a warning. Is assigning meaningless - will not be considered. — αλεχολυτ, Oct 16 '15 at 10:14
`S& operator=(const S&) & = default;` is a better option than returning a const instance — Piotr Skotnicki, Oct 16 '15 at 10:16
@PiotrSkotnicki is declaring `operator=` as default for lvalue object implicitly deletes `operator=` for rvalue object? — αλεχολυτ, Oct 16 '15 at 15:32
@alexolut it doesn't *delete* the rvalue version. the compiler will simply not generate any other assignment operator — Piotr Skotnicki, Oct 16 '15 at 15:34
@PiotrSkotnicki I thought that `not generate any other assignment operator` includes deleting it for rvalue object too. Am I right? — αλεχολυτ, Oct 16 '15 at 15:40
@alexolut a deleted function is considered as an existing one — Piotr Skotnicki, Oct 16 '15 at 16:18
https://bugs.llvm.org/show_bug.cgi?id=50438 is the bug report for getting this in clang. — David Stone, May 21 '21 at 20:49
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100721 is the bug report for getting this in gcc. — David Stone, May 21 '21 at 20:52

score 0 · Answer 1 · answered Oct 16 '15 at 10:18

0

The compiler may not be able to tell useful side effects.

Compilers do warn for

int test( S & data );

test( S.op());

answered Oct 16 '15 at 10:18

mksteve

2

That's not valid C++, even if a certain compiler who shall not be named allows it. – TartanLlama Oct 16 '15 at 10:20

1 Answers1