Assignment operator and a temporary

Question

I am currently using VS2012 and was expecting statement B in this code to fail since we are passing a temp which is a constant to the assignment operator method in the foo class. Surprisingly that doesnt fail why is that ? Statement A fails and that is fine. Why doesnt statement B fail ?

struct foo
{
    int a;
    foo& operator=(foo& that)
    {
        a=12;
        return *this;
    }
};

int main()
{
    const foo a;
    foo b;
    //b = a;      //statement A
    b = foo();    //Statement B
}

Latest VS with high warning should warn about this. – Neil Kirk Nov 24 '14 at 18:34 — Neil Kirk, Nov 24 '14 at 18:34

Columbo · Accepted Answer · 2014-11-24T19:16:43.483

5

This works because VC++, with language extensions enabled, is not standard compliant and allows binding a non-const lvalue reference to a temporary.

After adding a default constructor in foo, both GCC and Clang issue an appropriate error message:

main.cpp:18:7: error: no viable overloaded '='
  b = foo();    //Statement B
  ~ ^ ~~~~~
main.cpp:6:10: note: candidate function not viable: expects an l-value for 1st argument
foo& operator=(foo&)
     ^

edited Nov 24 '14 at 19:16

answered Nov 24 '14 at 18:33

Columbo

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And VC++ does issue an error as well when language extensions are disabled. – Nov 24 '14 at 18:36
Thanks for clearing that up – MistyD Nov 24 '14 at 18:37
@hvd Good to know. Don't work with VS actually :o) – Columbo Nov 24 '14 at 18:37

Assignment operator and a temporary

1 Answers1