Why does this compile on MS Visual C++?
struct myClass{};
void func(myClass& arg){}
void main() {
func( myClass() ); // works even though func only takes myClass&
} // (not const myClass&!!)
Does this work on other compilers as well or is this MSVC specific (or even a compiler bug?). I can even get the reference on this rvalue like this:
void func(myClass* arg){}
int main() {
func( &myClass() );
}
This works ONLY on Objects that are temporarily created with a constructor. This wouldn’t work with any other rvalue like (myClass() + myClass()) for example..
1
It compiles because MSVC has a non-standard compliant “extension” that allows binding non-const references to temporaries.
The first example should not compile on a standards compliant compiler.
In the second example, you are taking the address of a temporary to set the value of a pointer. This should also result in an error.
Clang 3.2 produces:
error: taking the address of a temporary object of type ‘Foo’ [-Waddress-of-temporary]
while GCC 4.7.3 produces
error: taking address of temporary [-fpermissive]
2
MSVC 2017 with default project settings (that now include /permissive- flag) yields an error:
error C2664: ‘void func(myClass &)’: cannot convert argument 1 from ‘myClass’ to ‘myClass &’
note: A non-const reference may only be bound to an lvalue
Even if /permissive- option is not viable for some reason using /W4 will yield a warning:
warning C4239: nonstandard extension used: ‘argument’: conversion from ‘myClass’ to ‘myClass &’
note: A non-const reference may only be bound to an lvalue