Reference a temporary in msvc

Question

Why does this compile on MS Visual C++?

struct myClass{};

void func(myClass& arg){}

void main() {
   func( myClass() );  // works even though func only         


        

Answer 1

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

Answer 2

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]