Is using an array as a conditional expression valid in C++?

Question

I have this code:

int main()
{
    char buffer[10];
    if( buffer ) {
       return 1;
    }
    return 0;
}

which Visual C++ 10 interprets like this: buffer decays to a pointer, then a pointer is compared against null. When this is compiled with /O2 the check gets eliminated and the code gets equivalent to just return 1;.

Is the code above valid? Does Visual C++ compile it right (I mean the decaying part, not the optimization)?

Answer 1

C++11, 6.4/4:

The value of a condition that is an expression is the value of the expression, contextually converted to bool for statements other than switch; if that conversion is ill-formed, the program is ill-formed.

So the standard says that the compiler has to perform any implicit conversions at its disposal to convert the array to a boolean. Decaying the array to pointer and converting the pointer to boolean with a test against against equality to null is one way to do that, so yes the program is well-defined and yes it does produce the correct result -- obviously, since the array is allocated on the stack, the pointer it decays to can never be equal to the null pointer.

Update: As to why this chain of two conversions is followed:

C++11, 4.2/1:

An lvalue or rvalue of type “array of N T” or “array of unknown bound of T” can be converted to a prvalue of type “pointer to T”. The result is a pointer to the first element of the array.

So, the only legal conversion from an array type is to a pointer to element type. There is no choice in the first step.

C++11, 4.12/1:

A prvalue of arithmetic, unscoped enumeration, pointer, or pointer to member type can be converted to a prvalue of type bool. A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true. A prvalue of type std::nullptr_t can be converted to a prvalue of type bool; the resulting value is false.

There is an implicit conversion directly from bare pointer to boolean; so the compiler picks that as the second step because it allows the desired result (conversion to boolean) to be immediately reached.