prvalue vs xvalue for class types

Question

I've got more confused when I see this question: Is a class instantiation--class_name() a xvalue or a prvalue? I'm trying to understand what does it mean by a class prvalue and a class xvalue. Someone tell me that they're called value category. But I think it would be better if I provide an example, because I'm very confused.

class myclass { public: myclass() {}; };

void myfunc(myclass c1){  }

int main(void)
{
   myfunc(myclass());
}

So what's myclass()? Is it a prvalue or xvalue?

I need a rule from the standard so that I wouldn't ask more questions.

Anyone can add language-lawyer tag? because the question says `I need a rule from the standard so that I wouldn't ask more questions.` — mada, Oct 01 '22 at 12:02

score 0 · Accepted Answer · answered Oct 01 '22 at 11:58

The expression myclass() is explicit type conversion using functional notation, per [expr.type.conv]/1:

A simple-type-specifier or typename-specifier followed by a parenthesized optional expression-list or by a braced-init-list (the initializer) constructs a value of the specified type given the initializer [..]

Here, the simple-type-specifier myclass is followed by parenthesized expression-list () with an empty initializer-list. This expression constructs a value of type myclass from the empty initializer-list.

So you might ask, Is that constructed value is prvalue or xvalue or lvalue. So here we've to invoke the immediately next paragraph: [expr.type.conv]/2:

If the initializer is a parenthesized single expression, the type conversion expression is equivalent to the corresponding cast expression. Otherwise, if the type is cv void and the initializer is () or {} (after pack expansion, if any), the expression is a prvalue of the specified type that performs no initialization. Otherwise, the expression is a prvalue of the specified type whose result object is direct-initialized with the initializer.

Our initializer is empty initializer-list, and the type is not cv void, so we've ended with the sentence that says:

Otherwise, the expression is a prvalue of the specified type whose result object is direct-initialized with the initializer.

Hence, the expression myclass() is a prvalue, whose result object is direct-initialized, from the empty initializer-list, by calling the default constructor.

prvalue vs xvalue for class types

1 Answers1