C ++, what is the * this value category?
Section 9.3.2.1 of the C ++ standard states the following:
In the body of a non-static (9.3) member function, the keyword this is a prvalue expression whose value is the address of the object for which the function is called. The type of this in a member function class X is X *. If a member function is declared const, the type is const X *, if the member function is declared volatile, the type of that is volatile X *, and if the member function is declared const volatile, the type is const volatile X *.
So if this
is a prvalue, what is the category of the values *this
? The following assumes that even when the object is an rvalue, *this
it is always an lvalue. It's right? Please refer to the standard if possible.
struct F;
struct test
{
void operator()(F &&) { std::cout << "rvalue operator()" << std::endl; }
void operator()(F const &&) { std::cout << "const rvalue operator()" << std::endl; }
void operator()(F &) { std::cout << "lvalue operator()" << std::endl; }
void operator()(F const &) { std::cout << "const lvalue operator()" << std::endl; }
};
struct F
{
void operator ()()
{
struct test t;
t(*this);
}
};
int main()
{
struct F f;
f();
std::move(f)();
}
Output:
lvalue operator()
lvalue operator()
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From [basic.lval]:
The l value (so called historically, since lvalues can appear on the left side of an expression assignment) denotes a function or object. [Example: if
E
is an expression of type pointer, then*E
is an lvalue expression referring to the object or function to which it pointsE
. As another example, the result of a call to a function whose return type is an lvalue reference is an lvalue. -end example]
And from [expr.unary.op]:
The unary operator
*
performs indirect direction: the expression to which it applies must be a pointer to an object type or a pointer to a function type, and the result is an lvalue referencing the object or function that the expression points to.
Pointer dereferencing is an lvalue. Thus, *this
is an lvalue.
Alternatively, anything that is not an lvalue is an r value. R value:
The value of r (so called historically, since rvalues can appear on the right side of an expression statement) is an x value, a temporary object (12.2), or its subobject, or a value that is not associated with an object.
And *this
certainly none of these things.
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