c++ - 隐式对象参数和 this 指针

Question

引用Non-static member functions , 在

const-, volatile-, and ref-qualified member functions

提到:

A non-static member function can be declared with no ref-qualifier, with an lvalue ref-qualifier (the token & after the parameter list) or the rvalue ref-qualifier (the token && after the parameter list). During overload resolution, non-static cv-qualified member function of class X is treated as follows:

no ref-qualifier: the implicit object parameter has type lvalue reference to cv-qualified X and is additionally allowed to bind rvalue implied object argument

lvalue ref-qualifier: the implicit object parameter has type lvalue reference to cv-qualified X

rvalue ref-qualifier: the implicit object parameter has type rvalue reference to cv-qualified X

Note: unlike cv-qualification, ref-qualification does not change the properties of the this pointer: within a rvalue ref-qualified function, *this remains an lvalue expression.

在这种情况下，隐式对象参数和*this有什么区别？

Answer 1

我认为区别可以用下面的例子来说明:

当重载决议选择依赖于 CV 限定符时，*this 将具有适当的 CV 限定符，就像隐式对象参数一样:

struct t_Tester
{
    constexpr auto Probe(/* t_Tester & param */) { return 1; }
    constexpr auto Probe(/* t_Tester const & param */) const { return 2; }
    constexpr auto Test(/* t_Tester const & param */) const { return (*this).Probe(); }
};

static_assert(2 == t_Tester{}.Test());

然而，当重载决议选择依赖于右值/左值引用限定符时，即使隐式对象参数是右值引用并且对象本身是右值，*this 仍将是左值:

struct t_Tester
{
    constexpr auto Probe(/* t_Tester & param */) & { return 1; }
    constexpr auto Probe(/* t_Tester && param */) && { return 2; }
    constexpr auto Test(/* t_Tester && param */) && { return (*this).Probe(); }
};

static_assert(1 == t_Tester{}.Test());

注意:在类范围内调用函数而不显式使用 this 指针(如 return Probe();)将使用隐含对象参数 (这与 (*this)) 相同，不应与隐式对象参数(它只是重载解析期间使用的成员函数签名的不可见部分)混淆。