c++ - 带有指针 this 的模板函数 "is_same"，意外行为？

Question

相关问题:template-function-is-same-in-template-classes

我对指针“this”的类型感到有点不安(gcc 4.7.2，c++11)。原则上，例如C类型的非常量对象的指针“this”的类型是“C * const”，因此“*this”的类型是“C”。但是“is_same”类的行为让我感到困惑。

测试:

// this-type.cpp
#include <iostream>
#include <type_traits>

using namespace std;

class C
{
public:
   void test()
   {
       cout << boolalpha;

       cout << "'this' const?" << "              "
            << is_const<decltype(this)>::value << endl;

       cout << "'*this' const?" << "             "
            << is_const<decltype(*this)>::value << endl;

       cout << "'*this' and 'C' the same?" << "  "
            << is_same<decltype(*this), C>::value << endl;

       cout << "'this' and 'C*' the same?" << "  "
            << is_same<decltype(this), C*>::value << endl;
   }
};

int main()
{
    C c;

    c.test();
}

输出:

$ g++ --version | grep g++
g++ (Ubuntu/Linaro 4.7.2-2ubuntu1) 4.7.2
$ g++ -std=c++11 this-type.cpp
$ ./a.out
'this' const?              false
'*this' const?             false
'*this' and 'C' the same?  false
'this' and 'C*' the same?  true

然而，预期的输出是:

$./a.out
'this' const?              true   // C* const
'*this' const?             false  // C
'*this' and 'C' the same?  true
'this' and 'C*' the same?  false  // C* const vs. C*

这里发生了什么？

Answer 1

9.3.2 The this pointer [class.this]

1 - [...] 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 of a class X is X*.

this 不是const 左值，它是一个纯右值表达式，因此它不需要是const。你不能给它赋值，因为它是纯右值，这与你不能写 2 + 2 = 5 的原因相同。