'this'类型为“Class * const”，即使方法不是const

Question

Today I noticed something strange about the type of 'this'. If you have something like this:

class C {
    void funcA() {
        funcB(this);
    }

    void funcB(C obj) {
        //do something
    }
};

You will of course get an error, because funcB() expects an object, while 'this' is a pointer. I accidentally did forget the asterisk, but was surprised by the error message, as it said:

no matching function for call to 'C::funcB(C* const)'

Where does the const come from, when funcA() is not constant?

Answer 1

That's saying that the this pointer itself as const -- ie, you can't modify the pointer to point to different memory.

Back in the very early history of C++, before you could overload new and delete , or placement new was invented, this was a non-const pointer (at least inside the ctor). A class that wanted to handle its own memory management did so by allocating space for an instance in the constructor, and writing the address of that memory into this before exiting the constructor.

In a const member function the type you'd be dealing would be Class const *const this , meaning that what this points at is const (as well as the pointer itself being const ).

Answer 2

C* const does not mean that the object of type C is constant. That would be C const* or const C* .

C* const means that the pointer itself is constant.

Which makes sense, since you cannot do

this = &something_else;

Answer 3

Note that it's not C const * , but C* const . Ie (reading from right to left) constant pointer to C .

Answer 4

There is a difference between C const* and C * const . You need to understand the difference:

• C const* means it is the object (pointed to by the pointer) which is constant.
• C * const means it is the pointer itself which is constant.

So this by definition is a pointer of C * const type, so it cannot be modified, though the object pointed to by it can still be modified.

Answer 5

指针本身是常量，而不是指向的数据。