What is the ‘this’ Pointer in C++?

c++classpointersthis

I'm fairly new to C++, and I don't understand what the this pointer does in the following scenario:

void do_something_to_a_foo(Foo *foo_instance);


void Foo::DoSomething()
{
  do_something_to_a_foo(this);
}

I grabbed that from someone else's post on here.

What does this point to? I'm confused. The function has no input, so what is this doing?

Best Answer

this refers to the current object.

The keyword this identifies a special type of pointer. Suppose that you create an object named x of class A, and class A has a non-static member function f(). If you call the function x.f(), the keyword this in the body of f() stores the address of x.

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C++ – Understanding Type of ‘this’ Pointer

The type of this pointer is either ClassName * or const ClassName *, depending on whether it is inspected inside a non-const or const method of the class ClassName. Pointer this is not an lvalue.

class ClassName {
  void foo() {
    // here `this` has `ClassName *` type
  }

  void bar() const {
    // here `this` has `const ClassName *` type
  }
};

The observation you mentioned above is misleading. Pointer this is not an lvalue, which means that it cannot possibly have ClassName * const type, i.e. it cannot possible have a const to the right of the *. Non-lvalues of pointer type cannot be const or non-const. There's simply no such concept in C++ language. What you observed must be an internal quirk of the specific compiler. Formally, it is incorrect.

Here are the relevant quotes from the language specification (emphasis mine)

9.3.2 The this pointer

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 of a class X is X*. If the member function is declared const, the type of this is const X*, if the member function is declared volatile, the type of this is volatile X*, and if the member function is declared const volatile, the type of this is const volatile X*. [ Note: thus in a const member function, the object for which the function is called is accessed through a const access path. —end note ]

It is worth nothing that back in the C++98/C++03 times several compilers used an internal implementational trick: they interpreted their this pointers as constant pointers, e.g. ClassName *const in a non-constant method of class ClassName. This apparently helped them to ensure non-modifiablity of this. GCC and MSVC are known to have used the technique. It was a harmless trick, since at language level this was not an lvalue and its constness was undetectable. That extra const would generally reveal itself only in diagnostic messages issued by the compiler.

However, with the advent of rvalue references in C++11 it became possible to detect this extra const on the type of this. For example, the following code is valid in C++11

struct S
{
  void foo() { S *&&r = this; }
};

Yet it will typically fail to compile in implementations that still use the aforementioned trick. GCC has since abandoned the technique. MSVC++ still uses it (as of VS2017), which prevents the above perfectly valid code from compiling in MSVC++.

Best Answer

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C++ – Understanding Type of ‘this’ Pointer

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