What is the Correct Type of the 'this' Pointer in C : A Question of Constness and Implementation?

Type Analysis of 'this' Pointer

In C , the 'this' pointer is a special pointer that points to the current instance of the class. Its type is determined based on the context in which it is used.

Type of 'this' Pointer in Member Functions

Within non-const member functions, the type of 'this' is simply the name of the class followed by *. For instance, in a class named ClassName, the type of 'this' within its non-const methods would be ClassName *.

Type of 'this' Pointer in Const Member Functions

When used within a const member function, the 'this' pointer becomes a pointer to a constant object. Its type reflects this, becoming const ClassName *. This ensures that the object cannot be modified through the 'this' pointer.

Why Make 'this' Pointer Constant?

The primary reason for making the 'this' pointer constant in const member functions is to enforce the const-correctness of the class. By preventing the modification of the object through the 'this' pointer, it ensures that the object remains unchanged. This helps maintain the integrity of the class data and prevents accidental modifications.

Error in Interpretation

The observation that the type of 'this' pointer is ClassName * const this in Windows using VC 2008 is incorrect. The 'this' pointer is not an lvalue, and therefore, cannot have a const qualifier applied to the right of the asterisk (*). The correct type of 'this' in a non-const member function is ClassName *.

Internal Implementation Detail

In the past, some compilers have used an implementation detail where they interpreted the 'this' pointer as a constant pointer even in non-const member functions. However, this is not in line with the C language specification and has since been abandoned by most compilers, including GCC.

Note on C 11 Rvalue References

With the introduction of rvalue references in C 11, it became possible to detect the extra constness added to the 'this' pointer by implementations that still use the abovementioned trick. This has led to the deprecation of the technique, as it can lead to errors when working with rvalue references.

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