Is the 'this' Pointer Const or Non-const: A Tale of Two Types?

The Nature of the 'this' Pointer: Non-const or Constant?

As the title suggests, the 'this' pointer is a ubiquitous element in object-oriented programming languages like C . But what is its specific type?

The answer is twofold, depending on the context of usage: inside a non-const class method, the 'this' pointer is simply a pointer to the object's type, i.e., 'ClassName '. However, when used within a const method, the 'this' pointer becomes a constant pointer: 'const ClassName '.

The Rationale for Constancy

The purpose of this type difference is to enforce the constness of the object's state. Within a const method, the object's internal data is considered immutable, preventing any modification. By making the 'this' pointer a constant pointer, the compiler ensures that the non-modifiable object is always accessed through a const access path.

Compiler Quirks

While the 'this' pointer should theoretically have the type 'const ClassName *' within a const method, some compilers exhibit an internal quirk. They interpret the 'this' pointer as a constant pointer, even in non-constant methods. This practice, once prevalent in older compilers like GCC and MSVC, was intended to guarantee the non-modifiability of the 'this' pointer.

The Demise of the Constant Pointer Trick

With the introduction of rvalue references in C 11, the practice of interpreting 'this' as a constant pointer became impractical. Rvalue references can detect the extra const on the 'this' pointer type, leading to compilation errors in environments that still use the constant pointer technique. Modern compilers like GCC have abandoned this workaround, while MSVC persists in its implementation even today.

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