Why is this a pointer and not a reference

In the intricate world of C++ object-oriented programming, the this keyword stands as a fundamental concept, serving as a self-referential pointer to the current object instance. A common point of confusion for many developers, especially those transitioning from other languages or new to C++, revolves around its nature: why is ’this’ a pointer and not a reference? This distinction is not merely semantic; it underpins critical aspects of C++’s design, memory management, and polymorphic behavior. Understanding the core differences between pointers and references, and how they apply to this, is crucial for writing robust, efficient, and idiomatic C++ code. This article will delve into the technical reasons behind this design choice, explore the implications, and clarify common misconceptions to provide a comprehensive understanding of the this pointer’s role.

Understanding ’this’: The Implicit Parameter

The this pointer is an implicit, constant, and non-static pointer that is automatically passed to every non-static member function of a class. Its primary role is to allow a member function to access the specific instance of the object on which it was called. When you invoke a method like myObject.doSomething(), the compiler implicitly translates this into something conceptually similar to doSomething(&myObject), where &myObject becomes the value of the this pointer inside the doSomething function. This mechanism ensures that member functions operate on the correct data members belonging to that particular object instance.

Consider a class Car with a member function setSpeed(int newSpeed). When you call myCar.setSpeed(60), inside setSpeed, this points to myCar. This allows the function to modify myCar’s speed member variable without accidentally affecting other Car objects. The this pointer is always of type T const, where T is the class type, meaning it’s a constant pointer to an object of type T. You cannot change what this points to within a member function, but you can modify the object it points to (unless the member function is const, in which case this becomes const T const).

The existence of this is fundamental to object-oriented programming in C++, enabling distinct instances of a class to share the same member function definitions while maintaining their unique state. It’s the silent workhorse that ensures member functions know which object’s data they are operating on, a concept central to managing class instances effectively. This implicit passing mechanism simplifies the syntax for developers, allowing them to write member functions as if they are directly operating on the object’s members, even though under the hood, this->member is often what’s being accessed.

Pointers vs. References: A Fundamental Distinction

To fully grasp why this is a pointer, it’s essential to revisit the core differences between pointers and references in C++. While both can be used to indirectly access data, their semantic and behavioral characteristics diverge significantly. Understanding these distinctions clarifies the design choice for this.

Nullability: Pointers can be null (nullptr), meaning they don’t point to any valid memory location. References, on the other hand, must always refer to a valid object and cannot be null. This “non-null” guarantee is a strong feature of references.
Reassignment: Pointers can be reassigned to point to different objects after initialization. A reference, once initialized to an object, cannot be rebound to refer to a different object; it acts as an alias to its initial target throughout its lifetime.
Memory Footprint: Pointers typically occupy memory (e.g., 4 or 8 bytes depending on the architecture) to store an address. References often do not consume additional memory; they are conceptually just aliases for existing objects, though compilers might implement them using addresses internally for efficiency.
Dereferencing: Pointers require explicit dereferencing (using `` or ->) to access the value they point to. References implicitly dereference; you use them just like the object they refer to.

For example, a pointer allows you to perform pointer arithmetic, moving through memory, which references do not permit. While references offer a safer, more “syntactic sugar” way to pass objects by alias, their inability to be null or reassigned makes them less flexible in certain low-level scenarios. The choice between a pointer and a reference often boils down to whether the ability to be null, reassigned, or perform pointer arithmetic is necessary. As Bjarne Stroustrup, the creator of C++, notes, “A reference is essentially a constant pointer that is automatically dereferenced.” This succinct definition highlights their core relationship and their key difference.

When considering this, the context of object identity and potential for dynamic behavior becomes paramount. The ability for this to represent the address of an object, rather than just an alias, opens up possibilities that a reference would not allow, particularly concerning polymorphism and memory layout. The decision to make ’this’ a pointer aligns with C++’s philosophy of providing low-level control and flexibility where needed, especially in its object model.

The Flexibility of ’this’ as a Pointer

The primary reason why ’this’ is a pointer and not a reference lies in the inherent flexibility and power that pointers offer, especially in the context of C++’s object model and dynamic dispatch. The ability of a pointer to represent a memory address, and potentially point to different types through polymorphism, is crucial for this.

Polymorphism and Virtual Functions: Pointers are essential for runtime polymorphism. When you have a base class pointer pointing to a derived class object, the virtual function mechanism relies on the pointer’s ability to resolve the correct function implementation at runtime. If this were a reference, it would bind at compile time, potentially limiting the dynamic dispatch of virtual functions based on the actual object type. The this pointer allows for correct virtual function calls even when invoking methods on an object via a base class pointer or reference. As explained on cppreference.com, “In a non- Question & Answer :
I was reading the answers to this question C++ pros and cons and got this doubt while reading the comments.

programmers frequently find it confusing that “this” is a pointer but not a reference. another confusion is why “hello” is not of type std::string but evaluates to a char const* (pointer) (after array to pointer conversion) – Johannes Schaub - litb Dec 22 ‘08 at 1:56

That only shows that it doesn’t use the same conventions as other (later) languages. – le dorfier Dec 22 ‘08 at 3:35

I’d call the “this” thing a pretty trivial issue though. And oops, thanks for catching a few errors in my examples of undefined behavior. :) Although I don’t understand what info about size has to do with anything in the first one. A pointer is simply not allowed to point outside allocated memory – jalf Dec 22 ‘08 at 4:18

Is this a constant poiner? – yesraaj Dec 22 ‘08 at 6:35

this can be constant if the method is const int getFoo() const; <- in the scope of getFoo, “this” is constant, and is therefore readonly. This prevents bugs and provides some level of guarantee to the caller that the object won’t change. – Doug T. Dec 22 ‘08 at 16:42

you can’t reassign “this”. i.e you cannot do “this = &other;”, because this is an rvalue. but this is of type T*, not of type T const . i.e it’s a non-constant pointer. if you are in a const method, then it’s a pointer to const. T const . but the pointer itself is nonconst – Johannes Schaub - litb Dec 22 ‘08 at 17:53

think of “this” like this: #define this (this_ + 0) where the compiler creates “this_” as a pointer to the object and makes “this” a keyword. you can’t assign “this” because (this_ + 0) is an rvalue. of course that’s not how it is (there is no such macro), but it can help understand it – Johannes Schaub - litb Dec 22 ‘08 at 17:55

My question is, why is this a pointer a not a reference? Any particular reason for making it a pointer?

Some further arguments why this being a reference would make sense:
- Consider Item 1 from More Effective C++ : use references when it is guaranteed that we have a valid object i.e. not a NULL (my interpretation).
- Furthermore, references are considered safer than pointers (because we can’t screw the memory up with a stray pointer).
- Thirdly, the syntax for accessing references (.) is a little bit nicer and shorter than accessing pointers (-> or (*)).
When the language was first evolving, in early releases with real users, there were no references, only pointers. References were added when operator overloading was added, as it requires references to work consistently.

One of the uses of this is for an object to get a pointer to itself. If it was a reference, we’d have to write &this. On the other hand, when we write an assignment operator we have to return *this, which would look simpler as return this. So if you had a blank slate, you could argue it either way. But C++ evolved gradually in response to feedback from a community of users (like most successful things). The value of backward compatibility totally overwhelms the minor advantages/disadvantages stemming from this being a reference or a pointer.