What Does 'const' at the End of a C Function Declaration Mean?

Understanding "const" at the End of Function Declarations

When working with object-oriented programming in C , you may encounter code that includes "const" at the end of function declarations. This raises the question of what this keyword signifies.

What Does "const" Mean in This Context?

The "const" keyword indicates that the function being declared is a "constant function." This means that it cannot modify any data members of the class it belongs to. Conversely, the function is allowed to read and access data members.

Analogy of the "this" Pointer

To understand the concept better, consider a class function as a regular function that takes an implicit "this" pointer. For example, a function declared as "int Foo::Bar(int random_arg)" can be viewed as "int Foo_Bar(Foo* this, int random_arg)." By adding "const" at the end, you essentially create a declaration with a const "this" pointer, which restricts any modification of data members.

Exceptions Using "mutable"

It's important to note that the "const" restriction can be relaxed in some situations. By marking class variables as "mutable," they can become writable even within a "const" function. This allows selective modification of variables while maintaining the constant nature of the function.

Placement of "const" Matters

The placement of "const" in C is crucial. Adding it at the end of the function declaration has a specific meaning, distinct from using it in other parts of the statement.

Further Reading

For a deeper understanding of "const" in C , refer to the following resources:

• Const Correctness
• The C 'const' Declaration: Why & How

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