What is the use of const overloading in C++?

Question

In C++, a function's signature depends partly on whether or not it's const. This means that a class can have two member functions with identical signatures except that one is const and the other is not. If you have a class like this, then the compiler will decide which function to call based on the object you call it on: if it's a const instance of the class, the const version of the function will be called; if the object isn't const, the other version will be called.

In what circumstances might you want to take advantage of this feature?

Solution 1

This really only makes sense when the member function returns a pointer or a reference to a data member of your class (or a member of a member, or a member of a member of a member, ... etc.). Generally returning non-const pointers or references to data members is frowned upon, but sometimes it is reasonable, or simply very convenient (e.g. [] operator). In such cases, you provide a const and a non-const versions of the getter. This way the decision on whether or not the object can be modified rests with the function using it, which has a choice of declaring it const or non-const.

Solution 2

It's there so you can make the compiler enforce whether you return a const object or a regular one, and still maintain the same method signature. There's an in-depth explanation at Const Correctness.

Solution 3

Have a look at the behaviour of std::map::operator[]. The const version throws an error if you try to reference an invalid key, but the non-const version does an insert. The insertion behaviour is much handier than having to use std::map::insert (and will do an overwrite, moreover) but can't work for a const map.

Solution 4

You might want to use it to decide whether or not to return a const reference to an object or not. The STL's containers use a const overloaded begin() and end() function to decide whether to return a const_iterator or a normal iterator.

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Comments

[Nathan Smiechowski]

You have not explained why const qualifier is needed on member functions at all.