Const Correctness

Overview

const is a modifier in C++ which does the obvious, it indicates that something is constant.

Correct usage of the const keyword in C++ is important to get right (it is called const correctness). Is is hard to correct an existing code base because of the ripple effect when adding the const modifier.

Decoding The const Keyword

The const keyword can be confusing, especially when used with pointers. What is const int * myInt compared to int const * myInt compared to int * const myInt?

The official rule is:

The const keyword applies the item on it’s left, unless there is none, in which it refers to the item on the right.

The only time there will not be an item on its left is when it is at the start of an expression (e.g. the start of a line). Lets take a look at the following example:

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int const * myInt;

Since there is an int to the left of the const, this is saying that myInt is a pointer to a constant integer. You cannot change the value of the variable the pointer points to.

What about:

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int * const myInt;

Again, since there is a pointer (*) to the left of the const, this is saying that myInt is a constant pointer to an integer. You cannot change the value of the pointer (i.e. where the pointer is pointing at).

Our third and final example:

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const int * myInt;

Since there is no item to the left of the const, it applies to the int on its right. This is saying the exact same thing as the first example, myInt is a pointer to a constant integer.

So int const * myInt; is exactly the same as const int * myInt;. I recommend only using the first version, as then your code reads the same way in every instance, the const always applies to the item on the left.

Multiple Const’s

After learning the rule above, reading multiple const’s in a single expression should be easy. Exactly the same rules apply. Take for example:

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int const * const myInt;

This is saying that myInt is a constant pointer to a constant int. This means that you cannot change what the pointer is pointing at, nor change the value of the int that the pointer points to. This is the same as writing:

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const int * const myInt;

Again, I discourage the use of this syntax.