Advantages and Disadvantages of Using a Const Pointer in C/C

When developers use the term 'const pointer', they often refer to a 'non-const pointer to const data'. However, the term 'const pointer' is ambiguous enough to refer to any of the following cases: a non-const pointer to const data, a const pointer to non-const data, and a const pointer to const data. Understanding the nuances of these types of pointers is crucial for writing robust and efficient C/C code.

Non-const Pointer to Const Data

Consider the following:

const char *var_name1

This is equivalent to:

char const *var_name1

var_name1 is a non-const pointer to const data. This means the pointer can be used to read the data it points to but cannot be used to write or change the data. The pointer variable itself can be altered to point to other char values. The const keyword in this case refers to the constness of the data only. Note the flexibility of the position of the const keyword as long as it is to the left of the asterisk.

This kind of pointer is useful when a function receives a pointer to some data but makes the promise to the caller that the function will not alter that data it points to. This is the most common use of this type of pointer. You’ll often find it in input parameter declarations in function prototypes and function definitions.

Of course, you could get a similar effect by having the function receive the data by value. In this case, it sees a copy of the original data. However, for large structures or arrays, this would be an expensive operation. Therefore, passing a pointer to const data is often the more appropriate approach from a performance perspective.

Be cautious when "casting away constness," as the receiving function can potentially alter the data pointed to. Advertise that the pointer is to const data and then renege on that promise is not a good practice. Follow the principle of least astonishment (POLA) and treat the constness of the data pointed to as a contract with the caller.

Const Pointer to Non-const Data

Now consider the following:

char const *var_name2

var_name2 is a const pointer to non-const data. This means the pointer can be used to read the data it points to and can be used to write or change the data. However, the pointer variable itself cannot be altered to point to other char values. You cannot increment, decrement, or reassign the pointer.

The const keyword in this case refers to the constness of the pointer itself, not to the data it points to. This can be useful if you have a pointer to a data buffer and you want to ensure that the data can change but you do not want to accidentally alter the pointer. For instance, if you allocated a buffer and wanted to ensure you didn’t increment or decrement the pointer so that you could have the original pointer when it came time to free it back to the heap, a const pointer to non-const data would be the right choice.

Const Pointer to Const Data

Consider the following:

const char *const var_name3

This is equivalent to:

char const *const var_name3

var_name3 is a const pointer to const data. This means the pointer can be used to read the data it points to but cannot be used to write or change the data. Additionally, the pointer variable itself cannot be altered to point to other char values. You cannot increment, decrement, or reassign the pointer.

This type of pointer is often used when the function needs to ensure that the data pointed to remains unchanged and the pointer itself should not be changed.

Conclusion

In conclusion, the choice between a const pointer, a non-const pointer to const data, and a const pointer to non-const data depends on your specific use case. Using const pointers appropriately can help prevent unintended modifications and make your code more predictable and robust. Always keep in mind the principle of least astonishment and the constness of the data and the pointer itself to ensure your functions behave as expected.

References

For a deeper understanding, you might want to refer to C standard documentation and online resources such as the C Reference website and Stack Overflow for additional examples and discussions.