Practical use of pointer to const

Question

I understand the use of pointer to constant for strlen implementation.

size_t strlen ( const char * str );

Can anyone suggest other reasons or provide some scenarios where 'Pointer to Const value' is useful in practice.

Answer 1

Think of it this way. You want me to look at the value of a variable but you don't want me to alter that variable in any way, so you pass it to me as a constant. When I use your function and see that a parameter is constant then I know there is a contract between you and I that says I should not change the value of that variable nor can I do it directly.

When you write code you don't always know who will use your functions. So it is good practice to protect your code. It also protects you from yourself, you will get compiler errors the moment you tr to change the value of that variable.

A side note: It is true that in C you can still change the value even though the parameter says const, but it would take another pointer which would alter the content of that variable in memory.

Try compiling this code and notice how the compiler protects you from making a mistake.

const char *cookies(const char *s)
{
   return ('\0' == *s)? s: s + 1;
}

It won't let you compile, why? Because you are trying to change a const variable.

Another post with the same question here: const usage with pointers in C

Answer 2

Marking a pointer argument as const is a contract whereby you assert to the user you won't change the values pointed to. It's also a contract that you won't try to write to them, so that it is legal for them to give you a pointer to something that is read-only memory, and your function won't crash - as long as you fulfill this contract.

It's not just "useful", in some cases it is required, particularly when you are dealing with what amounts to write protected memory such as string literals or any value stored in the code section of your executable.

It's also valuable when you are using data that might be shared: If two threads want to call a function foo(char* x) each will need its own copy of the string, otherwise bad things would happen. If the function is foo(const char* x) then we know it is safe for both to share a single pointer to the input.

Consider, if you have a pointer to write-protected memory:

mprotect(ptr, sizeOfData, PROT_READ);

It is now not possible to call a function that attempts to write to ptr without a program exception (this is often done with things like caches when nobody owns a write-lock on the cache).

Thus you can only call const functions on ptr at this point.

Answer 3

Pointer to const of char is the type of the strings literals (those inside ""). This is the primary use for pointer to const value.