What's the point of const pointers?

Asked 2011-10-10T15:48:02.137

160

I'm not talking about pointers to const values, but const pointers themselves.

I'm learning C and C++ beyond the very basic stuff and just until today I realized that pointers are passed by value to functions, which makes sense. This means that inside a function I can make the copied pointer point to some other value without affecting the original pointer from the caller.

So what's the point of having a function header that says:

void foo(int* const ptr);

Inside such a function you cannot make ptr point to something else because it's const and you don't want it to be modified, but a function like this:

void foo(int* ptr);

Does the work just as well! because the pointer is copied anyways and the pointer in the caller is not affected even if you modify the copy. So what's the advantage of const?

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2 Answers

There is a lot to the const keyword, it is a rather complex one. Generally, adding a lot of const to your program is considered good programming practice, search the web for "const correctness" and you'll find plenty of info about that.

The const keyword is a so-called "type qualifier", others are volatile and restrict. At least volatile follows the same (confusing) rules as const.

First of all, the const keyword serves two purposes. The most obvious one is to protect data (and pointers) from intentional or accidental misuse by making them read-only. Any attempt to modify a const variable will be spotted by the compiler at compile-time.

But there is also another purpose in any system with read-only memory, namely to ensure that a certain variable is allocated inside such memory - it could be EEPROM or flash for example. These are known as non-volatile memories, NVM. A variable allocated in NVM will still of course follow all the rules of a const variable.

There are several different ways to use the const keyword:

Declare a constant variable.

This can be done either as

const int X=1; or
int const X=1;

These two forms are completely equivalent. The latter style is considered bad style and should not be used.

The reason why the second row is considered bad style, is probably because "storage-class specifiers" such as static and extern also can be declared after the actual type, int static etc. But doing so for storage-class specifiers is labelled as an obsolete feature by the C committee (ISO 9899 N1539 draft, 6.11.5). Therefore, for the sake of consistency one should not write type qualifiers in that manner either. It serves no other purpose but to confuse the reader anyhow.

Declare a pointer to a constant variable.

const int* pt


                        
                        
                            answered 2011-10-10T19:01:28.437


            
            
                
                    
                        
                            
                            
                        
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                        int iVal = 10;
int *const ipPtr = &iVal;

Just like a normal const variable, a const pointer must be initialized to a value upon declaration, and its value can not be changed. 

This means a const pointer will always point to the same value. In the above case, ipPtr will always point to the address of iVal. However, because the value being pointed to is still non-const, it is possible to change the value being pointed to via dereferencing the pointer:

*ipPtr = 6; // allowed, since pnPtr points to a non-const int

                        
                        
                            answered 2011-10-10T15:54:53.860



        
        
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