What is the purpose of a constant pointer in c++?

Question

I am a beginner to C++ and have the following question regarding the purpose of pointers where neither the address nor the value that it is pointing to can be changed (constEverything in the example).

Example:

int main()
{
    int i = 1;
    int j = 2;
    int* const constPoint = &i;
    *constPoint = 3; // legal
    constPoint = &j; // illegal
    const int* constVal = &i;
    *constVal = 3; // illegal
    constVal = &j; // legal
    const int* const constEverything = &i;
    *constEverything = 3; // illegal
    constEverything = &j; // illegal
}

In the example there are different types of pointers. With constVal you can change the address and with constPoint you can change the underlying value. For constEverything you can do neither.

To me the purpose of such a pointer would be to pass things by constant reference. Why should I not just use const type &val instead? To me a const reference seems a lot easier and it makes const type* const val obsolete.

A C api has no references. To use libs like libcurl or libtar in your C++ projects you should know how to work with pointers. — Thomas Sablik, Jul 16 '19 at 09:24

score 4 · Accepted Answer · answered Jul 16 '19 at 09:24

Constant pointers are useful for two principal reasons.

this is a const pointer in a const member function.
A const pointer can also be set to nullptr.

My first point is somewhat circular, this could have been a reference type or a const reference type, but references arrived late into the C++ standard, and any changes would have been breaking.

score 2 · Answer 2 · answered Jul 16 '19 at 09:29

2

Pointers are remainders of the C programming language. C didn't have references, thus pointers were your only option. Now you are free to use references in C++, but if you have to work with a C API (e.g. Win32 API) you will have to use pointers.

answered Jul 16 '19 at 09:29

Aykhan Hagverdili

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score 1 · Answer 3 · answered Jul 16 '19 at 09:34

There are two ways to pass arguments: by value, and by reference. Constness is especially relevant when passing by reference: it indicates whether the passed argument can be changed.

In that respect, there are two ways you can pass by reference in C++: using a reference (&) or using a pointer (*). In C, there is only one way to pass by reference: using a pointer (*).

So if you allow passing references as const type &val in C++, the equivalent in C would have to use pointers, leading to const type * const val. You could ommit the constness of the pointer itself (which in my experience is often seen in C APIs).

Regardless of personal preferences, since C code should also be valid C++, a pointer based version is required as well.

score 1 · Answer 4 · answered Jul 16 '19 at 09:40

In the following code :

#include <iostream>

struct BigObject {
    int a{};
};

BigObject* createBigObj()
{
    bool allGood = true ; // some real work here

    if ( allGood ) 
        return new BigObject{1};
    else 
        return nullptr;
}

void readBigObj(BigObject const * const bigOb)
{
    if (bigOb)
        std::cout << bigOb->a << std::endl;
}

int main()
{
    auto* big = createBigObj();
    readBigObj(big);

    return 0;
}

The funciton :

void readBigObj(BigObject const * const bigOb)

Can't take a const BigObject& because the object can be null. The function didn't change the value of the pointor nor the value, both should be const

Tada! A reason of const value const pointor :)

What is the purpose of a constant pointer in c++?

4 Answers4

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