Const and pointers in C++

The possible uses of the const qualifier with C++ pointers:

• pointer to const variable
• const pointer to variable
• const pointer to const variable

All examples below will be using the same struct MyType defined below:

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struct MyType {
    int _x;
    MyType(int x): _x(x) {}
    friend std::ostream& operator<<(std::ostream& out, const MyType &my_type)
    {
        out << my_type._x;
        return out;
    }
};

We declare two variables of type MyType as follows:

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MyType a{1};
MyType b{2};

Pointer to const variable

When using a pointer to a const variable, we cannot change the value of the variable pointed to by the pointer. However, we can reassign the pointer itself:

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// pointer to const variable
const MyType *ptr1 = &a;
std::cout << "ptr1: " << *ptr1 << std::endl;
// ptr1->_x = 33; // error
ptr1 = &b;
std::cout << "ptr1: " << *ptr1 << std::endl;

Output:

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ptr1: 1
ptr1: 2

Const pointer to variable

When using a const pointer to a variable, we can change the value of the variable pointed to by the pointer. However, we cannot reassign the pointer. It is the opposite of the situation above:

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// const pointer to variable
MyType* const ptr2 = &a;
std::cout << "ptr2: " << *ptr2 << std::endl;
ptr2->_x = 10;
std::cout << "ptr2: " << *ptr2 << std::endl;
// ptr2 = &b; // error

Output:

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ptr2: 1
ptr2: 10

Const pointer to const variable

When using a const pointer to a const variable, we can neither change the value of the variable pointed by the pointer, nor reassign the pointer:

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// const pointer to const variable
const MyType* const ptr3 = &b;
std::cout << "ptr3: " << *ptr3 << std::endl;
// ptr3->_x = 33; // error
// ptr3 = &a; // error

Output:

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ptr3: 2

Conclusion

Here is the recap in table form: