C++ | const keyword

Const local variables

Declaration and usage.

// a is const int, so it can't be changed
const int a = 15;
a = 12;           // Error: can't assign new value to const variable
a += 1;           // Error: can't assign new value to const variable

Binding of references and pointers

int &b = a;       // Error: can't bind non-const reference to const variable
const int &c = a; // OK; c is a const reference

int *d = &a;      // Error: can't bind pointer-to-non-const to const variable
const int *e = &a // OK; e is a pointer-to-const

int f = 0;
e = &f;           // OK; e is a non-const pointer-to-const,
                  // which means that it can be rebound to new int* or const int*

*e = 1            // Error: e is a pointer-to-const which means that
                  // the value it points to can't be changed through dereferencing e

int *g = &f;
*g = 1;           // OK; this value still can be changed through dereferencing
                  // a pointer-not-to-const

Const pointers

int a = 0, b = 2;

const int* pA = &a; // pointer-to-const. `a` can't be changed through this
int* const pB = &a; // const pointer. `a` can be changed, but this pointer can't.
const int* const pC = &a; // const pointer-to-const.

//Error: Cannot assign to a const reference
*pA = b;

pA = &b;

*pB = b;

//Error: Cannot assign to const pointer
pB = &b;

//Error: Cannot assign to a const reference
*pC = b;

//Error: Cannot assign to const pointer
pC = &b;

Const member functions

Member functions of a class can be declared const, which tells the compiler and future readers that this function will not modify the object:

class MyClass
{
private:
    int myInt_;
public:
    int myInt() const { return myInt_; }
    void setMyInt(int myInt) { myInt_ = myInt; }
};

In a const member function, the this pointer is effectively a const MyClass * instead of a MyClass *. This means that you cannot change any member variables within the function; the compiler will emit a warning. So setMyInt could not be declared const.

You should almost always mark member functions as const when possible. Only const member functions can be called on a const MyClass.

static methods cannot be declared as const. This is because a static method belongs to a class and is not called on object; therefore it can never modify object’s internal variables. So declaring static methods as const would be redundant.

Avoiding duplication of code in const and non-const getter methods.

In C++ methods that differs only by const qualifier can be overloaded. Sometimes there may be a need of two versions of getter that return a reference to some member.

Let Foo be a class, that has two methods that perform identical operations and returns a reference to an object of type Bar:

class Foo
{
public:
    Bar& GetBar(/* some arguments */)
    {
        /* some calculations */
        return bar;
    }

    const Bar& GetBar(/* some arguments */) const
    {
        /* some calculations */
        return bar;
    }

    // ...
};

The only difference between them is that one method is non-const and return a non-const reference (that can be use to modify object) and the second is const and returns const reference.

To avoid the code duplication, there is a temptation to call one method from another. However, we can not call non-const method from the const one. But we can call const method from non-const one. That will require as to use ‘const_cast’ to remove the const qualifier.

The solution is:

struct Foo
{
    Bar& GetBar(/*arguments*/)
    {
        return const_cast<Bar&>(const_cast<const Foo*>(this)->GetBar(/*arguments*/));
    }

    const Bar& GetBar(/*arguments*/) const
    {
        /* some calculations */
        return foo;
    }
};

In code above, we call const version of GetBar from the non-const GetBar by casting this to const type: const_cast<const Foo*>(this). Since we call const method from non-const, the object itself is non-const, and casting away the const is allowed.

Examine the following more complete example:

#include <iostream>

class Student
{
public:
    char& GetScore(bool midterm)
    {
        return const_cast<char&>(const_cast<const Student*>(this)->GetScore(midterm));
    }

    const char& GetScore(bool midterm) const
    {
        if (midterm)
        {
            return midtermScore;
        }
        else
        {
            return finalScore;
        }
    }

private:
    char midtermScore;
    char finalScore;
};

int main()
{
    // non-const object
    Student a;
    // We can assign to the reference. Non-const version of GetScore is called
    a.GetScore(true) = 'B';
    a.GetScore(false) = 'A';

    // const object
    const Student b(a);
    // We still can call GetScore method of const object,
    // because we have overloaded const version of GetScore
    std::cout << b.GetScore(true) << b.GetScore(false) << '\n';
}

Syntax

const Type myVariable = initial; // Declares a const variable; cannot be changed
const Type &myReference = myVariable; // Declares a reference to a const variable
const Type *myPointer = &myVariable; // Declares a pointer-to-const. The pointer can change, but the underlying data member cannot be changed through the pointer
Type * const myPointer = &myVariable; // Declares a const pointer. The pointer cannot be reassigned to point to something else, but the underlying data member can be changed
const Type * const myPointer = &myVariable; // Declares a const pointer-to-const.

Remarks

A variable marked as const cannot¹ be changed. Attempting to call any non-const operations on it will result in a compiler error.

_{1: Well, it can be changed through const_cast, but you should almost never use that}