7.6. const member functions

We have previously mentioned uses for const, and classes add another situation where the const keyword can be used.

An object can be created with the const cv qualifier, just like any other type:

const int x = 3;

const Fibonacci foo = {5, 8, 13};

When created const, then no changes are allowed to the object. It is OK to call a non-modifying member function.

How does the compiler know that a function is a non-modifying member function? When the function is declared as const.

Create a const member function by inserting the const keyword after the parameter list and before the function body:

bool verbose() const {
  return true;
}

Here const tells the compiler this function will not change the object state.

It is a promise. If a const function attempts to change any class member, then a compile error results.

Note

Only class member functions can be marked const.

Attempting to mark a free function as const is a compile error.

sc-1-1: Given the following:

#include <iostream>

class Foo {
  int value_ = 0;
  public:
    Foo() {}
    void value (const int x) {
      value_ = x;
    }
    int value() {
      return value_;
    }
};
int main () {
  const Foo a;
  a.value(13);
  std::cout << a.value() << '\n';
}

What is the output? Choose the one correct answer.

• Program prints 13

• It can’t print 13 because changing value_ is not allowed.

• Does not compile

• Calling a non-const function of an object declared const is a compile error.

• Program runs, but does not reach the end of main

• The program cannot run

• Program behavior is undefined

• The program doesn’t get this far. We know exactly what will happen.

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More to Explore

• From cppreference.com

  • Const and volatile type qualifiers.

  • Const member functions.

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