14.2 Constant Functions

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C++ lets you define two types of numbers : constant and nonconstant. For example:

 int index;                // Current index into the data array const int DATA_MAX(100);  // Maximum number of items in the array 

These two items are treated differently. For example, you can change the value of index , but you can't change DATA_MAX .

Now let's consider a class to implement a set of numbers from 0 to 31. The definition of this class is:

 // Warning: The member functions in this class are incomplete //          See below for a better definition of this class class int_set {     private:         // ... whatever     public:         int_set(  );                       // Default constructor         int_set(const int_set& old_set); // Copy constructor         void set(int value);             // Set a value         void clear(int value);           // Clear an element         int test(int value);             // See whether an element is set }; 

As with numbers, C++ will let you define two types of int_set objects: constant and nonconstant:

 int_set var_set;      // A variable set (we can change this) var_set.set(1);       // Set an element in the set // Define a constant version of the set (we cannot change this) const int_set const_set(var_set); 

In the int_set class, there are member functions such as set and clear that change the value of the set. There is also a function test that changes nothing.

Obviously you don't want to allow set and clear to be used on a constant. However, it is okay to use the test member function.

But how does C++ know what can be used on a constant and what can't? The trick is to put the keyword const at the end of the function header. This tells C++ that this member function can be used for a constant variable. So if you put const after the member function test , C++ will allow it to be used in a constant. The member functions set and clear do not have this keyword, so they can't be used in a constant.

 class int_set {     private:         // ... whatever     public:         int_set(  );             // Default constructor         int_set(const int_set& old_set); // Copy constructor         void set(int value);   // Set a value         void clear(int value); // Clear an element         int test(int value) const;   // See whether an element is set }; 

Thus, in your code you can do the following:

 int_set var_set;      // A variable set (we can change this) var_set.set(1);       // Set an element in the set (legal) // Define a constant version of the set (we cannot change this) const int_set const_set(var_set); // In the next statement we use the member function "test" legally std::cout << "Testing element 1. Value=" << const_set.test(  ) << '\n'; 

However, you cannot do the following:

 const_set.set(5);    // Illegal (set is not allowed on a const) 

The member function set was not declared const , so it cannot be invoked on a const int_set object.

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