We will take the code below as an example throughout this section:

 #include <vector>
   
 class iStack {
 public:
    iStack( int capacity )
    : _stack( capacity ), _top( 0 ) { }
    bool pop( int &top_value );
    bool push( int value );
    bool full();
    bool empty();
    void display();
    int size();
 private:
    int _top;
    vector< int > _stack;
 };

Exceptions are most often implemented using classes.

 // stackExcp.h:
 // stackExcp.h
 class popOnEmpty { /* ... */ };
 class pushOnFull { /* ... */ };

An exception is an object, and pop() must throw an object of class type. The expression in the throw expression cannot simply be a type. To create an object of class type, we need to call the class constructor.

We are now ready to provide the new implementations of iStack’s pop() and push() member functions:

 #include "stackExcp.h"
   
 void iStack::pop( int &top_value )
 {
    if ( empty() )
    throw popOnEmpty();
    top_value = _stack[ --_top ];
    cout « "iStack::pop(): " « top_value « endl;
 }
   
 void iStack::push( int value )
 {
    cout « "iStack::push( " « value « " )\n";
    if ( full() )
    throw pushOnFull();
    _stack[ _top++ ] = value;
 }

Because exceptions are now used to indicate the failure of the pop() and push() operations, the return values from these functions are now unnecessary.

Although exceptions are most often objects of class type, a throw expression can throw an object of any type.