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15.6. Pure Virtual Functions

A pure virtual function is specified by writing = 0 after the function parameter list:

class Disc_item : public Item_base {

public:

double net_price(std::size_t) const =0;

};

Defining a virtual as pure indicates that the function provides an interface for sub-sequent types to override but that the version in this class will never be called. As importantly, users will not be able to create objects of type Disc_item.

An attempt to create an object of an abstract base class is a compile-time error:

// Disc_item declares pure virtual functions

Disc_item discounted; // error: can't define a Disc_item object

Bulk_item bulk; // ok: Disc_item subobject within Bulk_item

Note:A  class containing (or inheriting) one or more pure virtual functions is an  abstract base class  .  We may not create objects of an abstract type except as parts of objects of classes derived from the abstract base.

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