Understanding Templates and Friends in Object-Oriented Programming (OOP) of C++, Slides of Object Oriented Programming

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Object-Oriented Programming

(OOP)

Lecture No. 38

Templates and Friends

• Like inheritance, templates or their

specializations are compatible with

friendship feature of C++

void doSomething( B< char >& ); class A { … }; template< class T > class B { int data; friend void doSomething( B& ); friend A; … };

void doSomething( B< char >& cb ) { B< int > ib; ib.data = 5; // OK cb.data = 6; // OK }

Templates and Friends – Rule 2

• When a friend function / class template is

instantiated with the type parameters of

class template granting friendship then its

instantiation for a specific type is a friend

of that class template instantiation for that

particular type

template< class U > void doSomething( U ); template< class V > class A { … }; template< class T > class B { int data; friend void doSomething( T ); friend A< T >; };

int main() { int i = 5; char c = ‘x’; doSomething( i ); // OK doSomething( c ); // OK return 0; }

template< class U > void doSomething( U u ) { B< int > ib; ib.data = 78; }

• Because doSomething() always

instantiates B< int >

class B< int > { int data; friend void doSomething( int ); friend A< int >; };

template< class T > class A { void method() { // Error! B< char > cb; cb.data = 8; B< int > ib; ib.data = 9; } };

template< class U > void doSomething( U ); template< class V > class A { … }; template< class T > class B { int data; template< class W > friend void doSomething( W ); template< class S > friend class A; };

template< class U > void doSomething( U u ) { B< int > ib; ib.data = 78; }

template< class T > class A { void method() { // OK! B< char > cb; cb.data = 8; B< int > ib; ib.data = 9; } };

Templates and Friends – Rule 4

• Declaring a template as friend implies that

all kinds of its specializations – explicit,

implicit and partial, are also friends of the

class granting friendship