Computer Notes - Templates and Friends

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Templates and Friends
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Templates and Friends

• Like inheritance, templates or their
  specializations are compatible with friendship
  feature of C

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Templates and Friends Rule 1

• When an ordinary function or class is declared as
  friend of a class template then it becomes friend
  of each instantiation of that template

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Templates and Friends Rule 1

• void doSomething( Blt char gt )
• class A
• templatelt class T gt class B
• int data
• friend void doSomething( Bltchargt )
• friend A

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Templates and Friends Rule 1

• void doSomething( Blt char gt cb )
• Blt int gt ib
• ib.data 5 // OK
• cb.data 6 // OK

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Templates and Friends Rule 1

• class A
• void method()
• Blt int gt ib
• Blt char gt cb
• ib.data 5 // OK
• cb.data 6 // OK

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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

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Templates and Friends Rule 2

• templatelt class U gt
• void doSomething( U )
• templatelt class V gt
• class A
• templatelt class T gt class B
• int data
• friend void doSomething( T )
• friend Alt T gt

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Templates and Friends Rule 2

• templatelt class U gt
• void doSomething( U u )
• Blt U gt ib
• ib.data 78

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Templates and Friends Rule 2

• int main()
• int i 5
• char c x
• doSomething( i ) // OK
• doSomething( c ) // OK
• return 0

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Templates and Friends Rule 2

• templatelt class U gt
• void doSomething( U u )
• Blt int gt ib
• ib.data 78

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Templates and Friends Rule 2

• int main()
• int i 5
• char c x
• doSomething( i ) // OK
• doSomething( c ) // Error!
• return 0

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Templates and Friends Rule 2

• Because doSomething() always instantiates Blt int
  gt
• class Blt int gt
• int data
• friend void doSomething( int )
• friend Alt int gt

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Templates and Friends Rule 2

• templatelt class T gt
• class A
• void method() // Error!
• Blt char gt cb
• cb.data 8
• Blt int gt ib
• ib.data 9

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Templates and Friends Rule 3

• When a friend function / class template takes
  different type parameters from the class template
  granting friendship then its each instantiation
  is a friend of each instantiation of the class
  template granting friendship

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Templates and Friends Rule 3

• templatelt class U gt
• void doSomething( U )
• templatelt class V gt
• class A
• templatelt class T gt class B
• int data
• templatelt class W gt
• friend void doSomething( W )
• templatelt class S gt
• friend class A

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Templates and Friends Rule 3

• templatelt class U gt
• void doSomething( U u )
• Blt int gt ib
• ib.data 78

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Templates and Friends Rule 3

• int main()
• int i 5
• char c x
• doSomething( i ) // OK
• doSomething( c ) // OK
• return 0

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Templates and Friends Rule 3

• templatelt class T gt
• class A
• void method() // OK!
• Blt char gt cb
• cb.data 8
• Blt int gt ib
• ib.data 9

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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

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Templates and Friends Rule 4

• templatelt class T gt
• class B
• T data
• templatelt class U gt
• friend class A

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Templates and Friends Rule 4

• templatelt class U gt
• class A
• A()
• Blt int gt ib
• ib.data 10 // OK

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Templates and Friends Rule 4

• templatelt class U gt
• class Alt U gt
• A()
• Blt int gt ib
• ib.data 10 // OK

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Templates and Friends Rule 4

• templatelt class T gt
• class B
• T data
• templatelt class U gt
• friend void doSomething( U )

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Templates and Friends Rule 4

• templatelt class U gt
• void doSomething( U u )
• Blt int gt ib
• ib.data 56 // OK

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Templates and Friends Rule 4

• templatelt gt
• void doSomethinglt char gt( char u )
• Blt int gt ib
• ib.data 56 // OK

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