Type Traits Generic programming and library development Michael - - PowerPoint PPT Presentation

Introduction Type Traits In C++98 Type Traits In boost And TR1

Type Traits

Generic programming and library development Michael Rasmussen og Klaus Birkelund Jensen

Department of Computer Science - University of Copenhagen

May 9, 2008

Michael Rasmussen og Klaus Birkelund Jensen Department of Computer Science - University of Copenhagen Type Traits

Introduction Type Traits In C++98 Type Traits In boost And TR1

Motivation

Improve performance of generic algorithms based on advanced type information Specialize certain operations to better fit the type Deduce more information about the type Transform types into other types

Michael Rasmussen og Klaus Birkelund Jensen Department of Computer Science - University of Copenhagen Type Traits

Introduction Type Traits In C++98 Type Traits In boost And TR1

Examples

template <typename I1 , typename I2> I2 copy impl ( I1 f i r s t , I1 l a s t , I2

• ut )

w h i l e ( f i r s t != l a s t ) { ∗ out = ∗ f i r s t ; ++out ; ++f i r s t ; } } template <typename T > T∗ copy impl (T∗ f i r s t , T const ∗ l a s t ) { memcpy( out , f i r s t , ( l a s t −f i r s t )∗ s i z e o f (T ) ) ; r e t u r n

• ut+( l a s t −f i r s t ) ;

}

Michael Rasmussen og Klaus Birkelund Jensen Department of Computer Science - University of Copenhagen Type Traits

Introduction Type Traits In C++98 Type Traits In boost And TR1

Examples

template <typename T > T f i n d r o o t ( T (∗ f )(T const &) ) { i f ( std : : n u m e r i c l i m i t s <T>:: i s i n t e g e r ) /∗ f i n d exact root ∗/ e l s e /∗ f i n d where c l o s e to zero ∗/ }

Michael Rasmussen og Klaus Birkelund Jensen Department of Computer Science - University of Copenhagen Type Traits

Introduction Type Traits In C++98 Type Traits In boost And TR1

Examples

template <typename T > T f i n d r o o t ( T (∗ f )(T const &) ) { i f ( std : : n u m e r i c l i m i t s <T>:: i s i n t e g e r ) /∗ f i n d exact root ∗/ e l s e /∗ f i n d where c l o s e to zero ∗/ } template <typename I> std : : i t e r a t o r t r a i t s <T>:: v a l u e t y p e second ( I i t ) { ++i t ; r e t u r n ∗ i t ; }

Michael Rasmussen og Klaus Birkelund Jensen Department of Computer Science - University of Copenhagen Type Traits

Introduction Type Traits In C++98 Type Traits In boost And TR1

Implementation

Use various tricks to deduce what the types are (e.g. pointers to member functions) Use template specialization (to determine if the type is const, volatile etc.) Explicitly specialize base types (numeric types)

Michael Rasmussen og Klaus Birkelund Jensen Department of Computer Science - University of Copenhagen Type Traits

Introduction Type Traits In C++98 Type Traits In boost And TR1

iterator traits

template <c l a s s T > s t r u c t i t e r a t o r t r a i t s <T > { t y p e d e f /∗ . . . ∗/ i t e r a t o r c a t e g o r y ; t y p e d e f /∗ . . . ∗/ v a l u e t y p e ; t y p e d e f /∗ . . . ∗/ d i f f e r e n c e t y p e ; t y p e d e f /∗ . . . ∗/ p o i n t e r ; t y p e d e f /∗ . . . ∗/ r e f e r e n c e ; };

Michael Rasmussen og Klaus Birkelund Jensen Department of Computer Science - University of Copenhagen Type Traits

Introduction Type Traits In C++98 Type Traits In boost And TR1

numeric limits

template <c l a s s T > c l a s s n u m e r i c l i m i t s { p u b l i c : s t a t i c const bool i s s p e c i a l i z e d ; s t a t i c T min () throw ( ) ; s t a t i c T max () throw ( ) ; s t a t i c const i n t r a d i x ; s t a t i c const i n t d i g i t s ; s t a t i c const i n t d i g i t s 1 0 ; s t a t i c const bool i s s i g n e d ; s t a t i c const bool i s i n t e g e r ; s t a t i c const bool i s e x a c t ; // s n i p

Michael Rasmussen og Klaus Birkelund Jensen Department of Computer Science - University of Copenhagen Type Traits

Introduction Type Traits In C++98 Type Traits In boost And TR1

The Type Traits Library

Included in the recently approved TR11 Uniform and portable interface Query, compare and modify type traits Based on boost’s type traits library

1Technical Report One Michael Rasmussen og Klaus Birkelund Jensen Department of Computer Science - University of Copenhagen Type Traits

Introduction Type Traits In C++98 Type Traits In boost And TR1

Design

Type traits implemented as distinct classes. If the queried type has the given property the trait inherits from the class true type. Otherwise the trait inherits from false type.

Michael Rasmussen og Klaus Birkelund Jensen Department of Computer Science - University of Copenhagen Type Traits

Introduction Type Traits In C++98 Type Traits In boost And TR1

Simple Example

template <typename T > s t r u c t i s v o i d : p u b l i c f a l s e t y p e {}; template < > s t r u c t i s v o i d <void> : p u b l i c t r u e t y p e {};

Michael Rasmussen og Klaus Birkelund Jensen Department of Computer Science - University of Copenhagen Type Traits

Introduction Type Traits In C++98 Type Traits In boost And TR1

Example Usage

template <typename I1 , typename I2> i n l i n e I2 copy ( I1 f i r s t , I1 l a s t , I2

• ut )

{ t y p e d e f typename std : : i t e r a t o r t r a i t s <I1 >:: v a l u e t y p e v a l u e t y p e ; r e t u r n copy imp ( f i r s t , l a s t ,

• ut ,

std : : h a s t r i v i a l a s s i g n <v a l u e t y p e >()); }

Michael Rasmussen og Klaus Birkelund Jensen Department of Computer Science - University of Copenhagen Type Traits

Introduction Type Traits In C++98 Type Traits In boost And TR1

Example Usage

template <typename I1 , typename I2> i n l i n e I2 copy ( I1 f i r s t , I1 l a s t , I2

• ut )

{ t y p e d e f typename std : : i t e r a t o r t r a i t s <I1 >:: v a l u e t y p e v a l u e t y p e ; r e t u r n copy imp ( f i r s t , l a s t ,

• ut ,

std : : h a s t r i v i a l a s s i g n <v a l u e t y p e >()); } template <typename T > T∗ copy imp ( const T∗ f i r s t , const T∗ l a s t , T∗ out , const t r u e t y p e &); //

• ptimized

v e r s i o n template <typename I1 , typename I2 , bool b> I2 copy imp ( I1 f i r s t , I1 l a s t , I2

• ut ,

const i n t e g r a l c o n s t a n t <bool , b>&); // d e f a u l t − always s a f e

Michael Rasmussen og Klaus Birkelund Jensen Department of Computer Science - University of Copenhagen Type Traits

Introduction Type Traits In C++98 Type Traits In boost And TR1

Other Queries

i s a r r a y i s c l a s s i s c o n s t i s f u n c t i o n i s i n t e g r a l i s m e m b e r o b j e c t p o i n t e r i s m e m b e r p o i n t e r i s p o d i s r e f e r e n c e

Michael Rasmussen og Klaus Birkelund Jensen Department of Computer Science - University of Copenhagen Type Traits

Introduction Type Traits In C++98 Type Traits In boost And TR1

Comparisons

Compare base and derived classes:

is base of c l a s s Base ; c l a s s Derived : p u b l i c Base ; is base of <Base, Derived> inherits from true type

Michael Rasmussen og Klaus Birkelund Jensen Department of Computer Science - University of Copenhagen Type Traits

Introduction Type Traits In C++98 Type Traits In boost And TR1

Transformations

Add/remove const and volatile qualifiers.

template <typename T > s t r u c t remove const { t y p e d e f T type ; }; template <typename T > s t r u c t remove const <const T > { t y p e d e f T type ; };

Note: remove const<const int∗>::type evaluates to const int∗

Michael Rasmussen og Klaus Birkelund Jensen Department of Computer Science - University of Copenhagen Type Traits

Introduction Type Traits In C++98 Type Traits In boost And TR1

Sources

Bjarne Stroustrup: The C++ Programming Language Special Edition Vandevoorde, Josuttis: C++ Templates - The Complete Guide boost http://www.boost.org TR1 articles: http://www.informit.com

Michael Rasmussen og Klaus Birkelund Jensen Department of Computer Science - University of Copenhagen Type Traits