C++ Concepts :Introduction, Goals, and Applications

Mohammad SoryaniMazandaran University of Science and Technology

Soryani@ustmb.ac.ir

C++ Concepts: Introduction, Goals, and Applications 1

• Preface • C++ template system• Generic programming• Concepts goals and applications• Concepts considerations• Concepts structure• Concept maps• Concept refinement• Concept based overloading• Optimized algorithms• History and current situation

Outline

C++ Concepts: Introduction, Goals, and Applications 2

Preface

-All of our discussion will be inside C++

-Which part of it?

-Template system

-What does it have to do with CBSE?

-We’ll get to it soon

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Templates-Let’s take a look at a normal function that gets two integers and returns the min :

int getmin (int a, int b) {

return (a<b ? a : b);

}

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Templates

Template <typename T>

T getmin (T a, T b) {

return (a<b ? a : b);

} - There is a library of templates in C++ called STL(Standard Template Library)

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Templates

- How can we use this template?

- Let’s see some examples : int x,y; getmin <int> (x,y); double i,j; getmin <double> (i,j); - What happens when we use it?

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Templates Some questions about the current C++ template system :

- Is a user allowed to use a template with any type?

- Can we write their requirements in C++?

- What would happen if a user uses a wrong type? - Who is responsible for the error? - Will we get informed about the cause of the error?

- Is it possible to turn a wrong type into a right one?

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Generic programming-A methodology for the development of reusable software libraries

- Three primary tasks:- Categorize the abstractions in a domain into concepts- Implement generic algorithms based on the concepts- Build concrete models of the concepts

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Generic programming

- Lifting is a very important tool

Lifting in a glance : 1- Study the concrete implementations of an algorithm 2- Lift away unnecessary requirements to produce a more abstract algorithm and bundle these requirements into concepts. 3- Repeat the lifting process until we have obtained a generic algorithm

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Concepts Goals and Applications- Bringing the full power of Generic programming into C++

- Coding the requirements of data types directly directly into C++

- Better errors for template system

- adoption

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Considerations

- Simplicity

- Backward compatibility

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Concepts Structure - Recall the getmin() function; let’s write the requirements of it’s type parameters. concept LessThanComparable<typename T> { bool operator<(T x, T y); }

- Now let’s make our template constrained template<typename T> requires LessThanComparable<T> T getmin(T a, T b) { return a < b ? a : b; }

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Concept Maps - Consider we have a type called color for wich there is no “+” operator

- We want to apply an add algorithm to two variables with type color

- We make it possible by defining a map

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Concept Maps - If some operations are not defined for a type we can define them in a concept map

concept_map Addable<color} < color operator+(color x, color y)

} return x.mix(y){ ; {

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Concept Refinement - It seems like inheritance - The refining concept inherits all of the base concept's requirements - For example we can refine Polygon concept to make EquilateralPolygon concept : concept EquilateralPolygon<typename P> : Polygon<P> { … }

- A type that is an EquilateralPolygon can be used in any algorithm that expects a Polygon

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Concept Based Overloading - We have different algorithms with an identical name

- The compiler chooses which algorithm should be used

- Choosing the algorithm is based on the concept(requirements) that used types meet.

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Optimized algorithms - We have an algorithm that calculates the perimeter of a polygon by adding the side lengths of it (only the types that are Polygon can use this algorithm)

- For equilateral polygons we can write a better algorithm that has just one multiplication (only the types that are EquilateralPolygon can use this algorithm)

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Optimized algorithms - We make EquilateralPolygon concept by refining the Polygon concept (concept refinement)

- We give both of the algorithms(functions) that do the same thing an identical name (overloaded versions)

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Optimized algorithms - The compiler uses the better algorithm for types that meet the refined concept (Multiplication for EquilateralPolygon in this case) - If the used type just meets the base concept the normal algorithm is used

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History and Current Situation - Bjarne Stroustrup says that he has worked on concepts for more than seven years [3]

- Concepts is the work of many people, but the two universities that are doing much of the work are Texas A&M And Indiana university [2]

- At the July 2009 meeting in Frankfurt, Germany, the C++ Standards Committee voted to remove concepts from C++0x[3] - Maybe a significantly improved version of "concepts" will be available in five years [3]

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References[1] Douglas. Gregor, “Easier C++: An Introduction to Concepts”,http://www.devx.com/SpecialReports/Article/38864/1954, August 18, 2008.[2] Douglas. Gregor, “Concepts: Extending C++ Templates ForGeneric Programming”, Google TechTalks , February 21, 2007.[3] Bjarne. Stroustrup, “The C++0x Remove Concepts Decision”,Dr. Dobb’s Journal, http://www.ddj.com/cpp/218600111,Jul 22, 2009.[4] Douglas. Gregor, “ConceptsC++ Tutorial”, April 11, 2007.