Pairs of STL

What is pair?

The introduction of pair classes is introduced in C ++ standard library as follows:

The classPairIs providedTreat two values as a single unit. It is used in several places within the C ++ standard library. In particle,The container classesMapAndMultimapUsePairS to manage their elements, Which are key/value pairs (see section 6.6). Another exampleThe usagePairsIs functions that return two values.

 

Pair Definition

The structure pair is defined in<Utility>As follows:

namespace std {    template <class T1, class T2>    struct pair     {        //type names for the values        typedef T1 first_type;        typedef T2 second_type;        //member        T1 first;        T2 second;        /* default constructor        * - T1 () and T2 () force initialization for built-in types        */        pair(): first(T1()), second(T2())         {        }        //constructor for two values        pair(const T1& a, const T2& b): first(a), second(b)         {        }        //copy constructor with implicit conversions        template<class U, class V>        pair(const pair<U,V>& p): first(p.first), second(p.second)         {        }    };    //comparisons    template <class T1, class T2>    bool operator== (const pair<T1,T2>&, const pair<T1,T2>&);    template <class T1, class T2>    bool operator< (const pair<T1,T2>&, const pair<T1,T2>&);        //similar: !=, <=, >, >=        //convenience function to create a pair    template <class T1, class T2>    pair<T1,T2> make_pair (const T1&, const T2&);}namespace std {    //comparisons    template <class T1, class T2>    bool operator== (const pair<T1,T2>& x, const pair<T1,T2>& y)     {        return x.first == y.first && x.second == y.second;    }    //similar: !=, <=, >, >=    template <class T1, class T2>    bool operator< (const pair<T1,T2>& x, const pair<T1,T2>& y)     {        return x.first < y.first || (!(y.first < x.first) && x.second < y.second);    }    //create value pair only by providing the values    template <class T1, class T2>    pair<Tl,T2> make_pair (const T1& x, const T2& y) {        return pair<T1,T2>(x, y);    }}

 

Use of pair

std::pair<int,float> p;    //initialize p. first and p.second with zerovoid f(std::pair<int,const char*>);void g(std::pair<const int.std::string>);void foo {    std::pair<int,const char*> p(42,"hello");    f(p);    //OK: calls built-in default copy constructor    g(p);    //OK: calls template constructor}std::make_pair(42, ‘@‘); //or std::pair<int,char>(42,‘@‘);void f(std::pair<int,const char*>);void g(std::pair<const int,std::string>);void foo {    f(std::make_pair(42,"hello"));     //pass two values as pair    g(std::make_pair(42,"hello"));     //pass two values as pair                                        // with type conversions}std::pair<int,float>(42,7.77);//does not yield the same asstd::make_pair(42,7.77);

The C ++ standard library uses pairs a lot.

For example,MapAndMultimapContainers usePairAs a type to manage their elements, which areKey/value pairs. See section 6.6, for a general description of maps and multimaps, and in particle page 91 for an example that shows the usage of TypePair.Objects of TypePairAre also used inside the C ++ standard library in functions that return two values (see page 183 For an example ).

Pairs of STL