std::pair主要的作用是将两个数据组合成一个数据，两个数据可以是同一类型或者不同类型。例如std::pair<int,float> 或者 std：：pair<double,double>等。pair实质上是一个结构体，其主要的两个成员变量是first和second，这两个变量可以直接使用。初始化一个pair可以使用构造函数，也可以使用std::make_pair函数，make_pair函数的定义如下：
template pair make_pair(t1 a, t2 b) {
return pair(a, b); }
return pair(a, b); }
一般make_pair都使用在需要pair做参数的位置，可以直接调用make_pair生成pair对象。另一个使用的方面就是pair可以接受隐式的类型转换，这样可以获得更高的灵活度。但是这样会出现如下问题：例如有如下两个定义：
std::pair<int,
float>(1,
1.1);
1.1);
float>(1,
1.1);
std::make_pair(
1,1.1);
其中第一个的second变量是float类型，而make_pair函数会将second变量都转换成double类型。这个问题在编程是需要引起注意。下面是一段pair与make_pair的例子程序：
1 #include
<iostream>
2 #include
<utility>
3 #include
<string>
4 using
namespace std;
5
6 int
main () {
7 pair
<string,double>
product1 ("tomatoes",3.25);
8 pair
<string,double>
product2;
9 pair
<string,double>
product3;
10
11 product2.first
= "lightbulbs";
// type of first is string
12 product2.second
= 0.99;
// type of second is double
13
14 product3
= make_pair ("shoes",20.0);
15
16 cout
<< "the price of
" <<
product1.first <<
" is $"
<< product1.second
<< "\n";
17 cout
<< "the price of
" <<
product2.first <<
" is $"
<< product2.second
<< "\n";
18 cout
<< "the price of
" <<
product3.first <<
" is $"
<< product3.second
<< "\n";
19 return
0;
20 }
<iostream>
2 #include
<utility>
3 #include
<string>
4 using
namespace std;
5
6 int
main () {
7 pair
<string,double>
product1 ("tomatoes",3.25);
8 pair
<string,double>
product2;
9 pair
<string,double>
product3;
10
11 product2.first
= "lightbulbs";
// type of first is string
12 product2.second
= 0.99;
// type of second is double
13
14 product3
= make_pair ("shoes",20.0);
15
16 cout
<< "the price of
" <<
product1.first <<
" is $"
<< product1.second
<< "\n";
17 cout
<< "the price of
" <<
product2.first <<
" is $"
<< product2.second
<< "\n";
18 cout
<< "the price of
" <<
product3.first <<
" is $"
<< product3.second
<< "\n";
19 return
0;
20 }
其运行结果如下：
1 the price of tomatoes
is $3.25
2 the price of lightbulbs
is $0.99
3 the price of shoes
is $20
is $3.25
2 the price of lightbulbs
is $0.99
3 the price of shoes
is $20
为了更好的了解pair与make_pair的机制，下面是其定义：
1 //
template struct pair
2 template<class
_ty1,class
_ty2> struct
pair
3 {
// store a pair of values
4 typedef pair<_ty1, _ty2>
_myt;
5 typedef _ty1 first_type;
6 typedef _ty2 second_type;
7
8 pair(): first(_ty1()), second(_ty2())
9 {
// construct from defaults
10 }
11
12 pair(const
_ty1&
_val1, const
_ty2& _val2): first(_val1), second(_val2)
13 {
// construct from specified values
14 }
15
16 template<class
_other1,
17 class
_other2>
18 pair(const
pair<_other1, _other2>&
_right)
19 : first(_right.first), second(_right.second)
20 {
// construct from compatible pair
21 }
22
23 void
swap(_myt&
_right)
24 {
// exchange contents with _right
25 std::swap(first, _right.first);
26 std::swap(second, _right.second);
27 }
28
29 _ty1 first;
// the first stored value
30 _ty2 second;
// the second stored value
31 };
32
33
34 template<class
_ty1,class
_ty2> inline
35 pair<_ty1, _ty2>
make_pair(_ty1 _val1, _ty2 _val2)
36 {
// return pair composed from arguments
37 return
(pair<_ty1, _ty2>(_val1, _val2));
38 }
template struct pair
2 template<class
_ty1,class
_ty2> struct
pair
3 {
// store a pair of values
4 typedef pair<_ty1, _ty2>
_myt;
5 typedef _ty1 first_type;
6 typedef _ty2 second_type;
7
8 pair(): first(_ty1()), second(_ty2())
9 {
// construct from defaults
10 }
11
12 pair(const
_ty1&
_val1, const
_ty2& _val2): first(_val1), second(_val2)
13 {
// construct from specified values
14 }
15
16 template<class
_other1,
17 class
_other2>
18 pair(const
pair<_other1, _other2>&
_right)
19 : first(_right.first), second(_right.second)
20 {
// construct from compatible pair
21 }
22
23 void
swap(_myt&
_right)
24 {
// exchange contents with _right
25 std::swap(first, _right.first);
26 std::swap(second, _right.second);
27 }
28
29 _ty1 first;
// the first stored value
30 _ty2 second;
// the second stored value
31 };
32
33
34 template<class
_ty1,class
_ty2> inline
35 pair<_ty1, _ty2>
make_pair(_ty1 _val1, _ty2 _val2)
36 {
// return pair composed from arguments
37 return
(pair<_ty1, _ty2>(_val1, _val2));
38 }