C++ pair的定义及使用

 

声明为pair类型的变量可以有三种赋值方法：

1. 通过初始化赋值 直接声明的时候 后面加括号并且数据，如下a
2. 通过.first  .second 来赋值 如下b
3. 通过 = make_pair() 来赋值， 如下c

  a. pair <string,double> product1 ("tomatoes",3.25);
  pair <string,double> product2;
  pair <string,double> product3;

　b.  product2.first = "lightbulbs"; // type of first is string 
　　　 product2.second = 0.99; // type of second is double


　c. product3 = make_pair ("shoes",20.0);


template <class T1, class T2> struct pair;

Pair of values

This class couples together a pair of values, which may be of different types (T1 and T2). The individual values can be accessed through the public members first and second.

The class is defined as:

1 2 3 4 5 6 7 8 9 10 11 12

template <class T1, class T2> struct pair { typedef T1 first_type; typedef T2 second_type; T1 first; T2 second; pair() : first(T1()), second(T2()) {} pair(const T1& x, const T2& y) : first(x), second(y) {} template <class U, class V> pair (const pair<U,V> &p) : first(p.first), second(p.second) { } }

Members

first_type, second_type

Alises of template parameters T1 and T2 respectively.

first, second

Data members containing the first and second values stored in the pair.

pair()

Constructs a pair object with each of its members first and second constructed with their respective default constructors.

pair(const T1& x, const T2& y)

Constructs a pair object with its members first and second initialized to x and y, respectively.

template <class U, class V> pair (const pair<U,V> &p)

Constructs a pair object with its members first and second initialized to the corresponding elements in p, which must be of any couple of implicitly-convertible types (including the same types).

Example

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

#include <iostream> #include <utility> #include <string> using namespace std; int main () { pair <string,double> product1 ("tomatoes",3.25); pair <string,double> product2; pair <string,double> product3; product2.first = "lightbulbs"; // type of first is string product2.second = 0.99; // type of second is double product3 = make_pair ("shoes",20.0); cout << "The price of " << product1.first << " is $" << product1.second << "\n"; cout << "The price of " << product2.first << " is $" << product2.second << "\n"; cout << "The price of " << product3.first << " is $" << product3.second << "\n"; return 0; }

Output:

The price of tomatoes is $3.25 The price of lightbulbs is $0.99 The price of shoes is $20