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//

// Created by light on 19-11-3.

//

#include <iostream>

#include <map>

#include <set>

using namespace std;

// 1.used to declare return tyoes 返回类型

// c++2.0之前编译不出来

// template<typename T1, typename T2>

// decltype(x + y) add(T1 x, T2 y);

// 编译器编不出来在2.0之后变成下面这个,先用auto暂定,后面返回类型为decltype(x+y)

template <typename T1, typename T2>

auto add(T1 x, T2 y) -> decltype(x + y)

{ // -> 也用在lambda

return x + y;

}

class Person

{

public:

string firstname;

string lastname;

};

int main()

{

// 1.used to declare return tyoes

cout << add(1, 2) << endl;

// 2.模板元编程 例如在一个模板函数或类获取容器的value_type,这里就不封装了,直接写在main函数里面

// 获得表达式的type 有点像typeof()特点

map<string, float> coll;

// 获取上述类型

decltype(coll)::value_type m{"as", 1}; // value_type为pair<string,int> m

cout << m.first << " " << m.second << endl;

pair<string, int> p{"a", 2};

cout << p.first << " " << p.second << endl;

// 3.used to pass the type of a lambda

// 比大小

auto cmp = [](const Person &p1, const Person &p2)

{

return p1.lastname < p2.lastname;

};

// 对于lambda,我们往往只有object,很少有人能够写出它的类型，而有时就需要知道它的类型,要获得其type,就要借助其decltype

set<Person, decltype(cmp)> col(cmp);

return 0;

}