19.11    指向数据成员的指针与普通数据指针的区别：不仅要给出成员本身的类型，还需要给出所属类的类型。

19.12    

//定义成员指针，指向Screen类的cursor成员
Screen::pos Screen::*pm = &Screen::cursor;
//使用该指针获得Screen::cursor的值
Screen myscreen;
auto val = myscreen.*pm;
Screen *ps = &myscreen;
val = ps->*pm;

19.13    

std::string Sales_data::*pdata;

19.14    auto pmf = &Screen::get_cursor;    //合法，指向Screen的get_cursor成员函数的指针
             pmf = &Screen::get；    //合法，指向Screen中无参数版本的get成员函数

19.15    指向成员函数的指针，要给出成员函数本身的类型，还需要给出成员函数所属类的类型，并指明成员函数是否为const。

19.16    

using Avg = double (Sales_data::*)() const;
Avg pavg = &Sales_data::avg_price;

19.17    

using pmf1 = char (Screen::*)() const;
using pmf2 = char (Screen::*)(Screen::index, Screen::index) const;
using pmf3 = Screen& (Screen::*)();
using pmf4 = Screen& (Screen::*)(Screen::Directions);

19.18    用mem_fn实现：

#include <iostream>
#include <vector>
#include <algorithm>
#include <functional>

using namespace std;

unsigned int cnt(const vector<string> &svec)
{
	unsigned int ret = count_if(svec.begin(), svec.end(), mem_fn(&string::empty));
	
	return ret;
}

int main()
{
	vector<string> svec = {"s1", "", "s3", ""};
	auto c = cnt(svec);
	cout << c << endl;
	
	return 0;
}

19.19    需要先把Sales_data类的avg_price()成员函数改为public的

#include <iostream>
#include <vector>
#include <algorithm>
#include "Sales_data.h"

using namespace std;
using std::placeholders::_1;

bool high_price(Sales_data &item, double price)
{
	return item.avg_price() > price;
}

vector<Sales_data>::iterator first_high_price(vector<Sales_data> &sv, double price)
{
	auto f = bind(high_price, _1, price);
	
	return find_if(sv.begin(), sv.end(), f);
}

int main()
{
	vector<Sales_data> svec;
	Sales_data record;
	
	while (cin >> record) {
		svec.push_back(record);
	}
	
	auto iter = first_high_price(svec, 10);
	
	cout << *iter << endl;
	
	return 0;
}