C++进阶--this指针-优快云博客

本文链接：https://blog.youkuaiyun.com/weixin_42237429/article/details/129730457

文章通过一个Stock类的例子展示了C++中this指针的作用。每个成员函数内部，this指针指向调用该函数的对象。在Stock类的成员函数topval中，this用于区分调用对象和传入的参数对象，根据total_val值比较并返回最大的对象引用。程序创建并初始化了多个Stock对象，演示了如何使用this指针进行对象间的比较和操作。

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参考资料--《C++ Primer Plus 第六版》

this指针：每个成员函数都有this指针，this指针表示调用该成员函数的对象；

单独使用this：表示调用this的对象的地址；

加上＊的this：表示整个调用对象；

示例程序：

// stock20.cpp -- augmented version
#include <iostream>
#include "stock20.h"
using namespace std;
// constructors
Stock::Stock()        // default constructor
{
    shares = 0;
    share_val = 0.0;
    total_val = 0.0;
}

Stock::Stock(const std::string & co, long n, double pr)
{
    company = co;
    if (n < 0)
    {
        std::cout << "Number of shares can't be negative; "
                   << company << " shares set to 0.\n";
        shares = 0;
    }
    else
        shares = n;
    share_val = pr;
    set_tot();
}

// class destructor
Stock::~Stock()        // quiet class destructor
{
}

// other methods
void Stock::buy(long num, double price)
{
     if (num < 0)
    {
        std::cout << "Number of shares purchased can't be negative. "
             << "Transaction is aborted.\n";
    }
    else
    {
        shares += num;
        share_val = price;
        set_tot();
    }
}

void Stock::sell(long num, double price)
{
    using std::cout;
    if (num < 0)
    {
        cout << "Number of shares sold can't be negative. "
             << "Transaction is aborted.\n";
    }
    else if (num > shares)
    {
        cout << "You can't sell more than you have! "
             << "Transaction is aborted.\n";
    }
    else
    {
        shares -= num;
        share_val = price;
        set_tot();
    }
}

void Stock::update(double price)
{
    share_val = price;
    set_tot();
}

void Stock::show() const
{
    using std::cout;
    using std::ios_base;
    // set format to #.###
    ios_base::fmtflags orig = 
        cout.setf(ios_base::fixed, ios_base::floatfield); 
    std::streamsize prec = cout.precision(3);

    cout << "Company: " << company
        << "  Shares: " << shares << '\n';
    cout << "  Share Price: $" << share_val;
    // set format to #.##
    cout.precision(2);
    cout << "  Total Worth: $" << total_val << '\n';

    // restore original format
    cout.setf(orig, ios_base::floatfield);
    cout.precision(prec);
}

const Stock & Stock::topval(const Stock & s) const
{
    if (s.total_val > total_val)
        return s;
    else
        return *this; 
}

这里的成员函数 Stock::topval 用来比较两个对象的 total_val 值大小；

其中对象s作为成员函数参数；

s.total_val是对象s成员变量；total_val是调用对象成员变量；

返回值 *this 返回整个调用对象本身；

// stock20.h -- augmented version
#ifndef STOCK20_H_
#define STOCK20_H_
#include <string>

class Stock
{
private:
    std::string company;
    int shares;
    double share_val;
    double total_val;
    void set_tot() { total_val = shares * share_val; }
public:
    Stock();        // default constructor
    Stock(const std::string & co, long n = 0, double pr = 0.0);
    ~Stock();       // do-nothing destructor
    void buy(long num, double price);
    void sell(long num, double price);
    void update(double price);
    void show()const;
    const Stock & topval(const Stock & s) const;
};

#endif

// usestok2.cpp -- using the Stock class
// compile with stock20.cpp
#include <iostream>
#include "stock20.h"

const int STKS = 4;
int main()
{{
// create an array of initialized objects
    Stock stocks[STKS] = {
        Stock("NanoSmart", 12, 20.0),
        Stock("Boffo Objects", 200, 2.0),
        Stock("Monolithic Obelisks", 130, 3.25),
        Stock("Fleep Enterprises", 60, 6.5)
        };

    std::cout << "Stock holdings:\n";
    int st;
    for (st = 0; st < STKS; st++)
        stocks[st].show();
// set pointer to first element
    const Stock * top = &stocks[0];
    for (st = 1; st < STKS; st++)
        top = &top->topval(stocks[st]);
// now top points to the most valuable holding
    std::cout << "\nMost valuable holding:\n";
    top->show();}
    // std::cin.get();
    return 0; 
}

程序初始化4个对象，并分别显示；然后利用for循环比较4个对象的成员变量topval最大值；top->topval(stocks[st]) 表示两个对象的比较，并利用this指针返回topval最大对应的对象，然后再显示；

编译：

 g++ usestok2.cpp stock20.cpp

运行输出：

Stock holdings:
Company: NanoSmart  Shares: 12
  Share Price: $20.000  Total Worth: $240.00
Company: Boffo Objects  Shares: 200
  Share Price: $2.000  Total Worth: $400.00
Company: Monolithic Obelisks  Shares: 130
  Share Price: $3.250  Total Worth: $422.50
Company: Fleep Enterprises  Shares: 60
  Share Price: $6.500  Total Worth: $390.00

Most valuable holding:
Company: Monolithic Obelisks  Shares: 130
  Share Price: $3.250  Total Worth: $422.50

总结：

当成员函数只涉及一个对象时，可以不用this指针；

当成员函数涉及两个以上对象时，需要使用this指针；

比如 Stock::topval函数需要返回两个对象的某一个，就需要this区分；