[已解决]c++ 转换运算符问题

一减一 · 发表于 2023-1-22 21:59:22

我想能直接输出类，但是，如果返回const char* 可以运行，但是返回string就报错
所以cout 输出字符串的时候是把字符串看做char*插入到流中的吗，string不行吗

#include <iostream>
#include <string>
#include <sstream>

using namespace std;

class Date
{
    int day,month,year;
    string s;
public:
    Date()
    {

    }
    Date(int pday,int pmonth,int pyear)
    {
        day = pday;
        month = pmonth;
        year = pyear;
    }
    Date(Date& d)
    {
        day = d.day;
        month = d.month;
        year = d.year;
    }
    ~Date()
    {

    }
    /*operator string()
    {
        ostringstream p;
        p << year << "," << month << "," << day;
        s = p.str();
        return s;
    }*/
    operator const char*()
    {
        ostringstream p;
        p << year << "," << month << "," << day;
        return p.str().c_str();
    }
};

int main()
{
    Date d(29,2,2004);

    cout << d << endl;

    return 0;
}

最佳答案

月排行榜 / 总排行榜

人造人

2023-1-22 21:59:23

#include <iostream>
#include <string>
#include <sstream>

using namespace std;

class A {};

ostream &operator<<(ostream &os, const A &a) {
    return os;
}

class Date {
    int day, month, year;
    string s;
public:
    Date(int pday, int pmonth, int pyear) {
        day = pday;
        month = pmonth;
        year = pyear;
    }
    Date(Date &d) {
        day = d.day;
        month = d.month;
        year = d.year;
    }
    /*
    operator const string() {
        ostringstream p;
        p << year << "," << month << "," << day;
        s = p.str();
        return s;
    }
    */
    /*
    operator const char* ()
    {
        ostringstream p;
        p << year << "," << month << "," << day;
        return p.str().c_str();
    }
    */
    operator const A() {
        return A();
    }
};

int main() {
    Date d(29, 2, 2004);
    cout << d << endl;
    return 0;
}

上面这个代码可以通过编译，因为有这两个
operator const A();
ostream &operator<<(ostream &os, const A &a);

#include <iostream>
#include <string>
#include <sstream>

using namespace std;

class A {};

ostream &operator<<(ostream &os, const A &a) {
    return os;
}

class Date {
    int day, month, year;
    string s;
public:
    Date(int pday, int pmonth, int pyear) {
        day = pday;
        month = pmonth;
        year = pyear;
    }
    Date(Date &d) {
        day = d.day;
        month = d.month;
        year = d.year;
    }
    operator const string() {
        ostringstream p;
        p << year << "," << month << "," << day;
        s = p.str();
        return s;
    }
    /*
    operator const char* ()
    {
        ostringstream p;
        p << year << "," << month << "," << day;
        return p.str().c_str();
    }
    */
    /*
    operator const A() {
        return A();
    }
    */
};

ostream &operator<<(ostream &os, const string &s) {
    return os;
}

int main() {
    Date d(29, 2, 2004);
    cout << d << endl;
    return 0;
}

上面这个代码也可以通过编译
因为这两个
ostream &operator<<(ostream &os, const string &s);
operator const string();

#include <iostream>
#include <string>
#include <sstream>

using namespace std;

class A {};

ostream &operator<<(ostream &os, const A &a) {
    return os;
}

class Date {
    int day, month, year;
    string s;
public:
    Date(int pday, int pmonth, int pyear) {
        day = pday;
        month = pmonth;
        year = pyear;
    }
    Date(Date &d) {
        day = d.day;
        month = d.month;
        year = d.year;
    }
    operator const string() {
        ostringstream p;
        p << year << "," << month << "," << day;
        s = p.str();
        return s;
    }
    /*
    operator const char* ()
    {
        ostringstream p;
        p << year << "," << month << "," << day;
        return p.str().c_str();
    }
    */
    /*
    operator const A() {
        return A();
    }
    */
};

int main() {
    Date d(29, 2, 2004);
    cout << d << endl;
    return 0;
}

上面这段代码就无法通过编译了，因为没有这个
ostream &operator<<(ostream &os, const string &s);

跳转到最佳答案楼层

人造人 · 发表于 2023-1-22 21:59:23

#include <iostream>
#include <string>
#include <sstream>

using namespace std;

class A {};

ostream &operator<<(ostream &os, const A &a) {
    return os;
}

class Date {
    int day, month, year;
    string s;
public:
    Date(int pday, int pmonth, int pyear) {
        day = pday;
        month = pmonth;
        year = pyear;
    }
    Date(Date &d) {
        day = d.day;
        month = d.month;
        year = d.year;
    }
    /*
    operator const string() {
        ostringstream p;
        p << year << "," << month << "," << day;
        s = p.str();
        return s;
    }
    */
    /*
    operator const char* ()
    {
        ostringstream p;
        p << year << "," << month << "," << day;
        return p.str().c_str();
    }
    */
    operator const A() {
        return A();
    }
};

int main() {
    Date d(29, 2, 2004);
    cout << d << endl;
    return 0;
}

上面这个代码可以通过编译，因为有这两个
operator const A();
ostream &operator<<(ostream &os, const A &a);

#include <iostream>
#include <string>
#include <sstream>

using namespace std;

class A {};

ostream &operator<<(ostream &os, const A &a) {
    return os;
}

class Date {
    int day, month, year;
    string s;
public:
    Date(int pday, int pmonth, int pyear) {
        day = pday;
        month = pmonth;
        year = pyear;
    }
    Date(Date &d) {
        day = d.day;
        month = d.month;
        year = d.year;
    }
    operator const string() {
        ostringstream p;
        p << year << "," << month << "," << day;
        s = p.str();
        return s;
    }
    /*
    operator const char* ()
    {
        ostringstream p;
        p << year << "," << month << "," << day;
        return p.str().c_str();
    }
    */
    /*
    operator const A() {
        return A();
    }
    */
};

ostream &operator<<(ostream &os, const string &s) {
    return os;
}

int main() {
    Date d(29, 2, 2004);
    cout << d << endl;
    return 0;
}

上面这个代码也可以通过编译
因为这两个
ostream &operator<<(ostream &os, const string &s);
operator const string();

#include <iostream>
#include <string>
#include <sstream>

using namespace std;

class A {};

ostream &operator<<(ostream &os, const A &a) {
    return os;
}

class Date {
    int day, month, year;
    string s;
public:
    Date(int pday, int pmonth, int pyear) {
        day = pday;
        month = pmonth;
        year = pyear;
    }
    Date(Date &d) {
        day = d.day;
        month = d.month;
        year = d.year;
    }
    operator const string() {
        ostringstream p;
        p << year << "," << month << "," << day;
        s = p.str();
        return s;
    }
    /*
    operator const char* ()
    {
        ostringstream p;
        p << year << "," << month << "," << day;
        return p.str().c_str();
    }
    */
    /*
    operator const A() {
        return A();
    }
    */
};

int main() {
    Date d(29, 2, 2004);
    cout << d << endl;
    return 0;
}

上面这段代码就无法通过编译了，因为没有这个
ostream &operator<<(ostream &os, const string &s);

人造人 · 发表于 2023-1-22 22:35:57

#include <iostream>
#include <string>
#include <sstream>

using namespace std;

class Date
{
    int day,month,year;
    string s;
public:
    Date()
    {

    }
    Date(int pday,int pmonth,int pyear)
    {
        day = pday;
        month = pmonth;
        year = pyear;
    }
    Date(Date& d)
    {
        day = d.day;
        month = d.month;
        year = d.year;
    }
    ~Date()
    {

    }
    operator string()
    {
        ostringstream p;
        p << year << "," << month << "," << day;
        s = p.str();
        return s;
    }
    /*
    operator const char*()
    {
        ostringstream p;
        p << year << "," << month << "," << day;
        return p.str().c_str();
    }
    */
};

int main()
{
    Date d(29,2,2004);

    cout << (string)d << endl;

    return 0;
}

一减一 · 发表于 2023-1-22 23:01:27

人造人发表于 2023-1-22 22:35

我想问一下为啥char*不用这样写，string要这样写

dolly_yos2 · 发表于 2023-1-23 10:03:28

不是回答问题，只是好奇为什么不重载运算符（<<）而是自定义类型转换来实现这个效果？

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