使用cout中的链接时C ++ ostringstream奇怪的行为

Question

I am a C++ beginner ( came from Java ). I have the following code:

//#include <boost/algorithm/string.hpp>
#include <iostream>
#include <math.h>
#include <vector>
#include <string.h>
#include <string>
#include <bitset>
#include <algorithm>
#include <sstream>
#include <memory>
#include <assert.h>
#include <cctype>

using namespace std;

class Point{
private:
    int x;
    int y;
public:
    Point(int x,int y){
        this->x=x;
        this->y=y;
    }

    int getX(){
        return x;
    }

    int getY(){
        return y;
    }

    operator const char*(){
        return toString().c_str();
    }

    string toString(){
        ostringstream stream;
        stream<<"( "<<x<<", "<<y<<" )";
        return stream.str();
    }
};


class Line{
private:
    Point p1=Point(0,0);
    Point p2=Point(0,0);

public:
    Line(Point p1, Point p2){
        this->p1=p1;
        this->p2=p2;
    }

    Point getP1(){
        return p1;
    }

    Point getP2(){
        return p2;
    }

    operator const char*(){
        ostringstream stream;
        stream<<"[ "<<p1<<" -> "<<p2<<" ]";
        return stream.str().c_str();
    }

    //    operator const char*(){
    //        ostringstream stream;
    //        stream<<"[ "<<p1<<" -> ";
    //        stream<<p2<<" ]";
    //        return stream.str().c_str();
    //    }
};

int main()
{

    Line line=Line(Point(1,2), Point(3,4));
    cout<<line<<endl;


    cout<<"\nProgram exited successfully."<<endl;
    return 0;
}

I have redefined the operator const* so that I can use cout<

But, If I run the program as it is now, with the second block commented out ( I have 2 versions of operator const*, and by default the second one is commented out ) ,it will display

[ (1, 2) -> (1, 2) ]

But when running with the second block uncommented, the output is as expected:

[ (1, 2) -> (3, 4) ]

The issue seems to occur when I display both Point objects in the same line ( some kind of chaining, though I don't know if chaining is the right word here )

My question, is,why is this happening?

UPDATE

I have added the std::ostream& operator << function to my Line class but now I'm receiving the following errors:

/home/ryu/qt_workspace/hello/main.cpp:67: error: 'std::ostream& Line::operator<<(std::ostream&, const Line&)' must take exactly one argument

/home/ryu/qt_workspace/hello/main.cpp:77: error: cannot bind 'std::ostream {aka std::basic_ostream<char>}' lvalue to 'std::basic_ostream<char>&&'

Regards, Aurelian

Answer 1

If you want to use cout << , there is a more direct way to do that.

Add this function to Line .

friend std::ostream& operator << ( std::ostream & os, const Line & l ){
    os << "[ " << l.p1 << " -> " << l.p2 << " ]";
    return os;
}

You should also note that your approach was returning invalid memory - this is a significant way that Java differs from C++.

    return stream.str().c_str();  // Danger!

stream was declared in operator const char*() which limits its lifetime to that function. It is destroyed when that scope is exited. As a result, you are returning a pointer to something that no longer exists.

Answer 2

actually I think with C++11 returning the string by value is perfectly fine, so you can do the transfer there instead of using the cstring underneath.

What are move semantics?