C ++中编译类时出现错误

Question

I tried to create a class for 3-D vector in c++, but there are some errors. I have learnt a little bit of oop in python before, but I'm still pretty new to oop and c++. I created the header file threevector.h, the file for the class threevector.cpp and the main program file main.cpp. I just want to know what I have done wrong.

// threevector.h
#ifndef THREEVECTOR_H
#define THREEVECTOR_H
#include <iostream>

class threevector {
    private:
        double xcoord, ycoord, zcoord;

    public:
        threevector();
        threevector(double x, double y, double z, char type);
        void print ();      
};
#endif // THREEVECTOR_H


//threevector.cpp
#include "threevector.h"
#include <cmath>

threevector() {
    xcoord = 0.0;
    ycoord = 0.0;
    zcoord = 0.0;
}

threevector(double x, double y, double z, char type) {
    if (type == 'c') {
        // cartesian coordinate
        xcoord = x;
        ycoord = y;
        zcoord = z;
    } 
    else if (type == 'p') {
        // polar coordinate
        // x = r, y = phi, z = theta
        xcoord = x*sin(y)*cos(z);
        ycoord = x*sin(y)*sin(z);
        zcoord = x*cos(y);
        }
}

void print () {
    std::cout << xcoord << '\t' << ycoord << '\t' << zcoord << std::endl;
}


// main.cpp
#include "threevector.h"
#include <cmath>
#include <iostream>

using namespace std;

int main() {
    threevector v0;
    v0.print();

    threevector v1(-1,2,4.384,'c');
    cout << "v1 = ";
    v1.print();

    return 0;
}

The following are the error messages I get:

main.cpp(.text+0x15): undefined reference to 'threevector::threevector()'
main.cpp(.text+0x15): undefined reference to 'threevector::print()'
main.cpp(.text+0x15): undefined reference to 'threevector::threevector::threevector(double, double, double, char)'
main.cpp(.text+0x15): undefined reference to 'threevector::print()'
[Error] ld returned 1 exit status

Answer 1

You defined your threevector 's methods wrong. It should be:

threevector::threevector() {
    xcoord = 0.0;
    ycoord = 0.0;
    zcoord = 0.0;
}

threevector::threevector(double x, double y, double z, char type) {
    if (type == 'c') {
        // cartesian coordinate
        xcoord = x;
        ycoord = y;
        zcoord = z;
    } 
    else if (type == 'p') {
        // polar coordinate
        // x = r, y = phi, z = theta
        xcoord = x*sin(y)*cos(z);
        ycoord = x*sin(y)*sin(z);
        zcoord = x*cos(y);
        }
}

void threevector::print () {
    std::cout << xcoord << '\t' << ycoord << '\t' << zcoord << std::endl;
}

All threevector 's methods are in threevector 's scope.

Don't forget to compile it as same as main.cpp :

g++ threevector.cpp main.cpp

This will be linked properly as threevector is in same object file.

Answer 2

You have not defined your member function, you only declared them.

If you look at your class definition again:

 class threevector { private: double xcoord, ycoord, zcoord; public: threevector(); threevector(double x, double y, double z, char type); void print (); }; 

It contains declarations for three member functions:

• threevector::threevector()
• threevector::threevector(double, double, double, char)
• threevector::print()

The threevector:: part is important - since these are member functions, the class name is part of their own name (or so to speak).

The functions you defined, however, miss the threevector:: part. For example, you define a function called print() , but the name is just a coincidence.

threevector::print() and print() are different names and thus different functions.

Same with the two constructors.

The solution is therefore to use the correct, full names when you define the functions:

threevector::threevector() {
    xcoord = 0.0;
    ycoord = 0.0;
    zcoord = 0.0;
}

threevector::threevector(double x, double y, double z, char type) {
    if (type == 'c') {
        // cartesian coordinate
        xcoord = x;
        ycoord = y;
        zcoord = z;
    } 
    else if (type == 'p') {
        // polar coordinate
        // x = r, y = phi, z = theta
        xcoord = x*sin(y)*cos(z);
        ycoord = x*sin(y)*sin(z);
        zcoord = x*cos(y);
        }
}

void threevector::print () {
    std::cout << xcoord << '\t' << ycoord << '\t' << zcoord << std::endl;
}

While we're at it, constructors should use initialisation lists, not assignment, at least if that's easily possible:

threevector::threevector() :
    xcoord(0.0),
    ycoord(0.0),
    zcoord(0.0)
{
}

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PS: Do not confuse this with namespaces. All of this has nothing to do with namespaces.