How to prevent Redefinition of class?

Question

I have been implementing a custom Vector from one of the latest C++ books and I am coming upon an error in the vector.cpp and vector.h file at the Class Vector and Vector::Vector respectively: Redefinition of 'Vector'

Here is the code for each file:

main.cpp:

#include <iostream>
#include "Vector.h"
#include "Container.h"

int main(int argc, const char * argv[]) {
    return 0;
}

vector.h

class Vector {
public:
    Vector(int s) :elem{new double[s]}, sz{s}     // constructor: acquire resources
    {
        for (int i=0; i!=s; ++i)      // initialize elements
            elem[i]=0;
            }

    ~Vector() { delete[] elem; }                  // destructor: release resources

    double& operator[](int i);
    int size() const;
private:
    double* elem;     // elem points to an array of sz doubles
    int sz;
};

vector.cpp

Vector::Vector(int s) // definition of the constructor

:elem{new double[s]}, sz{s} // initialize members
{}
double& Vector::operator[](int i) // definition of subscripting
{
    return elem[i];
}

int Vector::size() const // definition of size()
{
    return sz;
}

container.cpp

class Container {
public:
     virtual double& operator[](int) = 0;     // pure virtual function
     virtual int size() const = 0;            // const member function (§4.2.1)
     virtual ~Container() {}                  // destructor (§4.2.2)
};


class Vector_container : public Container {   // Vector_container implements Container
public:
     Vector_container(int s) : v(s) { }   // Vector of s elements
     ~Vector_container() {}

     double& operator[](int i) override { return v[i]; }
     int size() const override { return v.size(); }
private:
     Vector v;
};

I tried enclosing the .h file with #ifndef Vector_h,#define Vector_h, and #endif but it the error still appears in Vector_cpp. If I encapsulate Vector.cpp with #ifndef Vector_cpp, #define Vector_h, and #endif the error still appears.

To add insult to injury, when I add Vector.h to container.cpp it tells me: Unknown type name 'Vector' at the bottom where I declared Vector v

Can anyone help me with this?

You do need the #ifndef's for sure, in each header file. Where is your Container.h? — ChrisMM
You implemented the Vector::Vector(int s) constructor in both the header and the .cpp file. Choose one. If your choice is the .cpp file, then strip the body and member initialization list of Vector(int s)` from the header, leaving only the decl in the header; just like you did with the size() member. — WhozCraig
You're defining the ctor both in the header and in the source file. Delete the definition in the header. — o11c

gsamaras gsamaras · Accepted Answer · 2019-12-07T20:15:00

You need to implement a header file for your Container class as well, so you need a Container.h too (note this should match the name of the file). In your case, it seems that it would suffice to just rename your file, from Container.cpp, to Container.h.

Moreover, you should implement your methods (e.g. the constructor of Vector) into only one file, not two. For example, you would declare the method in the header file, and then define it in the source file.

Putting everything together, you need:

main.cpp (no changes here)
Container.h (and not Container.cpp, but no changes in the content of the file, you did a good work! :))

Vector.h

class Vector {
public:
    Vector(int s);

    ~Vector();
    double& operator[](int i);
    int size() const;
private:
    double* elem;     // elem points to an array of sz doubles
    int sz;
};

Vector.cpp

#include "Vector.h"

Vector::Vector(int s) :elem{new double[s]}, sz{s} // initialize members
{}
double& Vector::operator[](int i) // definition of subscripting
{
    return elem[i];
}

int Vector::size() const // definition of size()
{
    return sz;
}

Vector::~Vector() { delete[] elem; }

Compile:

g++ main.cpp Vector.cpp

How to prevent Redefinition of class?

2 Answers