How to import a class in another C++ header file?

Question

I meet a problem when constructing a class. class "Graph" import a class "Bag" in another file and use "Bag" as its component.

//Graph.h
#ifndef GRAPH_H
#define GRAPH_H
#include <fstream>
#include <iostream>
#include <vector>
#include "Bag.h"
class Bag;
class Graph
{
public:
    Graph(int V);
    Graph(std::ifstream& in_file);
    int getV() { return V; }
    int getE() { return E; }
    void addEdge(int v, int w);
    void showadj() ;
private:
    int V;
    int E;
    std::vector<Bag> adj;
};
#endif

And "Bag.h" is as follow:

//Bag.h
#ifndef BAG_H
#define BAG_H
#include <vector>
#include <iostream>
class Bag
{
public: 
    Bag();
    void addBag(int i) { content.push_back(i); }
    void showBag();
private:
    std::vector<int> content;
};

#endif

Graph.cpp:

//Graph.cpp
#include "Graph.h"
#include "Bag.h"
Graph::Graph(int V) : V(V), E(0)
{
    for (int i = 0; i < V; i++)
    {
        Bag bag;
        adj.push_back(bag);
    }
}

Bag.cpp(sorry, forget it):

#include "Bag.h"
void Bag::showBag()
{
    for (int i : content)
    {
        std::cout << i << " ";
    }
}

When I try to complie these two classes, an error comes out saying:

C:\Users\ADMINI~1\AppData\Local\Temp\ccMj4Ybn.o:newtest.cpp:(.text+0x1a2): undef
ined reference to `Bag::Bag()'
collect2.exe: error: ld returned 1 exit status

You need to build `Bag.cpp` into `Bag.o` and link it with `Graph.o` — StoryTeller - Unslander Monica, Dec 15 '16 at 13:11
would it work if you just remove `class Bag;` from `Graph.h` and remove `#include "Bag.h"` from `Graph.cpp` (because `Graph.h` already includes `Bag.h`)? — Aemyl, Dec 15 '16 at 13:16
are you building that from command line ? What command are you using ? — Jean-Emmanuel, Dec 15 '16 at 13:18
see answer *Declared but did not define a variable or function* — NathanOliver, Dec 15 '16 at 13:31

score 4 · Answer 1 · answered Dec 15 '16 at 13:19

You need to also implement your constructor Bag::Bag() as it is missing in your Bag.cpp file.

This is what the error tells you. If you don't need the constructor, then you should remove it from your class definition, which would also solve this error.

An alternative would be to provide an empty constructor in your Bag.h file

class Bag
{
public: 
    Bag() {}
...
}

How to import a class in another C++ header file?

1 Answers1