I have a very simple program that doesn't compile due to multiple definition error. It is here:
main.cpp
#include <iostream>
#include "read_p.h"
using namespace std;
int main()
{
return 0;
}
read_p.cpp
#include "read_p.h"
using namespace std;
void read_p()
{
/*
some code here
*/
}
read_p.h
#ifndef READ_P_H
#define READ_P_H
#include "buildings.h"
void read_p();
#endif
buildings.h
#ifndef BUILDINGS_H
#define BUILDINGS_H
#include "flag.h"
using namespace std;
/*
some class here
*/
#endif
flag.h
#ifndef FLAG_H
#define FLAG_H
using namespace std;
class
Test
{
private:
public:
int test_var;
Test(int);
};
Test::Test(int a)
{
test_var = a;
}
#endif
The compiler gives me the error that the constructor Test::Test is defined multiple times. Unlike questions I find online, this error is not due to including the cpp-file instead of the h-file.
Question: Where does the multiple definition of the constructor occur? And is the proper way to circumvent the issue by making the constructur inline?
Change
Test(int);
to
inline Test(int);
Even better, fix your class definition to define member functions inline, which makes them implicitly inline:
class Test
{
public:
int test_var;
Test(int a) : test_var(a) {}
};
Otherwise, as always, defining a function in a header means that it gets defined in every translation unit that includes that header, which leads to multiple definitions.
Related
I´m building a program with several classes (data structures like stacks, lists,etc).
There is some class (Concesionario) that i need to use in another (ListaE). The class ListaE uses another class called NodoListaE, which uses two pointers, one to the value of the object (Concesionario) and another to the next position of the list (siguiente).
#ifndef NODOLISTAE_HPP
#define NODOLISTAE_HPP
#include "Concesionario.hpp"
class Concesionario;
class ListaE;
class NodoListaE
{
public:
NodoListaE(Concesionario* conc, NodoListaE* sig = NULL);
private:
Concesionario* conc;
NodoListaE* siguiente;
friend class ListaE;
};
typedef NodoListaE* pnodoListaE;
#endif // NODOLISTAE_HPP
#ifndef LISTAE_HPP
#define LISTAE_HPP
#include "NodoListaE.hpp"
#include "Automovil.hpp"
class Automovil;
class NodoListaE;
class ListaE
{
private:
NodoListaE* primero;
public:
ListaE();
void enlistarOrden(Automovil* automovil);
};
#endif // LISTAE_HPP
#ifndef CONCESIONARIO_HPP
#define CONCESIONARIO_HPP
#include <string>
#include "ListaE.hpp"
class ListaE;
class Concesionario
{
public:
Concesionario();
~Concesionario();
std::string mostrar();
void setZona(std::string letra);
void setNum();
int getNum();
private:
int nc=2;
int num_conc;
std::string zona;
int generadorNumsIntervalo(int min, int max);
ListaE automoviles;//ERROR HERE
};
#endif // CONCESIONARIO_HPP
All the cpp files are not implemented (empty constructor and destructor).
The compiler I´m currently using is MINGWx64.
I´ve tried using forward declarations and it worked for the rest of the classes but not for this one.
The program throws the following error in the **Concesionario ** hpp file: include\Concesionario.hpp|22|error: field 'automoviles' has incomplete type 'ListaE'|
Concesionario is implemented in other classes and the program runs perfectly.
Example of another class implementing Concesionario
#ifndef ARBOL_HPP
#define ARBOL_HPP
#include <iostream>
#include "NodoArbol.hpp"
#include "Concesionario.hpp"
using namespace std;
class Arbol {
public:
Arbol();
void Insertar(Concesionario* concesionario);
private:
pnodoArbol raiz;
pnodoArbol actual;
int contador;
int altura;
bool Vacio(pnodoArbol nodo);
};
#endif // ARBOL_HPP
I`ve also tried deleting this class and creating another one from 0 but the error remains.
Any solution to this problem? Thank you very much.
I made a smaller reproducible version of the code that gave me these errosr: 'MyNamespace::MySecondClass': 'class' type redefinition, 'print': is not a member of 'MyNamespace::MySecondClass'. Is there any way of working around this problem?
// MyClass.h
#pragma once
namespace MyNamespace {
class MySecondClass {};
}
// MyClass.cpp
#include "MyClass.h"
#include <iostream>
using namespace std;
class MyNamespace::MySecondClass
{
public:
void print(const char* msg)
{
cout << msg << endl;
}
};
The problem is that in MyClass.h you define a class MySecondClass as an empty class. When you the define your class in MyClass.cpp you give a different definition, which contains some new members. This infringes the One Definition Rule (ODR).
Solution 1
remove {} in the header. This will tell the compiler that you declare that such a class exists but that it will be defined later. Your code would compile. Unfortunately if you’d include the header in other cpp, these could make only a very very limited use of MySecondClass.
Solution 2
define in the header the class with all its members (but without providing the implementation of the member functions:the signature is sufficient). This would allow the class to be used in whichever cpp that
would include it:
// MyClass.h
#pragma once
namespace MyNamespace {
class MySecondClass {
public:
void print(const char* msg);
};
}
You’d then define the members of the class in its cpp in the appropriate namespace:
// MyClass.cpp
#include <iostream>
#include "MyClass.h"
using namespace std;
namespace MyNamespace {
// member functions
void MySecondClass::print(const char* msg)
{
cout << msg << endl;
}
}
Remark: the include sequence in the cpp should first include the standard library headers, then only your own headers. It makes no difference in your simple example, but better get used the good practices immediately.
I have a very simple program that doesn't compile due to multiple definition error. It is here:
main.cpp
#include <iostream>
#include "read_p.h"
using namespace std;
int main()
{
return 0;
}
read_p.cpp
#include "read_p.h"
using namespace std;
void read_p()
{
/*
some code here
*/
}
read_p.h
#ifndef READ_P_H
#define READ_P_H
#include "buildings.h"
void read_p();
#endif
buildings.h
#ifndef BUILDINGS_H
#define BUILDINGS_H
#include "flag.h"
using namespace std;
/*
some class here
*/
#endif
flag.h
#ifndef FLAG_H
#define FLAG_H
using namespace std;
class
Test
{
private:
public:
int test_var;
Test(int);
};
Test::Test(int a)
{
test_var = a;
}
#endif
The compiler gives me the error that the constructor Test::Test is defined multiple times. Unlike questions I find online, this error is not due to including the cpp-file instead of the h-file.
Question: Where does the multiple definition of the constructor occur? And is the proper way to circumvent the issue by making the constructur inline?
Change
Test(int);
to
inline Test(int);
Even better, fix your class definition to define member functions inline, which makes them implicitly inline:
class Test
{
public:
int test_var;
Test(int a) : test_var(a) {}
};
Otherwise, as always, defining a function in a header means that it gets defined in every translation unit that includes that header, which leads to multiple definitions.
Firstly, I am giving the codes. Then I am explaining the problem I am facing.
main.cpp
#include <iostream>
#include "acc.h"
using namespace std;
class mem;
int main()
{
show();
return 0;
}
acc.h
#ifndef ACC_H
#define ACC_H
#include "acc.cpp"
void show();
class mem{
int a;
public:
void showa();
void seta(int A);
};
#endif
acc.cpp
#include <iostream>
using namespace std;
void mem::showa(){cout<<a<<endl;}
void mem::seta(int A){a = A;}
void show()
{
mem m;
m.seta(22);
string ss;
cin>>ss;
cout<<"MY name is "<<ss<<" ";
m.showa();
}
"mem" class I declared in "acc.h" file already and added that "acc.h" into acc.cpp file also. But when I am calling that class from a function. It can't response. Showing "a" and "mem" not declared. How can I perfectly link that class definition and member functions of that class so that calling member functions of that class from another function can't create any problem?
If you remove the #include "acc.cpp" from the acc.h file it should compile without any errors. I tried and it compiles for me. I am using Visual Studio 2010 for the same.
Other than this, few more comments:
You can use #pragma once in you header file instead of #ifndef/#define macros. The former is more cleaner.
You dont need to forward declare class mem before main() as you are already including acc.h.
the show() can be moved to where main() is defined making the acc.h/acc.cppfiles dedicated for the mem class.
A header file should always be named after the class it is holding i.e. mem.h/mem.cpp in your case. This informs which file contains which class even without opening the file.
I've been trying to define an error class in C++. I have three files; main.cpp, my_class.h, my_class.cpp. If I tried to compile with the below code, all, of course, in three separate files, I would get a multiple definition error of my_exception.
Please note, if I move the entire definition and declaration of my_exception into my_class.cpp, everything compiles fine and works correctly. Why is this, and how should I write this so that I could put the definition of the exception into the .h file? Or should I even put an error class definition into a .h file?
main.cpp:
#include my_class.h
int main(){
my_class m;
/* stuff */
}
my_class.h:
#ifndef MY_CLASS_H
#define MY_CLASS_H
#include <iostream> //irrelevant for this
#include <exception>
using namespace std;
//taken from http://www.cplusplus.com/doc/tutorial/exceptions/
class my_exception : public exception{
virtual const char* what() throw() { return "message";}
} me_err;
class my_class{
my_class() };
};
#endif // MY_CLASS_H
my_class.cpp:
#include my_class.h
my_class::my_class(){
throw me_err;
}
The problem you are having is you created a global variable in your header file. When you compile main.cpp and my_class.cpp each one of them now has the same global variable. To fix this you can make me_err a member of my_class since that is what is using it.
Edit:
As pointed out by Fred Larson you could get rid of me_err entirely and just use throw my_exception;