I've got a problem when declaring a class in a separate object file and then using it in the main function of another file:
main.cpp:
#include <helloclass.hpp>
using namespace std;
int main() {
Hello hi;
hi.hello();
return 0;
}
helloclass.cpp:
#include <iostream>
using namespace std;
class Hello {
public:
void hello() {
cout << "Hello world\n";
}
Hello() {}
};
helloclass.hpp:
class Hello {
public:
void hello();
Hello();
};
Then I ran the following commands:
g++ -I. -c main.cpp
g++ -c helloclass.cpp
g++ -o main main.o helloclass.o
However, the last command gives the following output:
main.o: In function `main':
main.cpp:(.text+0x1f): undefined reference to `Hello::Hello()'
main.cpp:(.text+0x2b): undefined reference to `Hello::hello()'
collect2: error: ld returned 1 exit status
To me, it seems like I'm missing something pretty obvious. Does anyone know how to fix this?
You should not redefine the class in the separate source file. Instead include the header-file and implement the Hello::hello function (and the Hello::Hello constructor).
Related
Working with a Fedora g++, I've noticed something anoying for me.
I think I misundertands g++ behaviors with namesapce and header files.
You will see 2 versions of a very simple Class and a main function.
First one is fully embeded in single file named "mainfull.cpp" and compiles silently.
Second one is the same code distributed in three files "MaClass.hpp", "MaClass.cpp" and "main.cpp". The issue come from this version.
$ g++ -o doit MaClass.cpp main.cpp
/usr/bin/ld : /tmp/cc9IOJE8.o : in function « main » :
main.cpp:(.text+0x10) : undefined reference to « test::example::MaClass::doit() »
collect2: error: ld returned exit status 1
But the error vanishes with namespace suppression or when changing the include from the header file to the implementation file.
Here are my questions :
What is the good way to include multifiles classes with namespaces ?
How should one uses g++ in that case ?
But I am a chatterbox ; here is the code :
mainfull.cpp : no pb when compiling with : g++ -o mainfull mainfull.cpp
#include <iostream>
namespace test {
namespace example {
class MaClass {
public:
void doit();
};
}
}
int main() {
test::example::MaClass c;
c.doit();
return 0;
}
namespace test {
namespace example {
void MaClass::doit() {
::std::cout << "I did it !" << ::std::endl;
}
}
}
And here the version on witch I get doubts: (the whole remaining files)
MaClass.hpp
#ifndef MACLASS
#define MACLASS
#include <iostream>
namespace test {
namespace example {
class MaClass {
public:
void doit();
};
}
}
#endif
MaClass.cpp
#include "MaClass.hpp"
namespace test {
namespace example {
void MaClass::doit() {
::std::cout << "I did it !" << ::std::endl;
}
}
}
main.cpp
#include "MaClass.hpp"
int main() {
test::example::MaClass c;
c.doit();
return 0;
}
The issue occurs with :
g++ -o doit MaClass.cpp main.cpp -std=c++11
I don't know the reason of that. I know that the problem come from me and not from Fedora.
Any help would be apreciated.
I have no problem on Visual Studio 2019 neither with the one nor the other version ; everything is automatic, I should have miss something.
I get this /tmp/ccnL7Yz1.o: In function 'main':
main.cpp:(.text+0x70): undefined reference to 'dng::genDungeon()'
main.cpp:(.text+0xf0): undefined reference to 'dng::clrDungeon(char**)'
collect2: error: ld returned 1 exit status error when I'm trying to compile my program. It worked great before I added namespace functions. I'm compiling it like this: g++ -std=c++11 main.cpp Dungeon.cpp
Dungeon.h
namespace dng {
char** genDungeon();
void clrDungeon(char**);
class Dungeon {
//Methods and variables
}
}
Dungeon.cpp
#include "Dungeon.h"
using namespace dng;
char** genDungeon()
{
//Stuff
}
void clrDungeon(char** dungeon)
{
//Another Stuff
}
/*Implementation of class methods
void Dungeon::genStart(){} -> like this */
main.cpp
#include "Dungeon.h"
int main ()
{
//Stuff
auto dungeon = dng::genDungeon();
//Stuff
dng::clrDungeon(dungeon);
return 0;
}
I also tried to make .o files by myself g++ -std=c++11 -c main.cpp g++ -std=c++11 -c Dungeon.cpp and then link them, but got the same error. What can be the problem?
Enclose the function definitions in the namespace dng where they are declared.
#include "Dungeon.h"
namespace dng
{
char** genDungeon()
{
//Stuff
}
void clrDungeon(char** dungeon)
{
//Another Stuff
}
//...
}
Or use qualified names.
include "Dungeon.h"
using namespace dng;
//...
char** dng::genDungeon()
{
//Stuff
}
void dng::clrDungeon(char** dungeon)
{
//Another Stuff
}
Otherwise, the functions are defined in the global namespace, and as a result, you declared four functions: two in the namespace dng and another two in the global namespace.
I'm having one of those "undefined reference to " errors when compiling a c++ program. I know this is common pitfall, but so far was unable to figure out what I'm doing wrong.
Here's the relevant code. Ex1Two_Sum.h:
#ifndef EX1TWO_SUM_H
#define EX1TWO_SUM_H
#include <vector>
using namespace std;
namespace ddc {
class Ex1Two_Sum
{
public:
void f();
protected:
private:
};
}
#endif
Ex1Two_Sum.cpp:
#include <vector>
#include <cstddef>
#include <iostream>
using namespace std;
namespace ddc {
class Ex1Two_Sum {
public:
void f(){
cout << "works" << endl;
}
};
}
And finally, main.cpp:
#include <iostream>
#include "Ex1Two_Sum.h"
using namespace std;
using namespace ddc;
int main()
{
Ex1Two_Sum ex1;
ex1.f();
return 0;
}
I compile as follows:
g++ -std=c++11 -c Ex1Two_Sum.cpp
g++ -std=c++11 -c main.cpp
g++ Ex1Two_Sum.o main.o
yielding the following message:
main.o: In function `main':
main.cpp:(.text+0x2c): undefined reference to `ddc::Ex1Two_Sum::f()'
collect2: error: ld returned 1 exit status
Your source file redefines the whole class, with an inline function definition, when it just needs to provide a non-inline function definition.
#include "Ex1Two_Sum.h"
void ddc::Ex1Two_Sum::f() {
std::cout << "should work\n";
}
Also, please don't put using namespace std; in a header. Not everyone wants the global namespace polluted in potentially surprising ways.
First, which line of the command throws that error?
Second, I think you forgot to include the Ex1Two_Sum.h in the Ex1Two_Sum.cpp
Third you need to change class ....... in Ex1Two_Sum.cpp to:
void Ex1Two_Sum::f(){...}
I'm using Dev-C++ 5.2.0.1
I took an example of how to put a class in another file from a website but it resulted in an error.
In the file class.h I have:
class MyClass
{
public:
void foo();
int bar;
};
In the file class.cpp I have:
#include "class.h"
void MyClass::foo()
{
cout<< "test";
}
In the file main.cpp I have:
#include "class.h"
using namespace std;
int main()
{
MyClass a;
a.foo();
return 0;
}
Here is the error I get:
[Linker error] C:\Users\Matthew\AppData\Local\Temp\cccWe7ee.o:main.cpp:(.text+0x16): undefined reference to `MyClass::foo()'
collect2: ld returned 1 exit status
Is there something I'm doing wrong?
New answer.
Are you compiling and linking all of your files together? In gcc you would do something like:
gcc -o myExe class.cpp main.cpp
I'm not so sure about dev-c++, but I imagine it's not much different.
foo() only have definition. If you want to use a function that doesn't have a implement， Linker will give you this error "undefined reference".
class.h
#include <iostream>
#include <stdint.h>
using namespace std;
template <typename T>
class CIntegerType {
public:
void Show ( void );
private:
T m_Data;
};
class.cpp
#include "class.h"
template <typename T>
void CIntegerType<T> :: Show ( void ) {
cout << m_Data << endl;
}
main.cpp
#include "class.h"
int main ( void ) {
CIntegerType<uint32_t> UINT32;
UINT32 . Show ();
return 0;
}
This commands return:
g++ -Wall -pedantic -c main.cpp
g++ -Wall -pedantic -c class.cpp
g++ -Wall -pedantic -o class.o main.o
main.o: In function `main':
main.cpp:(.text+0x11): undefined reference to 'CIntegerType< unsigned int>::Show()'
collect2: ld returned 1 exit status
Try putting your template implementation in the header file.
See: Why can templates only be implemented in the header file?
Try g++ -Wall -pedantic -o main.o class.o instead. You are facing the same problem as in this question: g++ linking order dependency when linking c code to c++ code
The linker searches for functions in the order they appear. Since you have a template function, its use in main must be fed to the linker prior to the actual code to instantiate it in class.