I use Visual Studio 2017 RC. I added all these files to my solution:
main.cpp
#include "B.h"
#include "AAA.h"
int main()
{
AAA gen;
gen.Create();
return 0;
}
AAA.h
#ifndef _H_AAA
#define _H_AAA
class AAA
{
public:
void Function();
};
#endif // _H_AAA
AAA.cpp
#include "AAA.h"
#include "B.h"
void AAA::Function()
{
B obj;
obj.Function();
}
B.h
#ifndef _H_B
#define _H_B
class B
{
public:
void Function();
};
#endif // _H_B
B.cpp
#include "B.h"
void B::Function()
{}
When I try to compile it, VS shows that error:
LNK2019 unresolved external symbol "public:
void __thiscall B::Function(void)" (?Function#B##QAEXXZ)
referenced in function "public:
void __thiscall AAA::Function(void)" (?Function#AAA##QAEXXZ)`
If I use gcc, it doesn't show any errors.
Why is this happening and how to fix that problem?
#EDIT
I created new project and copied there all these files. It works fine now, but I still don't understand this strange behavior of VS.
You need to do
Right click on project > Add > Reference
Then add your library project as reference.
Related
I get a linker error - unresolved symbol - when using a (specialized) template class from a DLL (Visual Studio 2008 compiler). I tried to use the 'explicit template instantiation' trick described also here in Stackoverflow, but it didn't work. I broke it down to a very simple reproducable example:
I have a dynamic library (DLL) 'MyTemplates.lib' with a header file 'MyTemplates.h' (and a source file 'MyTemplates.cpp' without any code which simply includes this header file) with the following content:
template <class T>
class A
{
public:
A()
{ int x = 7; }
};
template <class T>
class B : public A<T>
{
public:
B()
{}
};
// do explicit template instantiation for classes A<int> and B<int>
// macro 'MYTEMPLATES_API' is defined in the usual way as:
//#ifdef MYTEMPLATES_EXPORTS
// #define MYTEMPLATES_API __declspec( dllexport )
//#else
// #define MYTEMPLATES_API __declspec(dllimport)
//#endif
template class MYTEMPLATES_API A<int>;
template class MYTEMPLATES_API B<int>;
Now i have another dynamic library 'UserLibary' (which links against 'MyTemplates.lib') with the files 'Util.h' and Util.cpp'. The file 'Util.h' is as follows:
#include "MyTemplates.h"
class UserClass
{
public:
UserClass();
public:
A<int> bla;
B<int> blubb;
};
and the content of the file 'Util.cpp' is:
#include "Util.h"
UserClass::UserClass()
{
}
The problem is now that my library 'UserLibrary' does compile well, but it gives two linker errors as follows:
1>Util.obj : error LNK2019: unresolved external symbol "__declspec(dllimport) public: __thiscall B<int>::B<int>(void)" (__imp_??0?$B#H##QAE#XZ) referenced in function "public: __thiscall UserClass::UserClass(void)" (??0UserClass##QAE#XZ)
1>Util.obj : error LNK2019: unresolved external symbol "__declspec(dllimport) public: __thiscall A<int>::A<int>(void)" (__imp_??0?$A#H##QAE#XZ) referenced in function "public: __thiscall UserClass::UserClass(void)" (??0UserClass##QAE#XZ)
So the linker can not find the symbols for the default constructors of classes A<int> and B<int>. Why is this possible, and how can i get rid of these linker errors ? I thought that the explict template instantiation of the class A<int> and B<int> (in file 'MyTemplates.h') would solve this, but unfortunately it doesn't seem to help - or am I using it in the wrong way ? My compiler is Visual Studio 2008, operating system is windows 7 64 bit, and code is compiled in 64bit.
Please have a look at the following code
Main.cpp
#include <iostream>
#include <string>
using namespace std;
int main()
{
system("pause");
return 0;
}
Magic.h
#pragma once
class Magic
{
public:
Magic();
~Magic();
virtual void display()=0;
};
Spell.h
#pragma once
#include "Magic.h"
#include <iostream>
#include <string>
using namespace std;
class Spell :
public Magic
{
public:
Spell(void);
Spell(string words);
~Spell(void);
void display();
private:
string words;
};
Spell.cpp
#include "Spell.h"
#include "Magic.h"
#include <iostream>
#include <string>
using namespace std;
Spell::Spell(void)
{
}
Spell::Spell(string words)
{
this->words = words;
}
Spell::~Spell(void)
{
cout << "Delete Spell" << endl;
}
void Spell::display()
{
cout << "Spell Words: " << words << endl;
}
Here, I am getting the error
1>------ Build started: Project: Revision1_1, Configuration: Debug Win32 ------
1>Spell.obj : error LNK2019: unresolved external symbol "public: __thiscall Magic::~Magic(void)" (??1Magic##QAE#XZ) referenced in function __unwindfunclet$??0Spell##QAE#XZ$0
1>Spell.obj : error LNK2019: unresolved external symbol "public: __thiscall Magic::Magic(void)" (??0Magic##QAE#XZ) referenced in function "public: __thiscall Spell::Spell(void)" (??0Spell##QAE#XZ)
1>C:\Users\yohan\Documents\Visual Studio 2010\Projects\Revision1_1\Debug\Revision1_1.exe : fatal error LNK1120: 2 unresolved externals
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========
I do not understand what to do here. Why is this happening? Please help! I am new to C++ anyway..
Magic is not implementing its constructor and destructor (which also should be virtual).
Don't even declare the constructor if not necessary, e.g.
class Magic {
public:
virtual ~Magic() {}
virtual void display() = 0;
};
Unrelated: I didn't know you can display magic.
You didn't implement
Magic();
~Magic();
You'll need to either implement them inline, in an implementation file, or mark them = default.
You have declared a destructor in your Magic class but did not define it. That's why the linker complains (and the compiler doesn't).
You don't have an implementation for Magic. If your intention is for Magic to be an abstract base class, then just change its declaration to:
#pragma once
class Magic
{
public:
virtual void display()=0;
};
Remember, any method that is not followed by = 0 in the interface must be implemented in the class.
This seems like a problem that is common. I defined an enum in classA and then included classA in classB. Then, in classB I defined a function which returns the enum type defined in classA...see below. I get the following error:
aFirst.obj : error LNK2019: unresolved external symbol "public: enum justEnum::things_t __thiscall usesTheEnum::returnsThings(void)" (?returnsThings#usesTheEnum##QAE?AW4things_t#justEnum##XZ) referenced in function _wmain
1>C:\Documents and Settings\Ben\My Documents\Visual Studio 2010\Projects\aFirst\Debug\aFirst.exe : fatal error LNK1120: 1 unresolved externals
#pragma once
class justEnum
{
public:
justEnum(void);
~justEnum(void);
enum things_t{ONE, TWO};
};
#pragma once
#include "justEnum.h"
class usesTheEnum
{
public:
usesTheEnum(void);
~usesTheEnum(void);
justEnum::things_t returnsThings(void);
};
#include "StdAfx.h"
#include "usesTheEnum.h"
#include "justEnum.h"
usesTheEnum::usesTheEnum(void)
{
}
usesTheEnum::~usesTheEnum(void)
{
}
justEnum::things_t returnsThings()
{
return justEnum::ONE;
}
// tester.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include <stdio.h>
#include <iostream>
#include "justEnum.h"
#include "usesTheEnum.h"
using namespace std;
int _tmain(int argc, _TCHAR* argv[])
{
usesTheEnum aUser;
justEnum anEnum;
justEnum::things_t anotherEnum;
anotherEnum = justEnum::ONE;
aUser.returnsThings();
cout << anotherEnum;
return 0;
}
You need to specify that your definition of returnsThings() is part of the usesTheEnum class.
justEnum::things_t usesTheEnum::returnsThings()
{
return justEnum::ONE;
}
error LNK2019: unresolved external symbol "public: enum justEnum::things_t __thiscall usesTheEnum::returnsThings(void)"
The compiler is complaining that usesTheEnum::returnThings() is not defined, and I cannot see a definition in the code you posted. You should provide a definition for the function in one translation unit.
I don't think I can emphasize enough how important it to learn to read error messages. The compiler is doing it's best to tell you what is wrong.
not shure, but can't you just move the enum out of the class?
Or on the .cpp of the class write something like
extern enum classname::things_t;
just to have the enum added to the generated lib wich is what will be linked against.
I have the following source code :
// main.cpp
#include "a.h"
int main() {
A::push(100);
}
// a.cpp
#include "a.h"
template <class T>
void A::push(T t) {
}
template void A::push(int t);
// a.h
#ifndef A_H
class A {
public:
template <class T>
static void push(T t);
};
#endif
The code compiled charming and no problem under VC2008.
But when come to my beloved VC6, it give me error :
main.obj : error LNK2001: unresolved
external symbol "public: static void
__cdecl A::push(int)" (?push#A##SAXH#Z)
Any workaround? I just want to ensure my function definition is re-inside cpp file.
The problem solved by using
// main.cpp
#include "a.h"
int main() {
A::push<int>(100);
}
It seems that you need to provide more hint to VC6, compared with VC2008.
I have a solution created using visual studio 2008 named, "Solution", and i have two projects in that solution, project "A" and project "B". when i do a thing like below it shows fatal errors in the bottom. I have given in project A->properties->Additional include Directries as ../B
project B
B.h
#include <iostream>
using namespace std;
class B
{
public:
B();
~B();
};
B.cpp
#include "B.h"
B::B()
{
}
B::~B()
{
}
project A
A.h
#include <iostream>
using namespace std;
class A
{
public:
A();
~A();
};
A.cpp
#include "A.h"
#include "B.h"
A::A()
{
B b;
}
A::~A()
{
}
Main.cpp in project A
#include "B.h"
int main()
{
B b;
system("pause");
}
when i run it says
Error 3 fatal error LNK1120: 2 unresolved externals H:\Sol\Debug\A.exe
Error 2 error LNK2001: unresolved external symbol "public: __thiscall B::B(void)" (??0B##QAE#XZ) A.obj
Error 1 error LNK2001: unresolved external symbol "public: __thiscall B::~B(void)" (??1B##QAE#XZ) A.obj
It doesn't look like you are exporting class B out of project B. So project A sees the declaration of class B but can't find its implementation. What does project B build?