Linking problem - "undefined reference"

Hi,

I try to Link my compiled object file with gcc, but
it gives me 50 lines of "undefined reference" errors, despite
the compile phase goes fine?!

Code:

---

...
Utils.o(.text+0xf):Utils.cpp: undefined reference to `operator new(unsigned long)'
Utils.o(.text+0x2e):Utils.cpp: undefined reference to `operator delete(void*)'
Utils.o(.text+0x5e):Utils.cpp: undefined reference to `std::ios_base::Init::Init()'
Utils.o(.text+0xbc):Utils.cpp: undefined reference to `std::ios_base::Init::~Init()'
Utils.o(.gnu.linkonce.t._ZN5Utils7printDescriptionEv+0xf):Utils.cpp: undefined reference to `std::cout'
...

---

well, I have installed all the required libraries for gcc, I think,

These are what I have included in the source file:

Code:

---

using namespace std;

#include <iostream>


#include <string>
#include <vector>
#include <fstream>

#include "FileUtils.cpp"

---

You have installed them, but do you reference them? You seem to be lacking a reference to the standard C++ library.

>You seem to be lacking a reference to the standard C++ library.

well, I´m migrating from Java to C++:D and I`m a bit puzzled.
Could you advice me a bit more.

Do I have to tweak gcc options or do I have something
wrong with my code?

It's the compiler options. Think of it as the class path.

gcc source.cpp -lstdc++
or something like that. Order matters!

Quote:

---

gcc source.cpp -lstdc++

---

ummmm... I`m lost. I`v tried to search to manual
for an answer...but no,

as far I understand, I have to inform the linker about
some libraries. But I don`t know what are the names of
those and where they reside?

If I try to do the whole thin as follows:

gcc -Wall -o myprog.exe mymain.c

-> "undefined reference to..."

Do I have to use the "-Xlinker" switch? If so, how to use it.


..ah, it`s allways pain in the beginning:(

I mean, with -l or -L flag. But what to include?

-lc for the standard C library.
I think -lstdc++ for the standard C++ library.

Then there are other libraries: to get libpng.a you'd write
-lpng




I just noticed you #include a .cpp. Don't ever do that!

I just tried the following

Code:

---

C:\projects\test>gcc -lstdc++ -o Test.exe Test.o

---

and with this

Code:

---

-Lc:\djgpp\lib\

---

and it produced:

Code:

---

c:/djgpp/bin/ld.exe: cannot find -lstdc++
collect2: ld returned 1 exit status

---

does this mean I don`t have that library??

Quote:

---

I just noticed you #include a .cpp. Don't ever do that!

---

hmm, I have seen two "ways" of coding with C++.

1. "Like" in java, you wite the .cpp file. No header file.
2. make a header file, and then the .cpp file, like:
MyClass::MyMethod()
{ ... }
whitout enclosing the functions in a class scope ( like in Java)

Been a long time since I used gcc this way. Never MingW. I really don't know.

Since you are using gcc, I suggest getting one of the IDE's that complement gcc. Dev C++ (aka Bloodshed C++) is an IDE for gcc, and it's free. You don't worry about all of these problems of linking, since they have built into the IDE the proper linker flags and paths.

Quote:

---

Originally posted by halmark6Z
hmm, I have seen two "ways" of coding with C++.
1. "Like" in java, you wite the .cpp file. No header file.
2. make a header file, and then the .cpp file, like:
MyClass::MyMethod()
{ ... }
whitout enclosing the functions in a class scope ( like in Java)

---

Please do not use Java as a model to programming C++. In C++, header files usually contain function prototypes and struct and class definitions, as well as #defined constants (aka macros). C++ header files are simply files that contain definitions that you would want to include in more than one CPP file. They should not be compared to Java's #import, which (at least to me) is more than just an "insert file here" command.

Just as an addendum, there was another poster a few weeks ago who was insistent on writing C++ like Java, since they came from a Java background. He/she was creating some sort of Object class similar to Java, so that they can create a Vector class. All of this was unnecessary, since C++ already has a vector class. I'm saying this because there are Java programmers who want to turn C++ into Java so as to be more "comfortable", but that isn't the way to go about learning proper C++.

Regards,

Paul McKenzie

instead of typing "gcc" try "g++"


g++ source.cpp

Finally, I got it working. Thank for helping, really!

I used gxx instead of g++:p

Well, I tried a bit further.

I made a simple class to do set/get stuff.
Then I created a file with a main function and I tried
to use my DerivedSimpleClass there ( compiled to DerivedSimpleClass.o ) but
the linker prompted:

Code:

---

C:\projects\test>gxx -o -lstdcxx Test.o DerivedSimpleClass.o SimpleClass.o
Test.o(.gnu.linkonce.t._ZN8SimpleClassC2Ev+0x8):Test.cpp: undefined reference to `vtable for SimpleClass
Test.o(.gnu.linkonce.t._ZN14DerivedSimpleClass15getContentEv+0xf):Test.cpp: undefined reference to `SimpleClass::getContent()'
Test.o(.gnu.linkonce.t._ZN8SimpleClassD2Ev+0x7):Test.cpp: undefined reference to `vtable for SimpleClass
collect2: ld returned 1 exit status

---

So I guess I`m using the linker wrong again?

Try switching changing the order of the object files. ld can be picky.

I tried, it complains about the same thing:

Code:

---

gxx -o Test.exe Test.o DerivedSimpleClass.o SimpleClass.o
gxx -o Test.exe SimpleClass.o DerivedSimpleClass.o Test.o
gxx -o Test.exe DerivedSimpleClass.o Test.o SimpleClass.o

---

they all produce:

Code:

---

Test.o(.gnu.linkonce.t._ZN14DerivedSimpleClass15getContentEv+0xf):Test.cpp: undefined reference to `SimpleClass::getContent()'
collect2: ld returned 1 exit status

---

and the code

Code:

---

//SimpleClass.cpp
class SimpleClass
{
public:
  virtual string getContent() = 0;
};

//DerivedSimpleClass.cpp
class DerivedSimpleClass : public SimpleClass
{
public:
  string getContent()
  {
    return "let me out";
  }
};

// Test.cpp

int main(int argc, char* argv[])
{
  // complains about this ?!
  DerivedSimpleClass *notworking = new DerivedSimpleClass();
  cout<<notworking->getContent();
}

---

Well, how does everybody in general deal with multiple .o files :confused:

fixed...I see light in the horizon....

inheritance bug.

Just to add:

Your SimpleClass base class will need a virtual destructor if you plan on doing something like this:

Code:

---

// Test.cpp

int main(int argc, char* argv[])
{
  // complains about this ?!
  SimpleClass *notworking = new DerivedSimpleClass();
  cout<<notworking->getContent();
  delete notworking; // undefined behavior
}

---

If SimpleClass does not have a virtual destructor, and you are destroying the object through a pointer to base, the behavior is undefined.

Regards,

Paul McKenzie