Question

Please help with the following noob question about C++ and g++ compilation and linking. Essentially I have 2 classes in 2 different files, and can compile them but when I attempt to link, one class can’t see the methods of the other, even though I am linking both. Order of object files does not help in this case.

The problem seems related to a non-default constructor that takes a parameter.

I have distilled and reproduced the problem in the following simple code:

File: a.cpp:

#include <iostream>

class A
{
  public:
  int my_int;
    A(int i) {
      my_int = i;
      std::cout << "A";
    }
};

File: a.hpp:

#ifndef __A_H_
#define __A_H_

class A
{
  public:
  A(int i);
};

#endif

File b.cpp:

#include <iostream>

using namespace std;

#include <a.hpp>

class B
{
  public:
  int my_int;
    B(int i) {
      my_int = i;
      A a(i);
      cout << "B\n";
    }
};

int main(int argc, char* argv[])
{
  B b(5);
  cout << "hello world: ";
  cout.flush();
  return 0;
}

Commands I use to build:

g++ -c -I. a.cpp
g++ -c -I. b.cpp
g++ -o c_test a.o b.o

Alternately, I’ve tried each of these:

g++ -o c_test b.o a.o
g++ -I. -o c_test a.cpp b.cpp
g++ -I. -o c_test b.cpp a.cpp

Error I get in any of above link scenarios:

b.o: In function `B::B(int)':
b.cpp:(.text._ZN1BC1Ei[B::B(int)]+0x1c): undefined reference to `A::A(int)'
collect2: ld returned 1 exit status

Thanks in advance for any insight.

(sorry if this is a re-post — I thought I posted it and don’t see it…)

Answer 1

It doesn’t work that way. What you’ve come across is technically an ODR violation, which roughly means that A in both a.cpp and b.cpp must be the same thing. It isn’t.

Moreover, the constructor is implicitly inline in a.cpp and therefore its code needn’t be emitted.

Changing a.cpp to

#include <iostream>
#include "a.hpp"

A::A(int i) {
      my_int = i;
      std::cout << "A";
}

will fix the error.