C++ program behaviour depending on source code file set - c++

Question

I want to create a program which behaves differently depending on a facultative additional source code file to compilation (where some people might add some differing functionality). I thought about function overloading, similar to the (non-compilable) following code:
file1.cpp:
#include <iostream>
#include <string.h>
using namespace std;
class base {
public:
void ausgeb() { cout<<"here output base"<<endl; }
};
class derive: public base;
int main(int argc, char** argv)
{
derive beisp;
beisp.ausgeb();
}
file2.cpp:
#include <iostream>
#include <string.h>
using namespace std;
class base;
class derive : public base
{
public:
void ausgeb() { cout<<"here output derive"<<endl; }
};
Now I wished that:
g++ -o o1 file1.cpp file2.cpp
and
g++ -o o2 file1.cpp
should produce executable files with differing output.
Might there be a possibilty to meet that need?

Answer 1

This solution is gcc specific, if you switch compiler, it most probably won't work any more...
file1.cpp:
#include <iostream>
void printOut() __attribute__((weak));
void printOut()
{
::std::cout << "weak" << ::std::endl;
}
int main(int, char*[])
{
printOut();
return 0;
}
file2.cpp:
#include <iostream>
void printOut()
{
::std::cout << "strong" << ::std::endl;
}
More advanced (left out printOut implementations):
file1.h:
class Base
{
virtual void printOut();
}
file1.cpp
#include "file1.h"
Base& getInstance() __attribute__((weak));
Base& getInstance()
{
static Base theInstance;
return theInstance;
}
int main(int, char*[])
{
Base& instance = getInstance();
instance.printOut();
}
file2.cpp:
#include "file1.h"
class Derived : public Base
{
virtual void printOut();
}
Base& getInstance()
{
static Derived theInstance;
return theInstance;
}
More general solution, via defining a preprocessor symbol:
file1.h:
class Base
{
virtual void printOut();
}
file1.cpp
#include "file1.h"
#ifdef USE_DERIVED
#include "file2.h"
#endif
void Base::printOut()
{
}
int main(int, char*[])
{
#ifdef USE_DERIVED
Derived instance;
#else
Base instance;
#endif
instance.printOut();
}
file2.h:
#include "file1.h"
class Derived : public Base
{
virtual void printOut();
}
file2.cpp:
void Derived::printOut()
{
}
and compile with g++ file1.cpp in one case and g++ -DUSE_DERIVED file1.cpp file2.cpp in the other one.