Question

Suppose I want to version the libs in binaries made. For static libs, I thought this approach would work but it does not:

LibInfo.h – Base class for all libinfo classes. Registers an object in gvLibInfo vector when a child is constructed.

#ifndef IFACE_H
#define IFACE_H

#include <vector>

class LibInfo;
extern std::vector<LibInfo*> gvLibInfo;

class LibInfo
{
    public:
        virtual int getversion() = 0;
        void reglib()
        {
            gvLibInfo.push_back(this);
        }

        LibInfo()
        {
            reglib();
        }
        virtual ~LibInfo()
        {}
};


#endif

Lib1.h – Derived from LibInfo and creates an object l1. Lib2.h is the same except getversion returns 2.

#ifndef LIB1_H
#define LIB1_H

#include "LibInfo.h"

class Lib1 : public LibInfo
{
    public:
        int getversion()
        {
            return 1;
        }

    private:
};

Lib1 l1;

#endif

main.cpp

#include "Lib1.h"
#include <iostream>
using namespace std;

vector<LibInfo*> gvLibInfo;

int main()
{       
    for(vector<LibInfo*>::iterator it = gvLibInfo.begin(); it != gvLibInfo.end(); it++)
    {       
        cout << (*it)->getversion() << endl;
    }

    return 0;
}

Compiling –

g++ -c Lib1.h -o Lib1.o
g++ -c Lib2.h -o Lib2.o
ar cr lib.a Lib1.o Lib2.o
g++ main.cpp -o app -L/home/duminda/statictest/lib.a

When I run, nothing happens. I thought this maybe due to one of several reasons:

1. At the time Lib1 l1 is made, gvLibInfo has not been constructed.
2. I saw somewhere that the linker will remove any unused variables from a static lib. However, when I run on the binary nm it shows up this :

0000000000603280 B gvLibInfo

0000000000603270 B l1

0000000000603278 B l2

I guess l1 & l2(The corresponding object of class Lib2 are there but what does that ‘B’ flag mean?
3. Something else I don’t know.

Answer 1

The first answer to this question propose to use the –whole-archive flag.
The main difference is, instead of referring to an external object, the library refers
to an external function.

Linking does not occur with -L, but with -l, i.e. it should be :

-L/path/to/libdir -lname

If your library is :

/path/to/libdir/libname.a