Re: Self-registration in a library

From:

Sam <sam@email-scan.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 25 Jan 2011 17:58:08 -0600

Message-ID:

<cone.1295999884.725002.1831.500@monster.email-scan.com>

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Kaba writes:

Hi,

Have a look at the following:

aFile.cpp
---------

#include <iostream>
#include <string>

namespace
{

    void libraryAddFunction(
        const std::string& name, void (*function)());

    struct CallFunction
    {
        CallFunction(void (*callback)()) {callback();}
    };

    void aFunction()
    {
        std::cout << "aFunction" << std::endl;
    }

    void addFunction()
    {
        libraryAddFunction("aFunction", aFunction);
    }

    CallFunction call(addFunction);

}

register.cpp
------------

#include <string>
#include <map>

typedef std::map<std::string, void (*)()> FunctionMap;

FunctionMap& functionMap()
{
    static FunctionMap theFunctionMap;
    return theFunctionMap;
}

void libraryAddFunction(const std::string& name,
                        void (*function)())
{
    functionMap().insert(
        std::make_pair(name, function));
}

void runFunction(const std::string& name)
{
    if (functionMap().count(name))
    {
        functionMap()[name]();
    }
}

I then compile aFile.cpp and register.cpp into a static library, say
library.lib. Next I build the following trivial main along with the
library.lib:

main.cpp
--------

#include <string>

void runFunction(const std::string& name);

int main()
{
    runFunction("aFunction");
    return 0;
}

And the result is... That nothing is printed. The first question is: why
not? When all of the three files are built together, then the result is
the string "aFunction" printed on screen.

Because your main() translation unit does not reference any undefined
symbols that are define in the aFile.cpp translation unit, so when you
statically link against it, the aFile.cpp translation unit is not added to
your binary.

This would work, as intended, if you build your module as a shared library,
and link against it.

This kind of a linkage issue is really platform specific. You did not
specify what platform you are using. This is not really a C++ question.
There is no such thing as a "library" in the C++ programming language.
Depending on your platform and C++ compiler, there may be compilation
switches that you can use to force the inclusion of a translation unit from
a statically-linked library.

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