CODEWITHSUNDEEP
c++cstatic-linkingextern
Emir Akaydın
Emir Akaydın

Reputation: 5823

Trying to use extern in reverse order

When we have an exe or dll and a static library attached to it, we are able to use extern keyword to access static library's variables and/or functions from the exe or dll. To make things simpler, let's assume ve have an exe and a lib attached to it.

What I am trying to do is to call a function of exe from lib.

Executable Code

void doSomething() {
    // do something here
}

Static Linked Library Code

void onSomeEvent() {
    doSomething(); // call doSomething() here
}

Vice versa is easy but I wonder if this can be done in a way like extern keyword. Or what is the best method?

What comes to my mind is to pass a function pointer (like void*) to one of the functions / methods in the lib (probably to a class constructor). I think this should work but I don't want to touch library's code too much since library is not mine and can be replaced with newer versions. I can add/remove a few lines of code to it but I want to prevent from changing function interfaces.

What is the better way?

Upvotes: 3

Views: 193

Answers (3)

tesla
tesla

Reputation: 1248

You may also declare it outside your function, you may have other functions need it. 

void doSomething(); // declares the function

void onSomeEvent() 
{
    doSomething(); // call doSomething() here
}

void onSomeEvent2() 
{
    doSomething(); // call doSomething() here
}

Upvotes: 4

trojanfoe
trojanfoe

Reputation: 122391

Given the static library is (probably) meant to be used in many different programs, it's not uncommon to use the callback approach where the exe initialises the library and passes it one or more function pointers to use to do things (like logging messages, or whatever). If the exe doesn't pass in the function pointers (or passes them as NULL) then the library can simply not call those functions and the library will work well in both environments.

This is much better than assuming the functions are always defined in the exe.

Upvotes: 1

avakar
avakar

Reputation: 32635

Of course, you merely have to declare the function in the library.

void onSomeEvent() {
    void doSomething(); // declares the function
    doSomething(); // call doSomething() here
}

Upvotes: 2

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