Object creation/destruction in C++

[samsogade]

Let's say my C++ code has some object that needs to be used throughout the program:

class API {
 API();
 ~API();
 void methodA() { std::cout << "called method A\n"; }
 void methodB() { std::cout << "called method B\n"; }
}

API api* = new API();

extern "C" K methodA(K x) { api->methodA(); R 0; }
extern "C" K methodB(K x) { api->methodB(); R 0; }

My Q code only knows about `methodA` and `methodB` loaded through the 2: function.

How do you go about managing to creating/destruction of API (and use of r0/r1)? I assume this is a common scenario when using C++ libraries in C.

Thanks

[kuentang]

I would use a std::map< std::uintptr_t , boost::shared_ptr< API > > repository to manage the API objects.

Code snippet taken from https://github.com/kimtang/bml/blob/master/bml.cpp :

kx::K uniform_01()
	{
	boost::random::uniform_01<>* dst = new boost::random::uniform_01<>();
	std::uintptr_t ptr = reinterpret_cast<long>(dst);
	bml::random::distribution_map[ptr] = dst;
	return kx::kj(ptr);
	}

Happy to hear other’s people opinion and solutions.

Kim

Von: personal-kdbplus@googlegroups.com [mailto:personal-kdbplus@googlegroups.com] Im Auftrag von Morten Sorensen
Gesendet: Samstag, 9. April 2016 20:22
An: Kdb+ Personal Developers
Betreff: [personal kdb+] Object creation/destruction in C++

Let's say my C++ code has some object that needs to be used throughout the program:

class API {
 API();
 ~API();
 void methodA() { std::cout << "called method A\n"; }
 void methodB() { std::cout << "called method B\n"; }
}

API api* = new API();

extern "C" K methodA(K x) { api->methodA(); R 0; }
extern "C" K methodB(K x) { api->methodB(); R 0; }

My Q code only knows about `methodA` and `methodB` loaded through the 2: function.

How do you go about managing to creating/destruction of API (and use of r0/r1)? I assume this is a common scenario when using C++ libraries in C.

Thanks

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[effbiae]

you can use the pattern that linux and kdb+ use for managing lifetimes:

  eg. in linux: : open/close  and in kdb+: hopen/hclose

so in the library:

extern "C"{

K open(K x){api=new API();R 0;}

K close(K x){delete api;R 0;}

K methodA(K x) { api->methodA(); R 0; }
K methodB(K x) { api->methodB(); R 0; }

}

also on linux (and i think on windows) there is a mechanism for functions to be called on opening and closing the library:

$ man dlopen

..

  libraries should export routines using the __attribute__((constructor)) and __attribute__((destructor)) function attributes.  See the gcc info pages for information on these.  Constructor routines are executed before dlopen() returns,  and  destructor  routines  are  executed  before dlclose() returns.

..

[samsogade]

Thanks both! I found the __attribute__((constructor)) / __attribute__((destructor)) quite straightforward to use and works on both GCC and clang:

__attribute__((constructor))
static void initialize_api() {
    api = new API();
}

__attribute__((destructor))
static void destroy_api() {
    delete api;
}

There's an equivalent function DllMain() available for MSVC: http://stackoverflow.com/questions/20724628/attribute-destructor-equivalent-in-vc