S
Stanisław Findeisen
Hi
I am trying to define an extern "C" friend function without external
linkage. The idea is to pass this function to pthread_once and use it
for initialization of a singleton object in a multithreaded program.
I don't want external linkage because I don't want other parts of the
program to be able to call this initialization function.
Here is an example that does no pthread_once call but uses extern "C".
The point, after all, is to have C linkage.
========== MyClass.h BEGIN ==========
#ifdef UGLY_UGLY_UGLY_MyClass_implementation_is_here
extern "C" {
static void changeX();
}
#endif
class MyClass {
public:
static void printX();
private:
friend void changeX();
static int x;
};
extern "C" void testChangeX();
========== MyClass.h END ============
========== MyClass.cpp BEGIN ==========
#include <iostream>
#define UGLY_UGLY_UGLY_MyClass_implementation_is_here
#include "MyClass.h"
int MyClass::x = 5;
void MyClass:
rintX() {
std::cout << "x is now: " << x << std::endl;
}
extern "C" void changeX() {
MyClass::x = 7;
}
extern "C" void testChangeX() {
changeX();
}
========== MyClass.cpp END ============
========== test-friend.cpp BEGIN ============
#include "MyClass.h"
int main() {
MyClass:rintX();
changeX();
testChangeX();
MyClass:rintX();
return 0;
}
========== test-friend.cpp END ==============
========== Makefile BEGIN ==============
..PHONY: default
default:
g++ -Wall -c MyClass.cpp -o MyClass.o
g++ -Wall -c test-friend.cpp -o test-friend.o
g++ -Wall -o test-friend.out MyClass.o test-friend.o
..PHONY: clean
clean:
rm -f MyClass.o
rm -f test-friend.o
rm -f test-friend.out
========== Makefile END ==============
Here changeX is the secret initialization function, and testChangeX
simulates pthreads library C code that changeX is called from
(testChangeX has external linkage and that's no problem).
It *WORKS* here (the program does not compile until I delete changeX
call from main()), but as you can see, this is quite ugly. The question
is how to simplify this.
Explanation:
1. changeX() declaration at the beginning of MyClass.h seems necessary,
because without it my compiler complains:
I think that's because of the Standard saying (11.4.3):
2. The ugly macro helps to fix the problem of other (than MyClass.cpp)
translation units encountering the declaration of changeX in MyClass.h.
Each such case generates a warning:
Sooo, does anyone has an idea how to simplify this?
You should be able to download the above source here:
http://eisenbits.homelinux.net/~stf/load/20100825-friend-1.tgz .
I am using g++ for this:
g++ (Debian 4.3.2-1.1) 4.3.2
Copyright (C) 2008 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
however I do care about POSIX compliance.
STF
http://eisenbits.homelinux.net/~stf/
OpenPGP: DFD9 0146 3794 9CF6 17EA D63F DBF5 8AA8 3B31 FE8A
I am trying to define an extern "C" friend function without external
linkage. The idea is to pass this function to pthread_once and use it
for initialization of a singleton object in a multithreaded program.
I don't want external linkage because I don't want other parts of the
program to be able to call this initialization function.
Here is an example that does no pthread_once call but uses extern "C".
The point, after all, is to have C linkage.
========== MyClass.h BEGIN ==========
#ifdef UGLY_UGLY_UGLY_MyClass_implementation_is_here
extern "C" {
static void changeX();
}
#endif
class MyClass {
public:
static void printX();
private:
friend void changeX();
static int x;
};
extern "C" void testChangeX();
========== MyClass.h END ============
========== MyClass.cpp BEGIN ==========
#include <iostream>
#define UGLY_UGLY_UGLY_MyClass_implementation_is_here
#include "MyClass.h"
int MyClass::x = 5;
void MyClass:
rintX() {std::cout << "x is now: " << x << std::endl;
}
extern "C" void changeX() {
MyClass::x = 7;
}
extern "C" void testChangeX() {
changeX();
}
========== MyClass.cpp END ============
========== test-friend.cpp BEGIN ============
#include "MyClass.h"
int main() {
MyClass:
rintX();changeX();
testChangeX();
MyClass:
rintX();return 0;
}
========== test-friend.cpp END ==============
========== Makefile BEGIN ==============
..PHONY: default
default:
g++ -Wall -c MyClass.cpp -o MyClass.o
g++ -Wall -c test-friend.cpp -o test-friend.o
g++ -Wall -o test-friend.out MyClass.o test-friend.o
..PHONY: clean
clean:
rm -f MyClass.o
rm -f test-friend.o
rm -f test-friend.out
========== Makefile END ==============
Here changeX is the secret initialization function, and testChangeX
simulates pthreads library C code that changeX is called from
(testChangeX has external linkage and that's no problem).
It *WORKS* here (the program does not compile until I delete changeX
call from main()), but as you can see, this is quite ugly. The question
is how to simplify this.
Explanation:
1. changeX() declaration at the beginning of MyClass.h seems necessary,
because without it my compiler complains:
I think that's because of the Standard saying (11.4.3):
2. The ugly macro helps to fix the problem of other (than MyClass.cpp)
translation units encountering the declaration of changeX in MyClass.h.
Each such case generates a warning:
Sooo, does anyone has an idea how to simplify this?
You should be able to download the above source here:
http://eisenbits.homelinux.net/~stf/load/20100825-friend-1.tgz .
I am using g++ for this:
g++ (Debian 4.3.2-1.1) 4.3.2
Copyright (C) 2008 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
however I do care about POSIX compliance.
STF
http://eisenbits.homelinux.net/~stf/
OpenPGP: DFD9 0146 3794 9CF6 17EA D63F DBF5 8AA8 3B31 FE8A