The question is the following: consider this piece of code:
#include <iostream>
class aClass
{
public:
void aTest(int a, int b)
{
printf("%d + %d = %d", a, b, a + b);
}
};
void function1(void (*function)(int, int))
{
function(1, 1);
}
void test(int a,int b)
{
printf("%d - %d = %d", a , b , a - b);
}
int main()
{
aClass a;
function1(&test);
function1(&aClass::aTest); // <-- How should I point to a's aClass::test function?
}
How can I use the a's aClass::test as an argument to function1? I would like to access a member of the class.
There isn't anything wrong with using function pointers. However, pointers to non-static member functions are not like normal function pointers: member functions need to be called on an object which is passed as an implicit argument to the function. The signature of your member function above is, thus
void (aClass::*)(int, int)
rather than the type you try to use
void (*)(int, int)
One approach could consist in making the member function static in which case it doesn't require any object to be called on and you can use it with the type void (*)(int, int).
If you need to access any non-static member of your class and you need to stick with function pointers, e.g., because the function is part of a C interface, your best option is to always pass a void* to your function taking function pointers and call your member through a forwarding function which obtains an object from the void* and then calls the member function.
In a proper C++ interface you might want to have a look at having your function take templated argument for function objects to use arbitrary class types. If using a templated interface is undesirable you should use something like std::function<void(int, int)>: you can create a suitably callable function object for these, e.g., using std::bind().
The type-safe approaches using a template argument for the class type or a suitable std::function<...> are preferable than using a void* interface as they remove the potential for errors due to a cast to the wrong type.
To clarify how to use a function pointer to call a member function, here is an example:
// the function using the function pointers:
void somefunction(void (*fptr)(void*, int, int), void* context) {
fptr(context, 17, 42);
}
void non_member(void*, int i0, int i1) {
std::cout << "I don't need any context! i0=" << i0 << " i1=" << i1 << "\n";
}
struct foo {
void member(int i0, int i1) {
std::cout << "member function: this=" << this << " i0=" << i0 << " i1=" << i1 << "\n";
}
};
void forwarder(void* context, int i0, int i1) {
static_cast<foo*>(context)->member(i0, i1);
}
int main() {
somefunction(&non_member, nullptr);
foo object;
somefunction(&forwarder, &object);
}
#Pete Becker's answer is fine but you can also do it without passing the class instance as an explicit parameter to function1 in C++ 11:
#include <functional>
using namespace std::placeholders;
void function1(std::function<void(int, int)> fun)
{
fun(1, 1);
}
int main (int argc, const char * argv[])
{
...
aClass a;
auto fp = std::bind(&aClass::test, a, _1, _2);
function1(fp);
return 0;
}
A pointer to member function is different from a pointer to function. In order to use a member function through a pointer you need a pointer to it (obviously ) and an object to apply it to. So the appropriate version of function1 would be
void function1(void (aClass::*function)(int, int), aClass& a) {
(a.*function)(1, 1);
}
and to call it:
aClass a; // note: no parentheses; with parentheses it's a function declaration
function1(&aClass::test, a);
Since 2011, if you can change function1, do so, like this:
#include <functional>
#include <cstdio>
using namespace std;
class aClass
{
public:
void aTest(int a, int b)
{
printf("%d + %d = %d", a, b, a + b);
}
};
template <typename Callable>
void function1(Callable f)
{
f(1, 1);
}
void test(int a,int b)
{
printf("%d - %d = %d", a , b , a - b);
}
int main()
{
aClass obj;
// Free function
function1(&test);
// Bound member function
using namespace std::placeholders;
function1(std::bind(&aClass::aTest, obj, _1, _2));
// Lambda
function1([&](int a, int b) {
obj.aTest(a, b);
});
}
(live demo)
Notice also that I fixed your broken object definition (aClass a(); declares a function).
I asked a similar question (C++ openframeworks passing void from other classes) but the answer I found was clearer so here the explanation for future records:
it’s easier to use std::function as in:
void draw(int grid, std::function<void()> element)
and then call as:
grid.draw(12, std::bind(&BarrettaClass::draw, a, std::placeholders::_1));
or even easier:
grid.draw(12, [&]{a.draw()});
where you create a lambda that calls the object capturing it by reference
Important to note that unless you can change the signature of the code taking the function, there is no (easy) way to do this. That would be trying to implement a closure in a language that does not have closures that are the same as functions (the signature for a closure in C++ is different).
There are two actual ways to achieve this:
Use some sort of singleton/global variable that you store the closure in, and then pass a helper function that calls the needed function using that closure. Here is an example:
#include <stdio.h>
template<class C, typename ReturnType, typename... Args>
class ClosureSingleton {
typedef ReturnType (C::*FuncType)(Args...);
public:
static ClosureSingleton& getInstance() {
static ClosureSingleton instance;
return instance;
}
void setClosure(C* obj, FuncType f) {
this->obj = obj;
this->function = f;
}
static ReturnType funcPtr(Args... args) {
C* obj = getInstance().obj;
auto func = getInstance().function;
return (obj->*func)(args...);
}
private:
ClosureSingleton() {}
C* obj;
FuncType function;
public:
ClosureSingleton(ClosureSingleton const&) = delete;
void operator=(ClosureSingleton const&) = delete;
};
class aClass {
public:
void aTest1(int a, int b) { printf("%d + %d = %d\n", a, b, a + b); }
int aTest2(int a, int b) { return a + b; }
};
void function1(void (*function)(int, int)) {
function(1, 1);
}
int function2(int (*function)(int, int)) {
return function(1, 1);
}
int main() {
aClass tmp;
ClosureSingleton<aClass, void, int, int>::getInstance().setClosure(
&tmp, &aClass::aTest1);
function1(&ClosureSingleton<aClass, void, int, int>::funcPtr);
ClosureSingleton<aClass, int, int, int>::getInstance().setClosure(
&tmp, &aClass::aTest2);
printf(
"function2: %d\n",
function2(&ClosureSingleton<aClass, int, int, int>::funcPtr));
return 0;
}
Of course, this has the obvious downside that the closure needs to be set before every call, as well as some thread safety issues. Not ideal, but potentially workable in specific circumstances
Use something like asmjit or dynamic compilation to dynamically compile and pass the function in to the C code. This will only work on machines that allow heap section to be marked as executable. It is also very much non-portable as you will be writing assembly code to accomplish this. However, if you get it working, you will indeed have a true closure, albeit a substantially higher cost to creating the closure compared to how most programming languages implement closures (they do not duplicate the function assembly, instead they use a context object)
Patch the lib/dll that has the function handler to change it's signature to allow a context object. Again, a very brittle and non optimal solution.
My original answer, which does not really answer the question, but people found it useful:
Not sure why this incredibly simple solution has been passed up:
#include <stdio.h>
class aClass
{
public:
void aTest(int a, int b)
{
printf("%d + %d = %d\n", a, b, a + b);
}
};
template<class C>
void function1(void (C::*function)(int, int), C& c)
{
(c.*function)(1, 1);
}
void function1(void (*function)(int, int)) {
function(1, 1);
}
void test(int a,int b)
{
printf("%d - %d = %d\n", a , b , a - b);
}
int main (int argc, const char* argv[])
{
aClass a;
function1(&test);
function1<aClass>(&aClass::aTest, a);
return 0;
}
Output:
1 - 1 = 0
1 + 1 = 2
I made the member function as static and all works:
#include <iostream>
class aClass
{
public:
static void aTest(int a, int b)
{
printf("%d + %d = %d\n", a, b, a + b);
}
};
void function1(int a,int b,void function(int, int))
{
function(a, b);
}
void test(int a,int b)
{
printf("%d - %d = %d\n", a , b , a - b);
}
int main (int argc, const char* argv[])
{
aClass a;
function1(10,12,test);
function1(10,12,a.aTest); // <-- How should I point to a's aClass::test function?
getchar();return 0;
}
If you actually don't need to use the instance a
(i.e. you can make it static like #mathengineer 's answer)
you can simply pass in a non-capture lambda. (which decay to function pointer)
#include <iostream>
class aClass
{
public:
void aTest(int a, int b)
{
printf("%d + %d = %d", a, b, a + b);
}
};
void function1(void (*function)(int, int))
{
function(1, 1);
}
int main()
{
//note: you don't need the `+`
function1(+[](int a,int b){return aClass{}.aTest(a,b);});
}
Wandbox
note: if aClass is costly to construct or has side effect, this may not be a good way.
You can stop banging your heads now. Here is the wrapper for the member function to support existing functions taking in plain C functions as arguments. thread_local directive is the key here.
http://cpp.sh/9jhk3
// Example program
#include <iostream>
#include <string>
using namespace std;
typedef int FooCooker_ (int);
// Existing function
extern "C" void cook_10_foo (FooCooker_ FooCooker) {
cout << "Cooking 10 Foo ..." << endl;
cout << "FooCooker:" << endl;
FooCooker (10);
}
struct Bar_ {
Bar_ (int Foo = 0) : Foo (Foo) {};
int cook (int Foo) {
cout << "This Bar got " << this->Foo << endl;
if (this->Foo >= Foo) {
this->Foo -= Foo;
cout << Foo << " cooked" << endl;
return Foo;
} else {
cout << "Can't cook " << Foo << endl;
return 0;
}
}
int Foo = 0;
};
// Each Bar_ object and a member function need to define
// their own wrapper with a global thread_local object ptr
// to be called as a plain C function.
thread_local static Bar_* Bar1Ptr = NULL;
static int cook_in_Bar1 (int Foo) {
return Bar1Ptr->cook (Foo);
}
thread_local static Bar_* Bar2Ptr = NULL;
static int cook_in_Bar2 (int Foo) {
return Bar2Ptr->cook (Foo);
}
int main () {
Bar1Ptr = new Bar_ (20);
cook_10_foo (cook_in_Bar1);
Bar2Ptr = new Bar_ (40);
cook_10_foo (cook_in_Bar2);
delete Bar1Ptr;
delete Bar2Ptr;
return 0;
}
Please comment on any issues with this approach.
Other answers fail to call existing plain C functions: http://cpp.sh/8exun
I have a C-style function, which stores another function as an argument. I also have an object, which stores a method that must be passed to the aforementioned function. I built an example, to simulate the desired situation:
#include <functional>
#include <iostream>
void foo(void(*f)(int)) {
f(2);
}
class TestClass {
public:
std::function<void(int)> f;
void foo(int i) {
std::cout << i << "\n";
}
};
int main() {
TestClass t;
t.f = std::bind(&TestClass::foo, &t, std::placeholders::_1);
foo( t.f.target<void(int)>() );
return 0;
}
What is expected is that it will be shown on screen "2". But I'm having trouble compiling the code, getting the following message on the compiler:
error: const_cast to 'void *(*)(int)', which is not a reference, pointer-to-object, or pointer-to-data-member
return const_cast<_Functor*>(__func);
As I understand the use of "target", it should return a pointer in the format void () (int), related to the desired function through std :: bind. Why didn't the compiler understand it that way, and if it is not possible to use "target" to apply what I want, what would be the alternatives? I don't necessarily need to use std :: function, but I do need the method to be non-static.
This is a dirty little hack but should work
void foo(void(*f)(int)) {
f(2);
}
class TestClass {
public:
void foo(int i) {
std::cout << i << "\n";
}
};
static TestClass* global_variable_hack = nullptr;
void hacky_function(int x) {
global_variable_hack->foo(x);
}
int main() {
TestClass t;
global_variable_hack = &t;
foo(hacky_function);
return 0;
}
//can also be done with a lambda without the global stuff
int main() {
static TestClass t;
auto func = [](int x) {
t->foo(x); //does not need to be captured as it is static
};
foo(func); //non-capturing lambas are implicitly convertible to free functions
}
My case is similar to another Question.
I would like to pass a function as argument and an integer value. Testing the case with a simplfied construction:
void print (int x, int y)
{ cout << "x = " << x << ", y = " << y << endl; }
void *interrupt (struct arg_struct args);
struct arg_struct {
void (*func)(int,int); // Argument 1
int a; // Argument 2
int b; // Argument 3
};
int _tmain(int argc, _TCHAR* argv[]){
int a = 1700, b = 1000;
struct arg_struct arguments;
arguments.func = print;
arguments.a = a;
arguments.b = b;
(*interrupt)(arguments);
cin.get(); return 0;
}
void *interrupt (struct arg_struct args) {
void (*func) (int,int) ;
func = args.func;
int x = args.a;
int y = args.b;
(*func)(x,y);
return 0; // Erfordert Rückgabewert
}
So now I want to create a thread to execute this passed function.
void *InThread_func(struct arg_struct *); // Prototype
struct arg_struct {
void (*func)(int); // Argument 1
int IntNumber; // Argument 2
};
int InThread(void (*func)(int), int IntNumber){
struct arg_struct arguments;
arguments.func = func;
arguments.IntNumber = IntNumber;
// Initialize handler
pthread_t InThread_thread;
// Create thread
pthread_create(&InThread_thread,NULL,&InThread_func,(void*) &arguments);
pthread_join(InThread_func,NULL);
return(errno);
}
Using
g++-4.6 -o test test.cpp
the compiler says
invalid conversion from void* (*)(arg_struct*) to void * (*)(void*)
referring to the last argument of pthread_create.
Why is that?
C++ is picky when it comes to casting.
Replace void *InThread_func(struct arg_struct *); by void *InThread_func(void *my_data); and it should solve the problem.
Since this is C++ I'd recommend using std::thread is those are available to you.
"Why is that?" Because you have an invalid conversion from
void* (*)( arg_struct* ) to void* (*)( void* ), perhaps. The third
argument to pthread_create (not the last) must be an
extern "C" void* (*)( void* ). (Some compilers will ignore the
necessity of the extern "C". They are broken in this regard.) So
your InThread_fnc (which I can't find in your code) must be something
like:
extern "C" void*
InThread_fnc( void* from_pthread_create )
{
arg_struct const* p = static_cast< arg_struct const* >( from_pthread_create );
(*p->func)( p->IntNumber );
return nullptr;
}
Of course, this only works if the last argument of pthread_create was
a arg_struct*. Which corresponds to your case, but beware if you
start deriving: passing a new Derived or a &someDerived when the
function you start casts to Base* results in undefined behavior.
I want the Windows thread pool (QueueUserWorkItem()) to call my class' member functions.
Unfortunately this cannot be done directly by passing a member function pointer as an argument to QueueUserWorkItem().
What makes it difficult is that more than one member function must be callable and they have different signatures (all return void though).
One probably need to add a few layers of abstraction to get this to work, but I'm not sure how to approach this. Any ideas?
This might help.
You can use tr1::function () and tr1::bind to "coalesce" various calls:
#include <iostream>
#include <tr1/functional>
using namespace std;
using namespace tr1;
class A
{
public:
void function(int i) { cout << "Called A::function with i=" << i << endl; }
};
void different_function(double c) {
cout << "Called different_function with c=" << c << endl;
}
int main(int argc, char* argv[])
{
function<void()> f = bind(different_function, 3.14165);
f();
A a;
f = bind(&A::function, a, 10);
f();
return 0;
}
The address of the function object can be passed as a single callable object (needing only one address).
Example:
In your class add:
char m_FuncToCall;
static DWORD __stdcall myclass::ThreadStartRoutine(LPVOID myclassref)
{
myclass* _val = (myclass*)myclassref;
switch(m_FuncToCall)
{
case 0:
_val->StartMyOperation();
break;
}
return 0;
}
Make a member for adding to queue then
void myclass::AddToQueue(char funcId)
{
m_FuncToCall=funcId;
QueueUserWorkItem(ThreadStartRoutine,this,WT_EXECUTEDEFAULT);
}
or create
typedef void (*MY_FUNC)(void);
typedef struct _ARGUMENT_TO_PASS
{
myclass* classref;
MY_FUNC func;
}ARGUMENT_TO_PASS;
and then
void myclass::AddToQueue(MY_FUNC func)
{
ARGUMENT_TO_PASS _arg;
_arg.func = func;
_arg.classref = this;
QueueUserWorkItem(ThreadStartRoutine,&_arg,WT_EXECUTEDEFAULT);
}
If you need further explanation feel free to ask :)
EDIT: You'll need to change the ThreadStartRoutine for the second example
and you can also change the struct to hold the passing argument
i want to do this simple piece of code work.
#include <iostream>
#include <windows.h>
void printSome (int i)
{
std::cout << i << std::endl;
}
void spawnThread (void (*threadName)(int i))
{
CreateThread
(
0, // default security attributes
0, // use default stack size
(LPTHREAD_START_ROUTINE)threadName, // thread function name
(LPVOID)i, // argument to thread function
0, // use default creation flags
0 // returns the thread identifier
);
}
int main ()
{
spawnThread(printSome(155));
}
i am on windows, using vs. Any help will be greatly appriciated.
CreateThread wants 2 arguments: pointer to the function to execute as a thread, and a DWORD argument that will be given to the thread. your spawnThread() function only has 1 argument (threadName); you think it has 2 args because of the "i", but that is really part of the definition of the "threadName" type. (you could just as well leave out the "i"; that is, you don't need to name the arguments to "threadName".)
anyway, given that you NEED 2 arguments, redefine spawnThread:
void spawnThread(void (*threadEntryPoint)(int), int argument)
{
CreateThread(0,0,
(LPTHREAD_START_ROUTINE)threadEntryPoint,
(LPVOID)argument,
0,0);
}
notice that i did not name the int argument to the threadEntryPoint; it is sufficient to tell the compiler that the function must have a single int argument.
and call it:
spawnThread(printSome, 155);
anyway, quick and dirty, this will do what you want.
hth.
reilly.
Personally, I wouldn't consider passing in a function pointer like you are trying to do as very C++ like. That's coding C in C++
Instead, I'd wrap that thing in a class. The big advantage there is you can just override the class to have however many members you want, rather than having to perform greazy casting tricks to get at your parameters every time.
The code's a little long-winded, so I pushed it to the end. But what it lets you do is something like this:
class print_some : public basic_thread {
private:
int i;
public:
print_some (int i) : i(i) {};
action_callback () {
std::cout << i << std::endl;
}
}
int main () {
print_some printer (155);
}
Here's some exerpted example code from one of our classes that does this:
class basic_thread :
{
public:
basic_thread();
protected:
unsigned long m_ThreadId;
virtual void action_callback () {};
// Internal routine used to bridge between OS callback format and
// action_callback. *Must* be static for the OS.
static unsigned long __stdcall self_calling_callback (void *parameter);
}
...and in the .cpp:
unsigned long __stdcall basic_thread::self_calling_callback (void *parameter) {
if (parameter) {
basic_thread * thread = reinterpret_cast<basic_thread *>(parameter);
thread->action_callback();
}
return 0; // The value returned only matters if someone starts calling GetExitCodeThread
// to retrieve it.
}
basic_thread::basic_thread () {
// Start thread.
m_Handle = CreateThread(NULL,
0,
self_calling_callback,
(PVOID)this,
0,
&m_ThreadId );
if( !IsHandleValid() )
throw StartException("CreateThread() failed", GetLastError());
}
You cannot pass parameter information in a function pointer; it must be passed separately. That is exactly why the CreateThread function provides a void* parameter that can point to whatever you want.
Additionally, you should use _beginthread instead of CreateThread for C++ applications.
Finally, your program is more than likely to terminate before the thread ever runs. Therefore, you must either enter an indefinite loop or use an API call to wait for the thread to finish.
The following is a working version using WaitForSingleObject to block until the thread completes.
#include <iostream>
#include <process.h>
#include <windows.h>
void
printSome(int i)
{
std::cout << i << std::endl;
}
HANDLE
spawnThread(void (*threadName)(int), int i)
{
return (HANDLE) _beginthread((void (*)(void*)) threadName, 0, (LPVOID) i);
}
int
main(int argc, char *argv[])
{
HANDLE threadHandle;
threadHandle = spawnThread(printSome, 155);
WaitForSingleObject(threadHandle, INFINITE);
return 0;
}
Here is a much more C++/object-oriented way of handling this same situation:
#include <iostream>
#include <process.h>
#include <windows.h>
class Thread {
static void proxy(void *arg) { (*(reinterpret_cast<Thread *> (arg)))(); }
HANDLE thread_;
public:
virtual ~Thread() {}
virtual void operator()() = 0;
void start() { thread_ = (HANDLE) _beginthread(Thread::proxy, 0, this);}
void waitForExit() { WaitForSingleObject(thread_, INFINITE); }
};
class Printer : public Thread {
int i_;
public:
Printer(int i) : i_(i) {}
void operator()() { std::cout << i_ << std::endl; }
};
int
main(int argc, char *argv[])
{
Printer p(155);
p.start();
p.waitForExit();
return 0;
}
As many people already mentioned here, you can't pass a function pointer and the argument it should be called with in one parameter.
Your line
spawnThread(printSome(155));
"should" (in DWIM world) mean "Invoke printSome on a separate thread with argument 155". However, it's not how C++ understands it. C++ sees "Pass the result of printSome invoked on 155 as a parameter to spawnThread". In other words, the sequence of steps is:
call prinotSome with 155 as argument. Store it in temporary memory.
call spawnThread with the contents of temporary memory as its argument.
In order to do what you really mean, you have to humor C++ and separate argument from function. How to do it is already explained in other answers. The short of it is:
callOnOtherThreadWithArgument(function, integer);
You can read how you do that here: http://www.newty.de/fpt/fpt.html
2.6 How to Pass a Function Pointer as an Argument ?
You can pass a function pointer as a
function's calling argument. You need
this for example if you want to pass a
pointer to a callback function. The
following code shows how to pass a
pointer to a function which returns an
int and takes a float and two char:
//------------------------------------------------------------------------------------
// 2.6 How to Pass a Function Pointer
// <pt2Func> is a pointer to a function which returns an int and takes a float and two char
void PassPtr(int (*pt2Func)(float, char, char))
{
int result = (*pt2Func)(12, 'a', 'b'); // call using function pointer
cout << result << endl;
}
// execute example code - 'DoIt' is a suitable function like defined above in 2.1-4
void Pass_A_Function_Pointer()
{
cout << endl << "Executing 'Pass_A_Function_Pointer'" << endl;
PassPtr(&DoIt);
}