Related
How do I obtain a function pointer for a class member function, and later call that member function with a specific object? I’d like to write:
class Dog : Animal
{
Dog ();
void bark ();
}
…
Dog* pDog = new Dog ();
BarkFunction pBark = &Dog::bark;
(*pBark) (pDog);
…
Also, if possible, I’d like to invoke the constructor via a pointer as well:
NewAnimalFunction pNew = &Dog::Dog;
Animal* pAnimal = (*pNew)();
Is this possible, and if so, what is the preferred way to do this?
Read this for detail :
// 1 define a function pointer and initialize to NULL
int (TMyClass::*pt2ConstMember)(float, char, char) const = NULL;
// C++
class TMyClass
{
public:
int DoIt(float a, char b, char c){ cout << "TMyClass::DoIt"<< endl; return a+b+c;};
int DoMore(float a, char b, char c) const
{ cout << "TMyClass::DoMore" << endl; return a-b+c; };
/* more of TMyClass */
};
pt2ConstMember = &TMyClass::DoIt; // note: <pt2Member> may also legally point to &DoMore
// Calling Function using Function Pointer
(*this.*pt2ConstMember)(12, 'a', 'b');
How do I obtain a function pointer for a class member function, and later call that member function with a specific object?
It's easiest to start with a typedef. For a member function, you add the classname in the type declaration:
typedef void(Dog::*BarkFunction)(void);
Then to invoke the method, you use the ->* operator:
(pDog->*pBark)();
Also, if possible, I’d like to invoke the constructor via a pointer as well. Is this possible, and if so, what is the preferred way to do this?
I don't believe you can work with constructors like this - ctors and dtors are special. The normal way to achieve that sort of thing would be using a factory method, which is basically just a static function that calls the constructor for you. See the code below for an example.
I have modified your code to do basically what you describe. There's some caveats below.
#include <iostream>
class Animal
{
public:
typedef Animal*(*NewAnimalFunction)(void);
virtual void makeNoise()
{
std::cout << "M00f!" << std::endl;
}
};
class Dog : public Animal
{
public:
typedef void(Dog::*BarkFunction)(void);
typedef Dog*(*NewDogFunction)(void);
Dog () {}
static Dog* newDog()
{
return new Dog;
}
virtual void makeNoise ()
{
std::cout << "Woof!" << std::endl;
}
};
int main(int argc, char* argv[])
{
// Call member function via method pointer
Dog* pDog = new Dog ();
Dog::BarkFunction pBark = &Dog::makeNoise;
(pDog->*pBark)();
// Construct instance via factory method
Dog::NewDogFunction pNew = &Dog::newDog;
Animal* pAnimal = (*pNew)();
pAnimal->makeNoise();
return 0;
}
Now although you can normally use a Dog* in the place of an Animal* thanks to the magic of polymorphism, the type of a function pointer does not follow the lookup rules of class hierarchy. So an Animal method pointer is not compatible with a Dog method pointer, in other words you can't assign a Dog* (*)() to a variable of type Animal* (*)().
The static newDog method is a simple example of a factory, which simply creates and returns new instances. Being a static function, it has a regular typedef (with no class qualifier).
Having answered the above, I do wonder if there's not a better way of achieving what you need. There's a few specific scenarios where you would do this sort of thing, but you might find there's other patterns that work better for your problem. If you describe in more general terms what you are trying to achieve, the hive-mind may prove even more useful!
Related to the above, you will no doubt find the Boost bind library and other related modules very useful.
I don't think anyone has explained here that one issue is that you need "member pointers" rather than normal function pointers.
Member pointers to functions are not simply function pointers. In implementation terms, the compiler cannot use a simple function address because, in general, you don't know the address to call until you know which object to dereference for (think virtual functions). You also need to know the object in order to provide the this implicit parameter, of course.
Having said that you need them, now I'll say that you really need to avoid them. Seriously, member pointers are a pain. It is much more sane to look at object-oriented design patterns that achieve the same goal, or to use a boost::function or whatever as mentioned above - assuming you get to make that choice, that is.
If you are supplying that function pointer to existing code, so you really need a simple function pointer, you should write a function as a static member of the class. A static member function doesn't understand this, so you'll need to pass the object in as an explicit parameter. There was once a not-that-unusual idiom along these lines for working with old C code that needs function pointers
class myclass
{
public:
virtual void myrealmethod () = 0;
static void myfunction (myclass *p);
}
void myclass::myfunction (myclass *p)
{
p->myrealmethod ();
}
Since myfunction is really just a normal function (scope issues aside), a function pointer can be found in the normal C way.
EDIT - this kind of method is called a "class method" or a "static member function". The main difference from a non-member function is that, if you reference it from outside the class, you must specify the scope using the :: scope resolution operator. For example, to get the function pointer, use &myclass::myfunction and to call it use myclass::myfunction (arg);.
This kind of thing is fairly common when using the old Win32 APIs, which were originally designed for C rather than C++. Of course in that case, the parameter is normally LPARAM or similar rather than a pointer, and some casting is needed.
typedef void (Dog::*memfun)();
memfun doSomething = &Dog::bark;
....
(pDog->*doSomething)(); // if pDog is a pointer
// (pDog.*doSomething)(); // if pDog is a reference
Minimal runnable example
main.cpp
#include <cassert>
class C {
public:
int i;
C(int i) : i(i) {}
int m(int j) { return this->i + j; }
};
int main() {
// Get a method pointer.
int (C::*p)(int) = &C::m;
// Create a test object.
C c(1);
C *cp = &c;
// Operator .*
assert((c.*p)(2) == 3);
// Operator ->*
assert((cp->*p)(2) == 3);
}
Compile and run:
g++ -ggdb3 -O0 -std=c++11 -Wall -Wextra -pedantic -o main.out main.cpp
./main.out
Tested in Ubuntu 18.04.
You cannot change the order of the parenthesis or omit them. The following do not work:
c.*p(2)
c.*(p)(2)
GCC 9.2 would fail with:
main.cpp: In function ‘int main()’:
main.cpp:19:18: error: must use ‘.*’ or ‘->*’ to call pointer-to-member function in ‘p (...)’, e.g. ‘(... ->* p) (...)’
19 | assert(c.*p(2) == 3);
|
C++11 standard
.* and ->* are a single operators introduced in C++ for this purpose, and not present in C.
C++11 N3337 standard draft:
2.13 "Operators and punctuators" has a list of all operators, which contains .* and ->*.
5.5 "Pointer-to-member operators" explains what they do
I came here to learn how to create a function pointer (not a method pointer) from a method but none of the answers here provide a solution. Here is what I came up with:
template <class T> struct MethodHelper;
template <class C, class Ret, class... Args> struct MethodHelper<Ret (C::*)(Args...)> {
using T = Ret (C::*)(Args...);
template <T m> static Ret call(C* object, Args... args) {
return (object->*m)(args...);
}
};
#define METHOD_FP(m) MethodHelper<decltype(m)>::call<m>
So for your example you would now do:
Dog dog;
using BarkFunction = void (*)(Dog*);
BarkFunction bark = METHOD_FP(&Dog::bark);
(*bark)(&dog); // or simply bark(&dog)
Edit:
Using C++17, there is an even better solution:
template <auto m> struct MethodHelper;
template <class C, class Ret, class... Args, Ret (C::*m)(Args...)> struct MethodHelper<m> {
static Ret call(C* object, Args... args) {
return (object->*m)(args...);
}
};
which can be used directly without the macro:
Dog dog;
using BarkFunction = void (*)(Dog*);
BarkFunction bark = MethodHelper<&Dog::bark>::call;
(*bark)(&dog); // or simply bark(&dog)
For methods with modifiers like const you might need some more specializations like:
template <class C, class Ret, class... Args, Ret (C::*m)(Args...) const> struct MethodHelper<m> {
static Ret call(const C* object, Args... args) {
return (object->*m)(args...);
}
};
A function pointer to a class member is a problem that is really suited to using boost::function. Small example:
#include <boost/function.hpp>
#include <iostream>
class Dog
{
public:
Dog (int i) : tmp(i) {}
void bark ()
{
std::cout << "woof: " << tmp << std::endl;
}
private:
int tmp;
};
int main()
{
Dog* pDog1 = new Dog (1);
Dog* pDog2 = new Dog (2);
//BarkFunction pBark = &Dog::bark;
boost::function<void (Dog*)> f1 = &Dog::bark;
f1(pDog1);
f1(pDog2);
}
Reason why you cannot use function pointers to call member functions is that
ordinary function pointers are usually just the memory address of the function.
To call a member function, you need to know two things:
Which member function to call
Which instance should be used (whose member function)
Ordinary function pointers cannot store both. C++ member function pointers are used
to store a), which is why you need to specify the instance explicitly when calling a member function pointer.
To create a new object you can either use placement new, as mentioned above, or have your class implement a clone() method that creates a copy of the object. You can then call this clone method using a member function pointer as explained above to create new instances of the object. The advantage of clone is that sometimes you may be working with a pointer to a base class where you don't know the type of the object. In this case a clone() method can be easier to use. Also, clone() will let you copy the state of the object if that is what you want.
I did this with std::function and std::bind..
I wrote this EventManager class that stores a vector of handlers in an unordered_map that maps event types (which are just const unsigned int, I have a big namespace-scoped enum of them) to a vector of handlers for that event type.
In my EventManagerTests class, I set up an event handler, like this:
auto delegate = std::bind(&EventManagerTests::OnKeyDown, this, std::placeholders::_1);
event_manager.AddEventListener(kEventKeyDown, delegate);
Here's the AddEventListener function:
std::vector<EventHandler>::iterator EventManager::AddEventListener(EventType _event_type, EventHandler _handler)
{
if (listeners_.count(_event_type) == 0)
{
listeners_.emplace(_event_type, new std::vector<EventHandler>());
}
std::vector<EventHandler>::iterator it = listeners_[_event_type]->end();
listeners_[_event_type]->push_back(_handler);
return it;
}
Here's the EventHandler type definition:
typedef std::function<void(Event *)> EventHandler;
Then back in EventManagerTests::RaiseEvent, I do this:
Engine::KeyDownEvent event(39);
event_manager.RaiseEvent(1, (Engine::Event*) & event);
Here's the code for EventManager::RaiseEvent:
void EventManager::RaiseEvent(EventType _event_type, Event * _event)
{
if (listeners_.count(_event_type) > 0)
{
std::vector<EventHandler> * vec = listeners_[_event_type];
std::for_each(
begin(*vec),
end(*vec),
[_event](EventHandler handler) mutable
{
(handler)(_event);
}
);
}
}
This works. I get the call in EventManagerTests::OnKeyDown. I have to delete the vectors come clean up time, but once I do that there are no leaks. Raising an event takes about 5 microseconds on my computer, which is circa 2008. Not exactly super fast, but. Fair enough as long as I know that and I don't use it in ultra hot code.
I'd like to speed it up by rolling my own std::function and std::bind, and maybe using an array of arrays rather than an unordered_map of vectors, but I haven't quite figured out how to store a member function pointer and call it from code that knows nothing about the class being called. Eyelash's answer looks Very Interesting..
How do I obtain a function pointer for a class member function, and later call that member function with a specific object? I’d like to write:
class Dog : Animal
{
Dog ();
void bark ();
}
…
Dog* pDog = new Dog ();
BarkFunction pBark = &Dog::bark;
(*pBark) (pDog);
…
Also, if possible, I’d like to invoke the constructor via a pointer as well:
NewAnimalFunction pNew = &Dog::Dog;
Animal* pAnimal = (*pNew)();
Is this possible, and if so, what is the preferred way to do this?
Read this for detail :
// 1 define a function pointer and initialize to NULL
int (TMyClass::*pt2ConstMember)(float, char, char) const = NULL;
// C++
class TMyClass
{
public:
int DoIt(float a, char b, char c){ cout << "TMyClass::DoIt"<< endl; return a+b+c;};
int DoMore(float a, char b, char c) const
{ cout << "TMyClass::DoMore" << endl; return a-b+c; };
/* more of TMyClass */
};
pt2ConstMember = &TMyClass::DoIt; // note: <pt2Member> may also legally point to &DoMore
// Calling Function using Function Pointer
(*this.*pt2ConstMember)(12, 'a', 'b');
How do I obtain a function pointer for a class member function, and later call that member function with a specific object?
It's easiest to start with a typedef. For a member function, you add the classname in the type declaration:
typedef void(Dog::*BarkFunction)(void);
Then to invoke the method, you use the ->* operator:
(pDog->*pBark)();
Also, if possible, I’d like to invoke the constructor via a pointer as well. Is this possible, and if so, what is the preferred way to do this?
I don't believe you can work with constructors like this - ctors and dtors are special. The normal way to achieve that sort of thing would be using a factory method, which is basically just a static function that calls the constructor for you. See the code below for an example.
I have modified your code to do basically what you describe. There's some caveats below.
#include <iostream>
class Animal
{
public:
typedef Animal*(*NewAnimalFunction)(void);
virtual void makeNoise()
{
std::cout << "M00f!" << std::endl;
}
};
class Dog : public Animal
{
public:
typedef void(Dog::*BarkFunction)(void);
typedef Dog*(*NewDogFunction)(void);
Dog () {}
static Dog* newDog()
{
return new Dog;
}
virtual void makeNoise ()
{
std::cout << "Woof!" << std::endl;
}
};
int main(int argc, char* argv[])
{
// Call member function via method pointer
Dog* pDog = new Dog ();
Dog::BarkFunction pBark = &Dog::makeNoise;
(pDog->*pBark)();
// Construct instance via factory method
Dog::NewDogFunction pNew = &Dog::newDog;
Animal* pAnimal = (*pNew)();
pAnimal->makeNoise();
return 0;
}
Now although you can normally use a Dog* in the place of an Animal* thanks to the magic of polymorphism, the type of a function pointer does not follow the lookup rules of class hierarchy. So an Animal method pointer is not compatible with a Dog method pointer, in other words you can't assign a Dog* (*)() to a variable of type Animal* (*)().
The static newDog method is a simple example of a factory, which simply creates and returns new instances. Being a static function, it has a regular typedef (with no class qualifier).
Having answered the above, I do wonder if there's not a better way of achieving what you need. There's a few specific scenarios where you would do this sort of thing, but you might find there's other patterns that work better for your problem. If you describe in more general terms what you are trying to achieve, the hive-mind may prove even more useful!
Related to the above, you will no doubt find the Boost bind library and other related modules very useful.
I don't think anyone has explained here that one issue is that you need "member pointers" rather than normal function pointers.
Member pointers to functions are not simply function pointers. In implementation terms, the compiler cannot use a simple function address because, in general, you don't know the address to call until you know which object to dereference for (think virtual functions). You also need to know the object in order to provide the this implicit parameter, of course.
Having said that you need them, now I'll say that you really need to avoid them. Seriously, member pointers are a pain. It is much more sane to look at object-oriented design patterns that achieve the same goal, or to use a boost::function or whatever as mentioned above - assuming you get to make that choice, that is.
If you are supplying that function pointer to existing code, so you really need a simple function pointer, you should write a function as a static member of the class. A static member function doesn't understand this, so you'll need to pass the object in as an explicit parameter. There was once a not-that-unusual idiom along these lines for working with old C code that needs function pointers
class myclass
{
public:
virtual void myrealmethod () = 0;
static void myfunction (myclass *p);
}
void myclass::myfunction (myclass *p)
{
p->myrealmethod ();
}
Since myfunction is really just a normal function (scope issues aside), a function pointer can be found in the normal C way.
EDIT - this kind of method is called a "class method" or a "static member function". The main difference from a non-member function is that, if you reference it from outside the class, you must specify the scope using the :: scope resolution operator. For example, to get the function pointer, use &myclass::myfunction and to call it use myclass::myfunction (arg);.
This kind of thing is fairly common when using the old Win32 APIs, which were originally designed for C rather than C++. Of course in that case, the parameter is normally LPARAM or similar rather than a pointer, and some casting is needed.
typedef void (Dog::*memfun)();
memfun doSomething = &Dog::bark;
....
(pDog->*doSomething)(); // if pDog is a pointer
// (pDog.*doSomething)(); // if pDog is a reference
Minimal runnable example
main.cpp
#include <cassert>
class C {
public:
int i;
C(int i) : i(i) {}
int m(int j) { return this->i + j; }
};
int main() {
// Get a method pointer.
int (C::*p)(int) = &C::m;
// Create a test object.
C c(1);
C *cp = &c;
// Operator .*
assert((c.*p)(2) == 3);
// Operator ->*
assert((cp->*p)(2) == 3);
}
Compile and run:
g++ -ggdb3 -O0 -std=c++11 -Wall -Wextra -pedantic -o main.out main.cpp
./main.out
Tested in Ubuntu 18.04.
You cannot change the order of the parenthesis or omit them. The following do not work:
c.*p(2)
c.*(p)(2)
GCC 9.2 would fail with:
main.cpp: In function ‘int main()’:
main.cpp:19:18: error: must use ‘.*’ or ‘->*’ to call pointer-to-member function in ‘p (...)’, e.g. ‘(... ->* p) (...)’
19 | assert(c.*p(2) == 3);
|
C++11 standard
.* and ->* are a single operators introduced in C++ for this purpose, and not present in C.
C++11 N3337 standard draft:
2.13 "Operators and punctuators" has a list of all operators, which contains .* and ->*.
5.5 "Pointer-to-member operators" explains what they do
I came here to learn how to create a function pointer (not a method pointer) from a method but none of the answers here provide a solution. Here is what I came up with:
template <class T> struct MethodHelper;
template <class C, class Ret, class... Args> struct MethodHelper<Ret (C::*)(Args...)> {
using T = Ret (C::*)(Args...);
template <T m> static Ret call(C* object, Args... args) {
return (object->*m)(args...);
}
};
#define METHOD_FP(m) MethodHelper<decltype(m)>::call<m>
So for your example you would now do:
Dog dog;
using BarkFunction = void (*)(Dog*);
BarkFunction bark = METHOD_FP(&Dog::bark);
(*bark)(&dog); // or simply bark(&dog)
Edit:
Using C++17, there is an even better solution:
template <auto m> struct MethodHelper;
template <class C, class Ret, class... Args, Ret (C::*m)(Args...)> struct MethodHelper<m> {
static Ret call(C* object, Args... args) {
return (object->*m)(args...);
}
};
which can be used directly without the macro:
Dog dog;
using BarkFunction = void (*)(Dog*);
BarkFunction bark = MethodHelper<&Dog::bark>::call;
(*bark)(&dog); // or simply bark(&dog)
For methods with modifiers like const you might need some more specializations like:
template <class C, class Ret, class... Args, Ret (C::*m)(Args...) const> struct MethodHelper<m> {
static Ret call(const C* object, Args... args) {
return (object->*m)(args...);
}
};
A function pointer to a class member is a problem that is really suited to using boost::function. Small example:
#include <boost/function.hpp>
#include <iostream>
class Dog
{
public:
Dog (int i) : tmp(i) {}
void bark ()
{
std::cout << "woof: " << tmp << std::endl;
}
private:
int tmp;
};
int main()
{
Dog* pDog1 = new Dog (1);
Dog* pDog2 = new Dog (2);
//BarkFunction pBark = &Dog::bark;
boost::function<void (Dog*)> f1 = &Dog::bark;
f1(pDog1);
f1(pDog2);
}
Reason why you cannot use function pointers to call member functions is that
ordinary function pointers are usually just the memory address of the function.
To call a member function, you need to know two things:
Which member function to call
Which instance should be used (whose member function)
Ordinary function pointers cannot store both. C++ member function pointers are used
to store a), which is why you need to specify the instance explicitly when calling a member function pointer.
To create a new object you can either use placement new, as mentioned above, or have your class implement a clone() method that creates a copy of the object. You can then call this clone method using a member function pointer as explained above to create new instances of the object. The advantage of clone is that sometimes you may be working with a pointer to a base class where you don't know the type of the object. In this case a clone() method can be easier to use. Also, clone() will let you copy the state of the object if that is what you want.
I did this with std::function and std::bind..
I wrote this EventManager class that stores a vector of handlers in an unordered_map that maps event types (which are just const unsigned int, I have a big namespace-scoped enum of them) to a vector of handlers for that event type.
In my EventManagerTests class, I set up an event handler, like this:
auto delegate = std::bind(&EventManagerTests::OnKeyDown, this, std::placeholders::_1);
event_manager.AddEventListener(kEventKeyDown, delegate);
Here's the AddEventListener function:
std::vector<EventHandler>::iterator EventManager::AddEventListener(EventType _event_type, EventHandler _handler)
{
if (listeners_.count(_event_type) == 0)
{
listeners_.emplace(_event_type, new std::vector<EventHandler>());
}
std::vector<EventHandler>::iterator it = listeners_[_event_type]->end();
listeners_[_event_type]->push_back(_handler);
return it;
}
Here's the EventHandler type definition:
typedef std::function<void(Event *)> EventHandler;
Then back in EventManagerTests::RaiseEvent, I do this:
Engine::KeyDownEvent event(39);
event_manager.RaiseEvent(1, (Engine::Event*) & event);
Here's the code for EventManager::RaiseEvent:
void EventManager::RaiseEvent(EventType _event_type, Event * _event)
{
if (listeners_.count(_event_type) > 0)
{
std::vector<EventHandler> * vec = listeners_[_event_type];
std::for_each(
begin(*vec),
end(*vec),
[_event](EventHandler handler) mutable
{
(handler)(_event);
}
);
}
}
This works. I get the call in EventManagerTests::OnKeyDown. I have to delete the vectors come clean up time, but once I do that there are no leaks. Raising an event takes about 5 microseconds on my computer, which is circa 2008. Not exactly super fast, but. Fair enough as long as I know that and I don't use it in ultra hot code.
I'd like to speed it up by rolling my own std::function and std::bind, and maybe using an array of arrays rather than an unordered_map of vectors, but I haven't quite figured out how to store a member function pointer and call it from code that knows nothing about the class being called. Eyelash's answer looks Very Interesting..
I am trying to write a class that I can store and use type information in without the need for a template parameter.
I want to write something like this:
class Example
{
public:
template<typename T>
Example(T* ptr)
: ptr(ptr)
{
// typedef T EnclosedType; I want this be a avaialable at the class level.
}
void operator()()
{
if(ptr == NULL)
return;
(*(EnclosedType*)ptr)(); // so i can cast the pointer and call the () operator if the class has one.
}
private:
void* ptr;
}
I am not asking how to write an is_functor() class.
I want to know how to get type information in a constructor and store it at the class level. If that is impossible, a different solution to this would be appreciated.
I consider this as a good and valid question, however, there is no general solution beside using a template parameter at the class level. What you tried to achieve in your question -- using a typedef inside a function and then access this in the whole class -- is not possible.
Type erasure
Only if you impose certain restrictions onto your constructor parameters, there are some alternatives. In this respect, here is an example of type erasure where the operator() of some given object is stored inside a std::function<void()> variable.
struct A
{
template<typename T>
A(T const& t) : f (std::bind(&T::operator(), t)) {}
void operator()() const
{
f();
}
std::function<void()> f;
};
struct B
{
void operator()() const
{
std::cout<<"hello"<<std::endl;
}
};
int main()
{
A(B{}).operator()(); //prints "hello"
}
DEMO
Note, however, the assumptions underlying this approach: one assumes that all passed objects have an operator of a given signature (here void operator()) which is stored inside a std::function<void()> (with respect to storing the member-function, see here).
Inheritance
In a sense, type erasure is thus like "inheriting without a base class" -- one could instead use a common base class for all constructor parameter classes with a virtual bracket operator, and then pass a base class pointer to your constructor.
struct A_parameter_base
{
void operator()() const = 0;
};
struct B : public A_parameter_base
{
void operator()() const { std::cout<<"hello"<<std::endl; }
};
struct A
{
A(std::shared_ptr<A_parameter_base> _p) : p(_p) {}
void operator()()
{
p->operator();
}
std::shared_ptr<A_parameter_base> p;
}
That is similar to the code in your question, only that it does not use a void-pointer but a pointer to a specific base class.
Both approaches, type erasure and inheritance, are similar in their applications, but type erasure might be more convenient as one gets rid of a common base class. However, the inheritance approach has the further advantage that you can restore the original object via multiple dispatch
This also shows the limitations of both approaches. If your operator would not be void but instead would return some unknown varying type, you cannot use the above approach but have to use templates. The inheritance parallel is: you cannot have a virtual function template.
The practical answer is to store either a copy of your class, or a std::ref wrapped pseudo-reference to your class, in a std::function<void()>.
std::function type erases things it stores down to 3 concepts: copy, destroy and invoke with a fixed signature. (also, cast-back-to-original-type and typeid, more obscurely)
What it does is it remembers, at construction, how to do these operations to the passed in type, and stores a copy in a way it can perform those operations on it, then forgets everything else about the type.
You cannot remember everything about a type this way. But almost any operation with a fixed signature, or which can be intermediaried via a fixed signature operation, can be type erased down to.
The first typical way to do this are to create a private pure interface with those operations, then create a template implementation (templated on the type passed to the ctor) that implements each operation for that particular type. The class that does the type erasure then stores a (smart) pointer to the private interface, and forwards its public operations to it.
A second typical way is to store a void*, or a buffer of char, and a set of pointers to functions that implement the operations. The pointers to functions can be either stored locally in the type erasing class, or stored in a helper struct that is created statically for each type erased, and a pointer to the helper struct is stored in the type erasing class. The first way to store the function pointers is like C-style object properties: the second is like a manual vtable.
In any case, the function pointers usually take one (or more) void* and know how to cast them back to the right type. They are created in the ctor that knows the type, either as instances of a template function, or as local stateless lambdas, or the same indirectly.
You could even do a hybrid of the two: static pimpl instance pointers taking a void* or whatever.
Often using std::function is enough, manually writing type erasure is hard to get right compared to using std::function.
Another version to the first two answers we have here - that's closer to your current code:
class A{
public:
virtual void operator()=0;
};
template<class T>
class B: public A{
public:
B(T*t):ptr(t){}
virtual void operator(){(*ptr)();}
T*ptr;
};
class Example
{
public:
template<typename T>
Example(T* ptr)
: a(new B<T>(ptr))
{
// typedef T EnclosedType; I want this be a avaialable at the class level.
}
void operator()()
{
if(!a)
return;
(*a)();
}
private:
std::unique_ptr<A> a;
}
I am having difficulty getting my head around how to pass a class member function to a subclass (not derived).
My top level class is like this:
class CTop
{
public:
CTop();
int func1(void);
private:
CFnList* _funcList;
};
CTop::CTop():
_funcList(0)
{
_funcList = new CFnList();
_funcList->addFnPtrToList(0, &CTop::func1);
}
int CTop::func1(void)
{
// Does some stuff...
}
My function list class is like this:
class CFnList
{
public:
// Public functions
CFnList();
void addFnPtrToList(int index, int (*fn)(void));
private:
// Fn pointer list
typedef struct
{
int index;
int (*fn) (void);
}fn_list_t;
// function pointer list
QVector<fn_list_t> _fn_list;
};
So basically here I have an instance of class CTop and one of its members is a pointer to a class CFnList. CFnList pointer is instantiated in the constructor of CTop. Then I want to pass in a pointer to one of CTop's member functions to CFnList by calling the following line:
"_funcList->addFnPtrToList(0, &CTop::func1);"
I get issue (quite rightly) that addFnPtrToList does not take the parameters (int, (CTop::*)()). So the compiler knows this function is a certain member function and not just a generic (maybe static) function.
Is there a way to pass the a pointer to the member function into the sub-class? In my case I want the sub-class to be able to call this function. I am thinking I probably have to make static member functions or something, but the syntax is eluding me on how to do this.
All help / advise appreciated.
Fodder
CTop::func1 is a member function. &CTop::func1 is NOT a function pointer, it is a pointer to member (function). Those can not be mixed either in storing or calling. it is not compatible with int (*fn)(void), as the latter takes no arguments and the former requires an object that is passed as the hidden this.
For these reasons you can't have a simple but uniform facility. You either can go with simple function pointers, or pairs of PTM+object pointer, or use wrappers -- handmade or stock like boost::function fueled by boost::bind. If you have C++11 or TR1 you can use the std:: equivalents of the latter.
A declaration in the form:
int (*fn)(void)
cannot point to a member function. It can only point to a free function. Philispophically, this is because the calling conventions for member functions are different then that for free functions. Consider for example the need for a this pointer in the context of a member function call.
The syntax for declaring a pointer-to-member-function is like this:
int (CTop::*fn)(void)
There is an entire section in the C++ FAQ dedicated to member function pointers. Check it out.
You are passing the member function as if it were a regular function. That fails to include the 'this' reference to the class. In order to pass member functions, you have to be able to re-reference it from the original 'this'. Take a look at the following, instead.
typedef void (CTop::*OBJFNC)(args);
_funcList = new CFnList();
_funcList->addFnPtrToList(0, this, &CTop::func1);
void addFnPtrToList(int index, CTop* pobj, OBJFNC pfnc)
{ ... Store both ...
}
Now elsewhere you can execute it with the following.
(pobj->*pfnc)(args);
Here is the final solution, it uses a mixture of passing the instance of the object CTop and usage of template class for CFnList:
My top level class is like this (more or less the same except for the declaration of _funcList to includes the class type and to pass in the "this" to the constructor:
class CTop
{
public:
CTop();
int func1(void);
private:
CFnList<CTop>* _funcList;
};
CTop::CTop():
_funcList(0)
{
_funcList = new CFnList(this);
_funcList->addFnPtrToList(0, &CTop::func1);
}
int CTop::func1(void)
{
// Does some stuff...
}
My function list class is like this:
template<class T>
class CFnList
{
public:
// Public functions
CFnList(T *parent);
void addFnPtrToList(int index, int (T::*fn)(void));
private:
// Pointer to the parent (or owner is perhaps more correct)
T* _parent;
// Fn pointer list
typedef struct
{
int index;
int (T::*fn) (void);
}fn_list_t;
// function pointer list
QVector<fn_list_t> _fn_list;
};
// Constructor
template <class T>
CFnList<T>::CFnList(T *parent) :
_parent(parent),
_fn_list(0)
{
}
// addFnPtrToList:
template <class T>
void CFnList<T>::addFnPtrToList(int index, int (T::*fn)(void))
{
_fn_list.append((fn_list_t){index, fn});
}
So the major changes are:
1. Pass the CTop type in by using changing CFnList into a template.
2. Pass in the instance of the object CTop (so that the pointer to the function can be called) by passing "this" into the constructor and then template class stores it as a pointer to the given template type.... vio-la!...easy :o
Thanks to all who contributed :))
I would like to know if it'd be possible (and if, how) to create a pointer of X value
Now, let's say I know which types would be possible to be assigned in this pointer.
For example, a pointer of X value (of course feel free to change the name of this value), that'd be able to point to variables of string, bool and a custom class
Usually what you describe is a bad idea.
void* works, for marginal values of works. It throws out all type safety, requiring you to keep track of it.
Creating a root type sort of works, but it doesn't work for primitive types, and is rather intrusive.
A boost::variant< bool*, std::string*, MyClass* > is a variable that can contain a pointer to any one of these 3 types (bool, std::string or MyClass). You will probably find it challenging to use, because it enforces type safety, and the syntax can get annoying.
Similarly, a pointer to boost::variant< bool, std::string, MyClass > may be what you want, but it doesn't let you point to bool variables you aren't fooling around with.
With full C++11 support, union can contain arbitrary types, together with an enum can let you do something very much like a boost::variant. As a downside, this requires the thing you be pointed to be a union. With or without full C++11 support, a union of pointers is reasonable. In both cases, you'll have to track the type manually and keep it in sync.
What you really need to think about is "why am I asking this?", because as with many questions, the motivation matters. You may not be asking the right question.
I learned a new expression today, so I'm going to use it, "This is an XY question", You want to do X, so you think the solution is Y, therefore you ask how to do Y. You PROBABLY should ask how to do Y instead. It's a bit like you find your front wheel on the car punctured, and you go talk to the mechanic, but you don't ask "how do I fix a puncture", you ask "How do I undo a wheelnut", and only after you've removed all wheelnuts and the car has fallen over do you realize that you should have put a jack under the car to be able to get the wheel off without the car falling over...
Sure, there are void pointers and unions, that's no big deal. However, you still need to know what your pointer is actually pointing at, otherwise you'll have bloody mess.
So generally, in C++, you probably shouldn't do that.
What you should do is wrap your "thing" in another object, that knows what the content is, and can handle it. That way, you don't have some other thing maintainng state of what the object is and how to use it.
For example, we can have this:
class AnyThing
{
public:
virtual ~AnyThing();
virtual std::string ToString() = 0;
... // other things you want to do with your "things".
};
class IntThing: public AnyThing
{
private:
int value;
public:
virtual std::string ToString() { ... convert int to string ... }
};
class StringThing : public Anything
{
private:
std::string value;
public:
virtual std::string ToString() { return value; }
}
You can use a pointer to void.
If you want a pointer that can only be used to point at those three types, then I see three options:
Create a wrapper class for each type that derives from some base-class:
class Thingy { protected: Thing() {} };
class BoolThingy : public Thingy { bool x; }
class StringThingy : public Thingy { String x; }
class CustomThingy : public Thingy { Custom x; }
...
Thingy *p = new BoolThingy;
Create a smart-pointer class, overloading assignment operators that take bool *, String *, Custom *, and also overloading the * operator (although what that would do, I don't know!)
Use a variant class (e.g. boost::variant).
But with any of the options, it's not clear how such a thing would be useful...
void* has the property to point to any data type. It actually works as a container or cubboard in which you put your variable of any data type and then pass the void* to a function.
You must know the data type of the data you passed, in order to use it.
For Example:
int main()
{
int a;
void *x=&a;
func(x);
....
}
void func(void *argument)
{
int i=*(int *)argument;
....
}
If you know the types before hand and they do not have constructors. Then you can use a union.
class CustomClass {};
union MyType
{
char const* a;
bool b;
float c;
};
MyType stuff;
MyType* ptrToStuff = &stuff;
int main()
{
ptrToStuff->a = "Plop";
ptrToStuff->b = false;
ptrToStuff->c = 12.0;
}
Boost also has an ANY type.
You can store anything in it.
boost::any x = 12;
x = new CustomClass;
x = new std::string("Hi There");
x = std::string("Plop"); // even works without pointers.
You can have a pointer to any type (object, pointer, primitive type etc.) but you can not have a pointer to a reference.
NO you cannot have a pointer pointing at objects of unspecified type. The whole point of a pointer is that it points at objects of specific type. int*x points at an int, A*a points at an A.
What you can do, of course, is to have a variable of type void* that points at nothing at all (void), but can hold any address. If you somehow remember what it is the address of, then you can use a static_cast<> to cast to an appropriate pointer. Alternatively, you can use dynamic_cast<> to find out at run-time whether your void* points to a given type. This could be implemented as follows
struct AnyPointerWrapper
{
struct null_pointer {};
struct wrong_pointer_type {};
AnyPointerWrapper() : ptr(0) {}
AnyPointerWrapper(AnyPointerWrapper const&) = default;
AnyPointerWrapper&operator=(AnyPointerWrapper const&) = default;
template<typename T>
explicit AnyPointerWrapper(T*p)
: ptr(p) {}
template<typename T>
AnyPointerWrapper&operator=(T*p)
{ ptr=p; return*this; }
bool is_null() const
{ return ptr==0; }
template<typename T>
bool is() const
{ return dynamic_cast<T*>(ptr) != 0; }
template<typename T>
T* pointer()
{
if(p==0) throw null_pointer;
T*p = dynamic_cast<T*>(ptr);
if(p==0) throw wrong_pointer_type;
return p;
}
private:
void*ptr;
};
and used like this
int X;
AnyPointerWrapper a;
assert(a.is_null());
a = &X;
assert(a.is<int>());
int*p = a.pointer<int>();
(you can add more functionality including support for const pointers). Note, however, that the dynamic_cast<> is not trivial, i.e. incurs some performance penalty. Note also that AnyPointerWrapper is not a pointer: you cannot use the -> operator to call a member function of the object whose address is stored in AnyPointerWrapper::ptr; it is merely a wrapper from which you can get an appropriate pointer.