I'm deriving a class from a base class outside my control. The class will get created by a factory function pointer I pass in (in a part of the code I also don't control).
In my derived class, I need to pass an additional constructor argument to the factory function.
My first stab was to try to adapt the factory function via a lambda, but that cannot capture the additional argument. Other answers explained why that doesn't work. Next I've tried to augment that with std::function which other answers have led me to believe would work, but I can't figure out the right syntax and have found the examples to be incomprehensible (and not sure I really understand what that is even doing).
What am I doing wrong here? Is there a better way to solve this?
Demonstration code:
#include <functional>
#include <string>
// I have no control over this
struct Base {
Base(int i) {}
};
void UseObject(Base *(*factory)(int i)) {
Base *instance = factory(5);
// Save created instance
}
// I control the rest
struct Derived : public Base {
Derived(const char *s, int i) : Base(i) { /* Store s for later use */ }
static Base *Factory(const char *s, int i) { return new Derived(s, i); }
};
void AddObject(const char *name)
{
// First stab
// UseObject([name] (int i) { return Derived::Factory(name, i); });
// Second stab
std::function<Base *(int i)> foo { [name] (int i) { return Derived::Factory(name, i); } };
UseObject(foo);
}
int main(int ac, char **av)
{
AddObject("some_name");
AddObject("another_name");
return 0;
}
The error I get from g++ (7.4.0) is:
tfunc.cpp: In function ‘void AddObject(const char*)’:
tfunc.cpp:28:18: error: cannot convert ‘std::function<Base*(int)>’ to ‘Base* (*)(int)’ for argument ‘1’ to ‘void UseObject(Base* (*)(int))’
UseObject(foo);
UseObject doesn't allow non capturing lambda or std::function or regular class functor. The only callable it accepts is function pointer (non capturing lambda can convert to).
So you might do:
UseObject([](int i) { return Derived::Factory("some_name", i); });
but not
auto name = "some_name";
UseObject([name] (int i) { return Derived::Factory(name, i); });
Possible (limited) workaround is to use global variable to store state. (So cannot be used concurrently).
void AddObject(const char *name)
{
// global usage as UseObject only accepts pointer function
static const char* instance = nullptr;
instance = name;
UseObject(+[](int i) { return Derived::Factory(instance, i); });
}
Related
I need to call a function when a button is pressed. The following function should take in the function to be called:
void ButtonLayer::LoadButton(void(*func)()) {
// do button loading stuff
// if button is clicked...
func();
}
This would work except for the fact that passing a function within a seperate namespace gives the following error:
argument of type "void(OtherLayer::*)()" is incompatiable with parameter of type "void(*)()"
I don't want to make every function I pass static to avoid this problem, so I need some way of converting a function within a namespace to be of type void(*). I have tried static casting but I'm unsure of the exact syntax as I'm new to C++
It seems that you want to pass a member function.
This example may help you.
class A {
public:
int i;
int fun(int j) {
return i + j;
};
};
void fun(int j, A ob, int (A::* p)(int)) {
std::cout << (ob.*p)(j);
}
void main() {
int (A:: * fp)(int); //declare fp as a function pointer in class A
fp = &A::fun; //init fp
A obj;
obj.i = 1;
fun(123, obj, fp);
}
Based on #Yksisarvinen and #MSalters comments, the solution was:
void ButtonLayer::LoadButton(std::function<void()>) {
// do button loading stuff
// if button is clicked...
func();
}
and then to call it:
LoadButton([this] { functionToCall; });
Here is what I try to achieve:
class MyClass
{
public:
using Callback = void(MyClass::*)(uint8_t idx);
void forEach(Callback callback);
private:
int m_buf[64];
int m_x;
int m_y;
MyObject m_object;
}
void MyClass::forEach(Callback callback)
{
size_t s = m_object.size();
for(size_t i = 0; i < s; i++)
callback(i);
}
void MyClass::f1()
{
forEach([this](uint8_t idx)
{
m_buf[idx]++;
});
}
void MyClass::f2()
{
forEach([this](uint8_t idx)
{
m_buf[idx] = m_x + m_y * idx;
});
}
So there are a bunch of ways to modify m_buf[]. In order to avoid copying and pasting 'get size + for loop', I want to add a forEach method and pass lambdas as callbacks.
this is captured to have access to class members.
What is the right way to achieve the same result?
Thanks.
PS: compilation of this example returns error 'cannot convert ::lambda ....'
ANSWER:
With "Passer By" answer, I finished with the code:
// Class declaration in header
using Callback = std::function<void(uint8_t)>;
void forEach(Callback callback);
// forEach() is as above
// forEach() call looks like
forEach([this](uint8_t idx) {
m_buf[idx] = m_x + m_y * idx;
});
Also I found some related questions-anwers which might be useful
Passing lambda as function pointer - "5gon12eder" answer.
C++ lambda with captures as a function pointer
You have mistaken the semantics of a member function pointer
void (MyClass::*)(uint8_t idx)
is a pointer to a member function of MyClass that accepts a uint8_t, it is not anything else. You call it as
MyClass::Callback c = &MyClass::SomeMemberFunction;
MyClass mc;
(mc.*c)(0); // equivalent to...
mc.SomeMemberFunction(0);
Secondly, the purpose of for_each is so that the callable object passed in need not know the internals of the object, as such, you shouldn't pass in an index in the loop.
What you actually want is to pass in a callable object accepting a reference called on each object. This can be done in two ways
template<typename Callable>
void for_each1(Callable&& call)
{
for(size_t i = 0; i < size(); i++)
call(m_buf[i]);
}
#include<functional>
void for_each2(std::function<void (int&)>&& fn)
{
for(size_t i = 0; i < size(); i++)
fn(m_buf[i]);
}
Both can be called with a lambda
MyClass mc;
mc.for_each1([](int& i) { i++; });
mc.for_each2([&mc](int& i) { i += mc.increment(); });
where mc.increment is what that instance wants to be incremented by.
I have to make some kind of bridge between two pieces of software, but am facing an issue I don't know how to deal with. Hopefully someone will have interesting and (preferably) working suggestions.
Here is the background : I have a C++ software suite. I have to replace some function within a given class with another function, which is ok. The problem is that the new function calls another function which has to be static, but has to deal with members of the class. This is this second function which is making me mad.
If the function is not static I get the following error :
error: argument of type ‘void (MyClass::)(…)’ does not match ‘void (*)(…)’
If I set it to static I get either the following error :
error: cannot call member function ‘void
MyClass::MyFunction(const double *)’ without object
or
error: ‘this’ is unavailable for static member functions
depending on if I use or not the "this" keyword ("Function()" or "this->Function()").
And finally, the class object requires some arguments which I cannot pass to the static function (I cannot modify the static function prototype), which prevents me to create a new instance within the static function itself.
How would you deal with such a case with minimal rewriting ?
Edit : Ok, here is a simplified sample on what I have to do, hoping it is clear and correct :
// This function is called by another class on an instance of MyClass
MyClass::BigFunction()
{
…
// Call of a function from an external piece of code,
// which prototype I cannot change
XFunction(fcn, some more args);
…
}
// This function has to be static and I cannot change its prototype,
// for it to be passed to XFunction. XFunction makes iterations on it
// changing parameters (likelihood maximization) which do not appear
// on this sample
void MyClass::fcn(some args, typeN& result)
{
// doesn't work because fcn is static
result = SomeComputation();
// doesn't work, for the same reason
result = this->SomeComputation();
// doesn't work either, because MyClass has many parameters
// which have to be set
MyClass *tmp = new MyClass();
result = tmp->SomeComputation();
}
Pointers to non-static member functions are a bit tricky to deal with. The simplest workaround would just be to add an opaque pointer argument to your function which you can then cast as a pointer to 'this', then do what you need with it.
Here's a very simple example:
void doSomething(int (*callback)(void *usrPtr), void *usrPtr)
{
// Do stuff...
int value = callback(usrPtr);
cout << value << "\n";
}
class MyClass
{
public:
void things()
{
value_ = 42;
doSomething(myCallback, this);
}
private:
int value_;
static int myCallback(void *usrPtr)
{
MyClass *parent = static_cast<MyClass *>(usrPtr);
return parent->value_;
}
};
int main()
{
MyClass object;
object.things();
return 0;
}
In this example myCallback() can access the private value_ through the opaque pointer.
If you want a more C++-like approach you could look into using Boost.Function and Boost.Bind which allow you to pass non-static member functions as callbacks:
void doSomething(boost::function<int ()> callback)
{
// Do stuff...
int value = callback();
cout << value << "\n";
}
class MyClass
{
public:
void things()
{
value_ = 42;
doSomething(boost::bind(&MyClass::myCallback, this));
}
private:
int value_;
int myCallback()
{
return value_;
}
};
int main()
{
MyClass object;
object.things();
return 0;
}
If you really can't change the function prototype you could use a global pointer, but that opens up all sorts of issues if you will ever have more than one instance of your class. It's just generally bad practice.
class MyClass;
static MyClass *myClass;
void doSomething(int (*callback)())
{
// Do stuff...
int value = callback();
cout << value << "\n";
}
class MyClass
{
public:
void things()
{
value_ = 42;
myClass = this;
doSomething(myCallback);
}
private:
int value_;
static int myCallback()
{
return myClass->value_;
}
};
int main()
{
MyClass object;
object.things();
return 0;
}
Following spencercw's suggestion below the initial question I tried the "static member variable that you set to point to this" solution (the global variable would have been tricky and dangerous within the context of the software suite).
Actually I figured out there was already something like this implemented in the code (which I didn't write) :
static void* currentObject;
So I just used it, as
((MyClass*)currentObject)->SomeComputation();
It does work, thanks !!!
non-reentrant and non-thread-safe way is to pass "this" address using global variable.
You can move the result = SomeComputation(); out of your static function and place it in BigFunction right before your call to the static function.
Suppose I have a class with 2 static functions:
class CommandHandler
{
public:
static void command_one(Item);
static void command_two(Item);
};
I have a DRY problem where I have 2 functions that have the exact same code for every single line, except for the function that it calls:
void CommandOne_User()
{
// some code A
CommandHandler::command_one(item);
// some code B
}
void CommandTwo_User()
{
// some code A
CommandHandler::command_two(item);
// some code B
}
I would like to remove duplication, and, ideally, do something like this:
void CommandOne_User()
{
Function func = CommandHandler::command_one();
Refactored_CommandUser(func);
}
void CommandTwo_User()
{
Function func = CommandHandler::command_one();
Refactored_CommandUser(func);
}
void Refactored_CommandUser(Function func)
{
// some code A
func(item);
}
I have access to Qt, but not Boost. Could someone help suggest a way on how I can refactor something like this?
You could use function pointers:
// type of the functions
typedef void Function(Item);
void CommandOne_User() {
// function pointer
Function *func = CommandHandler::command_one;
Refactored_CommandUser(func);
}
void CommandTwo_User() {
// can also be used directly, without a intermediate variable
Refactored_CommandUser(CommandHandler::command_two);
}
// taking a function pointer for the command that should be executed
void Refactored_CommandUser(Function *func) {
// calling the funcion (no explicit dereferencing needed, this conversion is
// done automatically)
func(item);
}
Besides the C way (passing a function pointer) or the C++ way mentioned by Jay here there is the other (modern) c++ way with boost or with a compiler with c++0x support:
void Refactored_CommandUser( boost::function<void (Item)> f ) {
// alternatively std::function with proper compiler support
}
With the advantage that this encapsulates a functor, and can be combined with boost::bind (or std::bind) to pass in not only free-function pointers that match the signature exactly, but also other things, like member pointers with an object:
struct test {
void f( Item );
};
void foo( Item i, std::string const & caller );
void bar( Item i );
int main() {
test t;
Refactored_CommandUser( boost::bind( &test::f, &t, _1 ) );
Refactored_CommandUser( boost::bind( foo, _1, "main" ) );
Refactored_CommandUser( bar ); // of course you can pass a function that matches directly
}
I posted a question very similar to this and this was the explanation I got:
Function Pointers
And here is the link to the question I posted: Function callers (callbacks) in C?
Another way to do this if you don't have access to tr1 or boost, is just to use function template. It's quite simple and obviously a C++ way.
Here's a compilable example similar to yours:
#include <iostream>
using namespace std;
class CommandHandler
{
public:
static void command_one(int i) { cout << "command_one " << i << endl; }
static void command_two(int i) { cout << "command_two " << i << endl; }
};
template <typename Func>
void CommandCaller(Func f)
{
f(1);
}
int main()
{
CommandCaller(&CommandHandler::command_one);
return 0;
}
I can think of two ways.
The C style way: pass the function to be called in as a function pointer.
The C++ way: create a base class that implements your code and replace the called function with a virtual method. Then derive two concrete classes from the base class, each one implementing the virtual function differently.
see this please
http://www.newty.de/fpt/fpt.html
Static member functions can be passed simply as function pointers.
Non-static can be passed as member-function pointer + this.
void Refactored_CommandUser(static void (*func)(Item))
{
// some code A
func(item);
// some code B
}
void CommandOne_User()
{
Refactored_CommandUser(&CommandHandler::command_one);
}
void CommandTwo_User()
{
Refactored_CommandUser(&CommandHandler::command_two);
}
So inspired by David Roriguez's answer, I tried it out on my own and, yup, it works:
Here's an example (stupid) code of the "modern" way to pass a function as a function parameter:
#include <functional>
#include <assert.h>
class Command
{
public:
static int getSeven(int number_)
{
return 7 + number_;
}
static int getEight(int number_)
{
return 8 - number_;
}
};
int func(std::tr1::function<int (int)> f, int const number_ )
{
int const new_number = number_ * 2;
int const mod_number = f(new_number);
return mod_number - 3;
}
int main()
{
assert( func(Command::getSeven, 5) == 14 );
assert( func(Command::getEight, 10) == -15 );
return 0;
}
I tried this on VS2008 with Intel C++ Compiler 11.1 with C++0X support on (don't know if C++0x support is really needed since it's in TR1).
I need to bind a method into a function-callback, except this snippet is not legal as discussed in demote-boostfunction-to-a-plain-function-pointer.
What's the simplest way to get this behavior?
struct C {
void m(int x) {
(void) x;
_asm int 3;
}};
typedef void (*cb_t)(int);
int main() {
C c;
boost::function<void (int x)> cb = boost::bind(&C::m, &c, _1);
cb_t raw_cb = *cb.target<cb_t>(); //null dereference
raw_cb(1);
return 0;
}
You can make your own class to do the same thing as the boost bind function. All the class has to do is accept the function type and a pointer to the object that contains the function. For example, this is a void return and void param delegate:
template<typename owner>
class VoidDelegate : public IDelegate
{
public:
VoidDelegate(void (owner::*aFunc)(void), owner* aOwner)
{
mFunction = aFunc;
mOwner = aOwner;
}
~VoidDelegate(void)
{}
void Invoke(void)
{
if(mFunction != 0)
{
(mOwner->*mFunction)();
}
}
private:
void (owner::*mFunction)(void);
owner* mOwner;
};
Usage:
class C
{
void CallMe(void)
{
std::cout << "called";
}
};
int main(int aArgc, char** aArgv)
{
C c;
VoidDelegate<C> delegate(&C::CallMe, &c);
delegate.Invoke();
}
Now, since VoidDelegate<C> is a type, having a collection of these might not be practical, because what if the list was to contain functions of class B too? It couldn't.
This is where polymorphism comes into play. You can create an interface IDelegate, which has a function Invoke:
class IDelegate
{
virtual ~IDelegate(void) { }
virtual void Invoke(void) = 0;
}
If VoidDelegate<T> implements IDelegate you could have a collection of IDelegates and therefore have callbacks to methods in different class types.
Either you can shove that bound parameter into a global variable and create a static function that can pick up the value and call the function on it, or you're going to have to generate per-instance functions on the fly - this will involve some kind of on the fly code-gen to generate a stub function on the heap that has a static local variable set to the value you want, and then calls the function on it.
The first way is simple and easy to understand, but not at all thread-safe or reentrant. The second version is messy and difficult, but thread-safe and reentrant if done right.
Edit: I just found out that ATL uses the code generation technique to do exactly this - they generate thunks on the fly that set up the this pointer and other data and then jump to the call back function. Here's a CodeProject article that explains how that works and might give you an idea of how to do it yourself. Particularly look at the last sample (Program 77).
Note that since the article was written DEP has come into existance and you'll need to use VirtualAlloc with PAGE_EXECUTE_READWRITE to get a chunk of memory where you can allocate your thunks and execute them.
#include <iostream>
typedef void(*callback_t)(int);
template< typename Class, void (Class::*Method_Pointer)(void) >
void wrapper( int class_pointer )
{
Class * const self = (Class*)(void*)class_pointer;
(self->*Method_Pointer)();
}
class A
{
public:
int m_i;
void callback( )
{ std::cout << "callback: " << m_i << std::endl; }
};
int main()
{
A a = { 10 };
callback_t cb = &wrapper<A,&A::callback>;
cb( (int)(void*)&a);
}
i have it working right now by turning C into a singleton, factoring C::m into C::m_Impl, and declaring static C::m(int) which forwards to the singleton instance. talk about a hack.