我经常听到很多关于c++中的函子。谁能给我一个概述,他们是什么,在什么情况下他们会有用?
当前回答
除此之外,我还使用了函数对象使现有的遗留方法适合命令模式;(我唯一感受到面向对象范式真正OCP之美的地方);在这里还添加了相关的函数适配器模式。
假设你的方法有签名:
int CTask::ThreeParameterTask(int par1, int par2, int par3)
我们将了解如何使它适合Command模式——为此,首先,必须编写一个成员函数适配器,以便可以将其作为函数对象调用。
注意-这是丑陋的,也许你可以使用Boost绑定助手等,但如果你不能或不想这样做,这是一种方法。
// a template class for converting a member function of the type int function(int,int,int)
//to be called as a function object
template<typename _Ret,typename _Class,typename _arg1,typename _arg2,typename _arg3>
class mem_fun3_t
{
public:
explicit mem_fun3_t(_Ret (_Class::*_Pm)(_arg1,_arg2,_arg3))
:m_Ptr(_Pm) //okay here we store the member function pointer for later use
{}
//this operator call comes from the bind method
_Ret operator()(_Class *_P, _arg1 arg1, _arg2 arg2, _arg3 arg3) const
{
return ((_P->*m_Ptr)(arg1,arg2,arg3));
}
private:
_Ret (_Class::*m_Ptr)(_arg1,_arg2,_arg3);// method pointer signature
};
同样,我们需要一个辅助方法mem_fun3来帮助调用上面的类。
template<typename _Ret,typename _Class,typename _arg1,typename _arg2,typename _arg3>
mem_fun3_t<_Ret,_Class,_arg1,_arg2,_arg3> mem_fun3 ( _Ret (_Class::*_Pm) (_arg1,_arg2,_arg3) )
{
return (mem_fun3_t<_Ret,_Class,_arg1,_arg2,_arg3>(_Pm));
}
现在,为了绑定参数,我们必须写一个绑定函数。所以,是这样的:
template<typename _Func,typename _Ptr,typename _arg1,typename _arg2,typename _arg3>
class binder3
{
public:
//This is the constructor that does the binding part
binder3(_Func fn,_Ptr ptr,_arg1 i,_arg2 j,_arg3 k)
:m_ptr(ptr),m_fn(fn),m1(i),m2(j),m3(k){}
//and this is the function object
void operator()() const
{
m_fn(m_ptr,m1,m2,m3);//that calls the operator
}
private:
_Ptr m_ptr;
_Func m_fn;
_arg1 m1; _arg2 m2; _arg3 m3;
};
并且,一个使用binder3类的辅助函数- bind3:
//a helper function to call binder3
template <typename _Func, typename _P1,typename _arg1,typename _arg2,typename _arg3>
binder3<_Func, _P1, _arg1, _arg2, _arg3> bind3(_Func func, _P1 p1,_arg1 i,_arg2 j,_arg3 k)
{
return binder3<_Func, _P1, _arg1, _arg2, _arg3> (func, p1,i,j,k);
}
现在,我们必须在Command类中使用它;使用以下类型定义:
typedef binder3<mem_fun3_t<int,T,int,int,int> ,T* ,int,int,int> F3;
//and change the signature of the ctor
//just to illustrate the usage with a method signature taking more than one parameter
explicit Command(T* pObj,F3* p_method,long timeout,const char* key,
long priority = PRIO_NORMAL ):
m_objptr(pObj),m_timeout(timeout),m_key(key),m_value(priority),method1(0),method0(0),
method(0)
{
method3 = p_method;
}
你是这样称呼它的:
F3 f3 = PluginThreadPool::bind3( PluginThreadPool::mem_fun3(
&CTask::ThreeParameterTask), task1,2122,23 );
注意:f3 ();将调用方法task1->ThreeParameterTask(21,22,23);
此模式的完整上下文在下面的链接中
其他回答
如上所述,函子是可以被视为函数的类(重载操作符())。
在需要将某些数据与对函数的重复或延迟调用相关联的情况下,它们非常有用。
例如,函子链表可用于实现基本的低开销同步协程系统、任务分派器或可中断文件解析。 例子:
/* prints "this is a very simple and poorly used task queue" */
class Functor
{
public:
std::string output;
Functor(const std::string& out): output(out){}
operator()() const
{
std::cout << output << " ";
}
};
int main(int argc, char **argv)
{
std::list<Functor> taskQueue;
taskQueue.push_back(Functor("this"));
taskQueue.push_back(Functor("is a"));
taskQueue.push_back(Functor("very simple"));
taskQueue.push_back(Functor("and poorly used"));
taskQueue.push_back(Functor("task queue"));
for(std::list<Functor>::iterator it = taskQueue.begin();
it != taskQueue.end(); ++it)
{
*it();
}
return 0;
}
/* prints the value stored in "i", then asks you if you want to increment it */
int i;
bool should_increment;
int doSomeWork()
{
std::cout << "i = " << i << std::endl;
std::cout << "increment? (enter the number 1 to increment, 0 otherwise" << std::endl;
std::cin >> should_increment;
return 2;
}
void doSensitiveWork()
{
++i;
should_increment = false;
}
class BaseCoroutine
{
public:
BaseCoroutine(int stat): status(stat), waiting(false){}
void operator()(){ status = perform(); }
int getStatus() const { return status; }
protected:
int status;
bool waiting;
virtual int perform() = 0;
bool await_status(BaseCoroutine& other, int stat, int change)
{
if(!waiting)
{
waiting = true;
}
if(other.getStatus() == stat)
{
status = change;
waiting = false;
}
return !waiting;
}
}
class MyCoroutine1: public BaseCoroutine
{
public:
MyCoroutine1(BaseCoroutine& other): BaseCoroutine(1), partner(other){}
protected:
BaseCoroutine& partner;
virtual int perform()
{
if(getStatus() == 1)
return doSomeWork();
if(getStatus() == 2)
{
if(await_status(partner, 1))
return 1;
else if(i == 100)
return 0;
else
return 2;
}
}
};
class MyCoroutine2: public BaseCoroutine
{
public:
MyCoroutine2(bool& work_signal): BaseCoroutine(1), ready(work_signal) {}
protected:
bool& work_signal;
virtual int perform()
{
if(i == 100)
return 0;
if(work_signal)
{
doSensitiveWork();
return 2;
}
return 1;
}
};
int main()
{
std::list<BaseCoroutine* > coroutineList;
MyCoroutine2 *incrementer = new MyCoroutine2(should_increment);
MyCoroutine1 *printer = new MyCoroutine1(incrementer);
while(coroutineList.size())
{
for(std::list<BaseCoroutine *>::iterator it = coroutineList.begin();
it != coroutineList.end(); ++it)
{
*it();
if(*it.getStatus() == 0)
{
coroutineList.erase(it);
}
}
}
delete printer;
delete incrementer;
return 0;
}
当然,这些例子本身并没有多大用处。它们只是展示了函子是如何有用的,函子本身是非常基础和不灵活的,这使得它们不如boost所提供的有用。
在gtkmm中使用函子将一些GUI按钮连接到实际的c++函数或方法。
如果你使用pthread库使你的应用程序多线程,Functors可以帮助你。 要启动一个线程,pthread_create(..)的参数之一是要在自己的线程上执行的函数指针。 但有一个不便之处。这个指针不能是指向方法的指针,除非它是一个静态方法,或者除非你指定了它的类,比如class::method。还有一件事,你方法的接口只能是:
void* method(void* something)
因此,如果不做一些额外的事情,你就不能(以一种简单明显的方式)在线程中运行你的类中的方法。
在c++中处理线程的一个很好的方法是创建你自己的Thread类。如果你想运行MyClass类的方法,我所做的是,把那些方法转换成Functor派生类。
同样,Thread类有这样的方法: 静态void* startThread(void*参数) 指向该方法的指针将用作调用pthread_create(..)的参数。startThread(..)在arg中应该接收到的是一个void*类型的引用,它指向任何Functor派生类的堆中的实例,在执行时将被强制转换回Functor*,然后调用它的run()方法。
函子基本上就是一个定义操作符()的类。这让你可以创建“看起来像”函数的对象:
// this is a functor
struct add_x {
add_x(int val) : x(val) {} // Constructor
int operator()(int y) const { return x + y; }
private:
int x;
};
// Now you can use it like this:
add_x add42(42); // create an instance of the functor class
int i = add42(8); // and "call" it
assert(i == 50); // and it added 42 to its argument
std::vector<int> in; // assume this contains a bunch of values)
std::vector<int> out(in.size());
// Pass a functor to std::transform, which calls the functor on every element
// in the input sequence, and stores the result to the output sequence
std::transform(in.begin(), in.end(), out.begin(), add_x(1));
assert(out[i] == in[i] + 1); // for all i
函子有几个优点。其一,与常规函数不同,它们可以包含状态。上面的例子创建了一个函数,无论你给它什么,它都会加上42。但是值42并不是硬编码的,它是在创建函数实例时作为构造函数参数指定的。我可以创建另一个加法器,只需要用不同的值调用构造函数,就可以加27。这使得它们可以很好地定制。
As the last lines show, you often pass functors as arguments to other functions such as std::transform or the other standard library algorithms. You could do the same with a regular function pointer except, as I said above, functors can be "customized" because they contain state, making them more flexible (If I wanted to use a function pointer, I'd have to write a function which added exactly 1 to its argument. The functor is general, and adds whatever you initialized it with), and they are also potentially more efficient. In the above example, the compiler knows exactly which function std::transform should call. It should call add_x::operator(). That means it can inline that function call. And that makes it just as efficient as if I had manually called the function on each value of the vector.
如果我传递的是一个函数指针,编译器不能立即看到它指向哪个函数,所以除非它执行一些相当复杂的全局优化,否则它必须在运行时解除对指针的引用,然后进行调用。
除了在回调中使用,c++函子还可以帮助为矩阵类提供Matlab喜欢的访问样式。这里有一个例子。
除此之外,我还使用了函数对象使现有的遗留方法适合命令模式;(我唯一感受到面向对象范式真正OCP之美的地方);在这里还添加了相关的函数适配器模式。
假设你的方法有签名:
int CTask::ThreeParameterTask(int par1, int par2, int par3)
我们将了解如何使它适合Command模式——为此,首先,必须编写一个成员函数适配器,以便可以将其作为函数对象调用。
注意-这是丑陋的,也许你可以使用Boost绑定助手等,但如果你不能或不想这样做,这是一种方法。
// a template class for converting a member function of the type int function(int,int,int)
//to be called as a function object
template<typename _Ret,typename _Class,typename _arg1,typename _arg2,typename _arg3>
class mem_fun3_t
{
public:
explicit mem_fun3_t(_Ret (_Class::*_Pm)(_arg1,_arg2,_arg3))
:m_Ptr(_Pm) //okay here we store the member function pointer for later use
{}
//this operator call comes from the bind method
_Ret operator()(_Class *_P, _arg1 arg1, _arg2 arg2, _arg3 arg3) const
{
return ((_P->*m_Ptr)(arg1,arg2,arg3));
}
private:
_Ret (_Class::*m_Ptr)(_arg1,_arg2,_arg3);// method pointer signature
};
同样,我们需要一个辅助方法mem_fun3来帮助调用上面的类。
template<typename _Ret,typename _Class,typename _arg1,typename _arg2,typename _arg3>
mem_fun3_t<_Ret,_Class,_arg1,_arg2,_arg3> mem_fun3 ( _Ret (_Class::*_Pm) (_arg1,_arg2,_arg3) )
{
return (mem_fun3_t<_Ret,_Class,_arg1,_arg2,_arg3>(_Pm));
}
现在,为了绑定参数,我们必须写一个绑定函数。所以,是这样的:
template<typename _Func,typename _Ptr,typename _arg1,typename _arg2,typename _arg3>
class binder3
{
public:
//This is the constructor that does the binding part
binder3(_Func fn,_Ptr ptr,_arg1 i,_arg2 j,_arg3 k)
:m_ptr(ptr),m_fn(fn),m1(i),m2(j),m3(k){}
//and this is the function object
void operator()() const
{
m_fn(m_ptr,m1,m2,m3);//that calls the operator
}
private:
_Ptr m_ptr;
_Func m_fn;
_arg1 m1; _arg2 m2; _arg3 m3;
};
并且,一个使用binder3类的辅助函数- bind3:
//a helper function to call binder3
template <typename _Func, typename _P1,typename _arg1,typename _arg2,typename _arg3>
binder3<_Func, _P1, _arg1, _arg2, _arg3> bind3(_Func func, _P1 p1,_arg1 i,_arg2 j,_arg3 k)
{
return binder3<_Func, _P1, _arg1, _arg2, _arg3> (func, p1,i,j,k);
}
现在,我们必须在Command类中使用它;使用以下类型定义:
typedef binder3<mem_fun3_t<int,T,int,int,int> ,T* ,int,int,int> F3;
//and change the signature of the ctor
//just to illustrate the usage with a method signature taking more than one parameter
explicit Command(T* pObj,F3* p_method,long timeout,const char* key,
long priority = PRIO_NORMAL ):
m_objptr(pObj),m_timeout(timeout),m_key(key),m_value(priority),method1(0),method0(0),
method(0)
{
method3 = p_method;
}
你是这样称呼它的:
F3 f3 = PluginThreadPool::bind3( PluginThreadPool::mem_fun3(
&CTask::ThreeParameterTask), task1,2122,23 );
注意:f3 ();将调用方法task1->ThreeParameterTask(21,22,23);
此模式的完整上下文在下面的链接中