see link： https://isocpp.org/wiki/faq/pointers-to-members
function vs template: http://stackoverflow.com/questions/14677997/stdfunction-vs-template

boost::functoin/std::function可用于全部 operator() 操作的对象（函数，类。成员函数。lambda表达式等等）。

用处就是能够使用一个函数指针调用不用的函数实体(仅仅要他们的signature一样)，实现回调函数。或者多种不同的算法等等。
关于 std::function的实现。 see link：http://stackoverflow.com/questions/18453145/how-is-stdfunction-implemented
非常好的样例：原文链接

#include <functional>
#include <iostream>
using namespace std;std::function< int(int)> Functional;// 普通函数
int TestFunc(int a)
{return a;
}// Lambda表达式
auto lambda = [](int a)->int{ return a; };// 函数对象(functor)
class Functor
{
public:int operator()(int a){return a;}
};// 1.类成员函数
// 2.类静态函数
class TestClass
{
public:int ClassMember(int a) { return a; }static int StaticMember(int a) { return a; }
};int main()
{// 普通函数Functional = TestFunc;int result = Functional(10);cout << "普通函数："<< result << endl;// Lambda表达式Functional = lambda;result = Functional(20);cout << "Lambda表达式："<< result << endl;// 仿函数Functor testFunctor;Functional = testFunctor;result = Functional(30);cout << "仿函数："<< result << endl;// 类成员函数TestClass testObj;Functional = std::bind(&TestClass::ClassMember, testObj, std::placeholders::_1);result = Functional(40);cout << "类成员函数："<< result << endl;// 类静态函数Functional = TestClass::StaticMember;result = Functional(50);cout << "类静态函数："<< result << endl;return 0;
}

function简化了函数指针的使用：

class FooClass {
public:void Print( int a ) {std::cout << "A FooClass, param = "<< a <<" this = " << this << std::endl;}
};void main() {FooClass *myFoo = new FooClass();void( FooClass::* oldFunc )(int) = &FooClass::Print; //C style function pointer(myFoo->*oldFunc)( 5 );boost::function newFunc = boost::bind( &FooClass::Print, myFoo, _1 ); //boost function      newFunc( 5 );
}