NS3的Callback例子讲解

以ns-3.25/ex1/examples下面的main-callback.cc为例，讲解Callback用法。

#include "ns3/callback.h"#include "ns3/assert.h"#include <iostream>using namespace ns3;static doubleCbOne (double a, double b){  std::cout << "invoke cbOne a=" << a << ", b=" << b << std::endl;  return a;}/** Example Callback class. */class MyCb {public:  /**   * Example Callback class method.   *   * \param [in] a The argument.   * \returns -5   */  int CbTwo (double a) {    std::cout << "invoke cbTwo a=" << a << std::endl;    return -5;  }};int main (int argc, char *argv[]){  // return type: double  // first arg type: double  // second arg type: double  Callback<double, double, double> one;  // build callback instance which points to cbOne function  one = MakeCallback (&CbOne);  // this is not a null callback  NS_ASSERT (!one.IsNull ());  // invoke cbOne function through callback instance  double retOne;  retOne = one (10.0, 20.0);  // callback returned expected value  NS_ASSERT (retOne == 10.0);  // return type: int  // first arg type: double  Callback<int, double> two;  MyCb cb;  // build callback instance which points to MyCb::cbTwo  two = MakeCallback (&MyCb::CbTwo, &cb);  // this is not a null callback  NS_ASSERT (!two.IsNull ());  // invoke MyCb::cbTwo through callback instance  int retTwo;  retTwo = two (10.0);  // callback returned expected value  NS_ASSERT (retTwo == -5);  two = MakeNullCallback<int, double> ();  // invoking a null callback is just like  // invoking a null function pointer:  // it will crash.  //int retTwoNull = two (20.0);  NS_ASSERT (two.IsNull ());#if 0  // The below type mismatch between CbOne() and callback two will fail to   // compile if enabled in this program.  two = MakeCallback (&CbOne);#endif#if 0  // This is a slightly different example, in which the code will compile  // but because callbacks are type-safe, will cause a fatal error at runtime   // (the difference here is that Assign() is called instead of operator=)  Callback<void, float> three;  three.Assign (MakeCallback (&CbOne));#endif  return 0;}

首先我们观察到第一个Callback与Cbone有这样的关系：

如果这个函数和Callback函数有相同的函数参数类型，那么我们可以把这个函数绑定为Callback函数。Callback函数第一个参数是返回类型，后边的是参数类型。

Callback只是一个声明的作用，它表示我们绑定任意符合要求的函数到Callback函数，那么我们究竟要绑定哪个函数呢？接下来我们用MakeCallback函数绑定到具体的函数上去。

// build callback instance which points to cbOne functionone = MakeCallback (&CbOne);

接下来：

NS_ASSERT (!one.IsNull ());

这句话为了保证Callback为非空，也就是说，Callback绑定到了特定的函数。

第二次调用Callback函数，是绑定到了类的成员函数上，原理一样。

我们可以看一下Callback函数和MakeCallback原型

参数：
memPtr Class method member pointer
objPtr Class instance

返回：
A wrapper Callback