请参阅上面的定义，其中声明将回调函数传递给其他函数，并在某个时刻调用它。

在c++中，让回调函数调用类方法是可取的。当您这样做时，您可以访问成员数据。如果你使用C语言定义回调函数，你必须将它指向一个静态成员函数。这不是很理想。

Here is how you can use callbacks in C++. Assume 4 files. A pair of .CPP/.H files for each class. Class C1 is the class with a method we want to callback. C2 calls back to C1's method. In this example the callback function takes 1 parameter which I added for the readers sake. The example doesn't show any objects being instantiated and used. One use case for this implementation is when you have one class that reads and stores data into temporary space and another that post processes the data. With a callback function, for every row of data read the callback can then process it. This technique cuts outs the overhead of the temporary space required. It is particularly useful for SQL queries that return a large amount of data which then has to be post-processed.

/////////////////////////////////////////////////////////////////////
// C1 H file

class C1
{
    public:
    C1() {};
    ~C1() {};
    void CALLBACK F1(int i);
};

/////////////////////////////////////////////////////////////////////
// C1 CPP file

void CALLBACK C1::F1(int i)
{
// Do stuff with C1, its methods and data, and even do stuff with the passed in parameter
}

/////////////////////////////////////////////////////////////////////
// C2 H File

class C1; // Forward declaration

class C2
{
    typedef void (CALLBACK C1::* pfnCallBack)(int i);
public:
    C2() {};
    ~C2() {};

    void Fn(C1 * pThat,pfnCallBack pFn);
};

/////////////////////////////////////////////////////////////////////
// C2 CPP File

void C2::Fn(C1 * pThat,pfnCallBack pFn)
{
    // Call a non-static method in C1
    int i = 1;
    (pThat->*pFn)(i);
}

@Pixelchemist已经给出了一个全面的答案。但作为一名网络开发人员，我可以给出一些建议。

通常我们使用tcp来开发一个web框架，所以通常我们有一个结构:

TcpServer listen port and register the socket to epoll or something
  -> TcpServer receive new connection 
    -> HttpConenction deal the data from the connection 
      -> HttpServer call Handler to deal with HttpConnection.
        -> Handler contain codes like save into database and fetch from db

我们可以按顺序开发框架，但它对只想关心Handler的用户并不友好。是时候使用回调了。

Mutiple Handler written by user
  -> register the handler as callback property of HttpServer
    -> register the related methods in HttpServer to HttpConnection
      -> register the relate methods in HttpConnection to TcpServer

所以用户只需要注册他们的处理程序到httpserver(通常用一些路径字符串作为键)，其他的事情是框架可以做的通用的。

你会发现我们可以把回调当成一种context，我们想委托给其他人为我们做。核心是我们不知道什么时候是调用函数的最佳时间，但我们委托的人知道。

Scott Meyers举了一个很好的例子:

class GameCharacter;
int defaultHealthCalc(const GameCharacter& gc);

class GameCharacter
{
public:
  typedef std::function<int (const GameCharacter&)> HealthCalcFunc;

  explicit GameCharacter(HealthCalcFunc hcf = defaultHealthCalc)
  : healthFunc(hcf)
  { }

  int healthValue() const { return healthFunc(*this); }

private:
  HealthCalcFunc healthFunc;
};

我认为这个例子说明了一切。

std::function<>是编写c++回调函数的“现代”方式。

C语言中也有回调的方法:函数指针

// Define a type for the callback signature,
// it is not necessary but makes life easier

// Function pointer called CallbackType that takes a float
// and returns an int
typedef int (*CallbackType)(float);

void DoWork(CallbackType callback)
{
  float variable = 0.0f;
  
  // Do calculations
  
  // Call the callback with the variable, and retrieve the
  // result
  int result = callback(variable);

  // Do something with the result
}

int SomeCallback(float variable)
{
  int result;

  // Interpret variable

  return result;
}

int main(int argc, char ** argv)
{
  // Pass in SomeCallback to the DoWork
  DoWork(&SomeCallback);
}

现在，如果你想将类方法作为回调函数传入，对这些函数指针的声明会有更复杂的声明，例如:

// Declaration:
typedef int (ClassName::*CallbackType)(float);

// This method performs work using an object instance
void DoWorkObject(CallbackType callback)
{
  // Class instance to invoke it through
  ClassName objectInstance;

  // Invocation
  int result = (objectInstance.*callback)(1.0f);
}

//This method performs work using an object pointer
void DoWorkPointer(CallbackType callback)
{
  // Class pointer to invoke it through
  ClassName * pointerInstance;

  // Invocation
  int result = (pointerInstance->*callback)(1.0f);
}

int main(int argc, char ** argv)
{
  // Pass in SomeCallback to the DoWork
  DoWorkObject(&ClassName::Method);
  DoWorkPointer(&ClassName::Method);
}

请参阅上面的定义，其中声明将回调函数传递给其他函数，并在某个时刻调用它。

在c++中，让回调函数调用类方法是可取的。当您这样做时，您可以访问成员数据。如果你使用C语言定义回调函数，你必须将它指向一个静态成员函数。这不是很理想。

Here is how you can use callbacks in C++. Assume 4 files. A pair of .CPP/.H files for each class. Class C1 is the class with a method we want to callback. C2 calls back to C1's method. In this example the callback function takes 1 parameter which I added for the readers sake. The example doesn't show any objects being instantiated and used. One use case for this implementation is when you have one class that reads and stores data into temporary space and another that post processes the data. With a callback function, for every row of data read the callback can then process it. This technique cuts outs the overhead of the temporary space required. It is particularly useful for SQL queries that return a large amount of data which then has to be post-processed.

/////////////////////////////////////////////////////////////////////
// C1 H file

class C1
{
    public:
    C1() {};
    ~C1() {};
    void CALLBACK F1(int i);
};

/////////////////////////////////////////////////////////////////////
// C1 CPP file

void CALLBACK C1::F1(int i)
{
// Do stuff with C1, its methods and data, and even do stuff with the passed in parameter
}

/////////////////////////////////////////////////////////////////////
// C2 H File

class C1; // Forward declaration

class C2
{
    typedef void (CALLBACK C1::* pfnCallBack)(int i);
public:
    C2() {};
    ~C2() {};

    void Fn(C1 * pThat,pfnCallBack pFn);
};

/////////////////////////////////////////////////////////////////////
// C2 CPP File

void C2::Fn(C1 * pThat,pfnCallBack pFn)
{
    // Call a non-static method in C1
    int i = 1;
    (pThat->*pFn)(i);
}

回调函数是传递给例程的方法，在某个时刻被传递给它的例程调用。

这对于开发可重用软件非常有用。例如，许多操作系统API(如Windows API)大量使用回调。

例如，如果你想处理文件夹中的文件，你可以用你自己的例程调用一个API函数，并且你的例程在指定的文件夹中对每个文件运行一次。这使得API非常灵活。