# CodeGuru Technical FAQs > CodeGuru Individual FAQs >  C++ Callback: How to use class member functions as callbacks?

## Andreas Masur

*Q:* How to use class member functions as callbacks?

*A:* The problem is that every callback function has its own prototype, which determines the parameters that gets passed from the operating system to it.

In C++ every member function has a hidden parameter - the so-called 'this' pointer which will be automatically passed to the function. C++ is able to associate a function with a particular instance of an object by means of the 'this' pointer. Member functions access member variables through the 'this' pointer...



```
class foo
{
public:
  void func() { integer_ = 0; }

private:
  int integer_;
};
```

If you compile this code it will be compiled as



```
class foo
{
public:
  void func(foo* this) { this->integer_ = 0; }

private:
  int integer_;
};
```

The operating system does not call callback functions through objects therefore it cannot handle the automatically added 'this' pointer... To get a member functions working as a callback routine you need to tell the compiler explicitly not to expect a 'this' pointer. To avoid the automatic 'this' pointer you have two possibilities:

Non-member functionsStatic member functions
Non-member functions are not part of a class and therefore do not have a 'this' pointer. Static member functions do not receive a 'this' pointer either...thus, if you want to use a member function as a callback routine you need to declare it as 'static'...

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