# Callbacks

**URL:** <https://discourse.glfw.org/t/callbacks/471>\
**Category:** support\
**Created:** [May 29, 2007, 4:44pm UTC](https://discourse.glfw.org/t/callbacks/471 "2007-05-29T16:44:27Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![system](https://canada1.discourse-cdn.com/flex035/uploads/glfw/original/1X/b8cdefe6b61b8d5aee6c43b2c56e9cf5eabecfe4.svg) [@system](https://discourse.glfw.org/u/system)\
**Post date:** [May 29, 2007, 4:44pm UTC](https://discourse.glfw.org/t/callbacks/471/1 "2007-05-29T16:44:27Z")

</div>

**[machee](https://sourceforge.net/u/machee/)** wrote on [Tuesday, May 29, 2007](https://sourceforge.net/p/glfw/discussion/247562/thread/cd8e6bac/#bc38):

This probably isn’t directly a GLFW issue, but I figure since it’s related I’d ask for help here. I’m using Visual C++ 2005 Express.

I’m trying to use a member function of a class as a KeyCallback function. The problem I’m having is that the way I get it working, I can’t access the other members of the class within the function.

class MyClass  
{  
public:  
static void GLFWCALL KeyCallback(int key, int action);  
bool Running;  
void Init();  
};

void GLFWCALL MyClass::KeyCallback(int key, int action)  
{  
Running = false;  
}

void MyClass::Init()  
{  
Running=true;

```
glfwInit\(\);
glfwOpenWindow\( 640, 480, 0,0,0,0,0,0, GLFW\_WINDOW \);
glfwSetKeyCallback \(KeyCallback\);

while \(Running\)
\{
    ...
    glfwSwapBuffers\(\);
\}

```

}

If I take “static” out, I can get to the other members of the class, but it won’t compile:

error C3867: ‘MyClass::KeyCallback’: function call missing argument list; use ‘&MyClass::KeyCallback’ to create a pointer to member

Changing the SetKeyCallback to &MyClass::KeyCallback also gives an error compiling:

error C2664: ‘glfwSetKeyCallback’ : cannot convert parameter 1 from ‘void (\_\_thiscall MyClass::\* )(int,int)’ to ‘GLFWkeyfun’

Changing the SetKeyCallback to just &KeyCallback gives:

error C2276: ‘&’ : illegal operation on bound member function expression

I’m not sure where to go from here…

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<div class="post-metadata">

**Author:** ![system](https://canada1.discourse-cdn.com/flex035/uploads/glfw/original/1X/b8cdefe6b61b8d5aee6c43b2c56e9cf5eabecfe4.svg) [@system](https://discourse.glfw.org/u/system)\
**Post date:** [May 29, 2007, 5:21pm UTC](https://discourse.glfw.org/t/callbacks/471/2 "2007-05-29T17:21:09Z")

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**[peterpp](https://sourceforge.net/u/peterpp/)** wrote on [Tuesday, May 29, 2007](https://sourceforge.net/p/glfw/discussion/247562/thread/cd8e6bac/#6aeb):

The solution I use is creating a singleton. The simplest implementation is:

class MyClass  
{  
public:  
MyClass();  
static void GLFWCALL KeyCallback(int key, int action);  
bool Running;  
static MyClass \*Instance;  
};

MyClass::MyClass()  
{  
Instance = this;  
}

void GLFWCALL MyClass::KeyCallback(int key, int action)  
{  
MyClass::Instance-\>Running = false;  
}

---

<div class="post-metadata">

**Author:** ![system](https://canada1.discourse-cdn.com/flex035/uploads/glfw/original/1X/b8cdefe6b61b8d5aee6c43b2c56e9cf5eabecfe4.svg) [@system](https://discourse.glfw.org/u/system)\
**Post date:** [August 25, 2008, 12:55pm UTC](https://discourse.glfw.org/t/callbacks/471/3 "2008-08-25T12:55:03Z")

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**[blundis](https://sourceforge.net/u/blundis/)** wrote on [Monday, August 25, 2008](https://sourceforge.net/p/glfw/discussion/247562/thread/cd8e6bac/#3a9f):

Hi.

I hope nobody minds me digging up this thread, but I have been trying to implement the solution shown here by Peter without much success and was hoping someone could help me.

My code looks like this:

class Input  
{  
public:  
Input();  
bool isKeyPressed(int key);  
// is called by glfw, do not call  
static void GLFWCALL \_keyCallback(int key, int action);

```
static Input \*Instance;

```

private:  
bool \_keys[GLFW\_KEY\_LAST];  
};

Input::Input()  
{  
Instance = this;  
glfwSetKeyCallback(\_keyCallback);  
}

bool Input::isKeyPressed(int key)  
{  
return \_keys[key];  
}

void GLFWCALL Input::\_keyCallback(int key, int action)  
{  
Input::Instance-\>\_keys[key] = (action == GLFW\_PRESS);  
}

I just keep getting the very unhelpful error, at least to me as I’m quite new to C++:  
/usr/libexec/gcc/i686-apple-darwin8/4.0.1/ld: Undefined symbols:  
Input::Instance

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<div class="post-metadata">

**Author:** ![system](https://canada1.discourse-cdn.com/flex035/uploads/glfw/original/1X/b8cdefe6b61b8d5aee6c43b2c56e9cf5eabecfe4.svg) [@system](https://discourse.glfw.org/u/system)\
**Post date:** [August 25, 2008, 1:01pm UTC](https://discourse.glfw.org/t/callbacks/471/4 "2008-08-25T13:01:14Z")

</div>

**[blundis](https://sourceforge.net/u/blundis/)** wrote on [Monday, August 25, 2008](https://sourceforge.net/p/glfw/discussion/247562/thread/cd8e6bac/#bc38/e047):

Sorry to double post but I think my problems might be related to me completely misunderstanding how ‘static’ works. Going to look in to that by myself and see if i can come up with something.