We value your feedback.
Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!
Course Of The Month
5 days, 17 hours left to enrollBecome a Premium Member and unlock a new, free course in leading technologies each month.
Solved
Wrapper C++ classes for API's with callbacks.
Posted on 2000-03-09
Hi.
Question: What's the best way to wrap an API call that receives a callback function pointer in a C++ class.
I'm asking because:
1. Callback functions have to be static.
2. They need to access the members of the class in which they reside. In order for them to do that, these have to be static as well...
I tried taking the callbacks outside the class definition, but that's plain ugly. It worked though - But I had to pass them a pointer to my class, which meant they could only access the public members...
Is there a smarter way of solving this?
Thanks,
Assaf.
Question: What's the best way to wrap an API call that receives a callback function pointer in a C++ class.
I'm asking because:
1. Callback functions have to be static.
2. They need to access the members of the class in which they reside. In order for them to do that, these have to be static as well...
I tried taking the callbacks outside the class definition, but that's plain ugly. It worked though - But I had to pass them a pointer to my class, which meant they could only access the public members...
Is there a smarter way of solving this?
Thanks,
Assaf.
[X]
Welcome to Experts Exchange
Add your voice to the tech community where 5M+ people just like you are talking about what matters.
- Help others & share knowledge
- Earn cash & points
- Learn & ask questions
-
3
-
3
7 Comments
Most windows callback function are passed a VOID * pointer that is specified when the callback is "registered". You can use this VOID * to specify a pointer to an object whose member function is called. The static callback function can convert this VOID * pointer back to a pointer to an object and then call the member function.
example follows.
example follows.
class EnmWnd
{
static BOOL CALLBACK EnmPrc(HWND WndHnd,LPARAM LngPrm)
{
EnmWnd *ObjPtr = (EnmWnd *)LngPrm; // Convert back to a object pointer.
return ObjPtr->WndFnd(WndHnd); // Call member procedure.
}
public:
void Enumerate()
{
EnumWindows((WNDENUMPROC) EnmPrc,(LPARAM) this);
}
BOOL WndFnd(HWND WndHnd)
{
// Do something with the window handle and this object.
return TRUE;
}
};
{
static BOOL CALLBACK EnmPrc(HWND WndHnd,LPARAM LngPrm)
{
EnmWnd *ObjPtr = (EnmWnd *)LngPrm; // Convert back to a object pointer.
return ObjPtr->WndFnd(WndHnd); // Call member procedure.
}
public:
void Enumerate()
{
EnumWindows((WNDENUMPROC) EnmPrc,(LPARAM) this);
}
BOOL WndFnd(HWND WndHnd)
{
// Do something with the window handle and this object.
return TRUE;
}
};
Redesign.
You might be able to use a global variable to store a pointer to the object. (Assuiming you don't have to worry about multi-threadding.)
But honestly, I dont't think that will ever be a problem. I can't think of any that doesn't take a parameter like this.
You might be able to use a global variable to store a pointer to the object. (Assuiming you don't have to worry about multi-threadding.)
But honestly, I dont't think that will ever be a problem. I can't think of any that doesn't take a parameter like this.
Featured Post
We value your feedback.
Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!
Question has a verified solution.
If you are experiencing a similar issue, please ask a related question
Written by John Humphreys
C++ Threading and the POSIX Library
This article will cover the basic information that you need to know in order to make use of the POSIX threading library available for C and C++ on UNIX and most Linux systems.
[s…
Container Orchestration platforms empower organizations to scale their apps at an exceptional rate. This is the reason numerous innovation-driven companies are moving apps to an appropriated datacenter wide platform that empowers them to scale at a …
The viewer will learn how to use the return statement in functions in C++. The video will also teach the user how to pass data to a function and have the function return data back for further processing.
The viewer will be introduced to the technique of using vectors in C++. The video will cover how to define a vector, store values in the vector and retrieve data from the values stored in the vector.
Suggested Courses
Course of the Month5 days, 17 hours left to enroll
707 members asked questions and received personalized solutions in the past 7 days.
Join the community of 500,000 technology professionals and ask your questions.