So I was reading the article on Handle in C and realized that we implement handles as void pointers so "whatever" Object/data type we get we can cast the void pointer to that kind of Object/data and get its value. So I have basically two concerns :
1.If lets say in following example taken from Handle in C
typedef void* HANDLE;
int doSomething(HANDLE s, int a, int b) {
Something* something = reinterpret_cast<Something*>(s);
return something->doit(a, b);
}
If we pass the value to the function dosomething(21,2,2), does that mean the value HANDLE points to is 21, if yes how does any object when we type cast it to that object, will it be able to use it, like in this example, so in otherwords where does pointer to the object Something, something, will store the value 21.
2.Secondly the link also says "So in your code you just pass HANDLE around as an opaque value" what does it actually mean? Why do we "pass handle around"? If someone can give more convincing example of handles that uses objects that will be great!
1.: A handle is an identifier for an object. Since "21" is no object, but simply a number, your function call is invalid. Your code will only work if 's' really points to a struct of type Something. Simply spoken, a handle is nothing than a pointer is nothing than a memory address, thus "21" would be interpreted as a memory address and will crash your program if you try to write to it
2.: "Opaque value" means that no developer that uses your code can't take any assumptions about the inner structure of an object the handle identifies. This is an advantage over a pointer to a struct, where a developer can look at the structure and take some assumptions that will not be true anymore after you changed your code. "Pass around" simply means: assigning it and using it as a function call parameter, like in:
HANDLE s = CreateAnObject();
DoSomethingWithObject( s );
HANDLE t = s;
etc.
By the way: In real code, you should give handles to your objects different names, like EMPLOYEE_HANDLE, ORDER_HANDLE etc. A typical example of a handle are window handles in Windows. They identify windows, without giving you any information about how a "Window" memory structure in the operating system is built, therefore Microsoft was able to change this inner structure without the risk of breaking other developer's code with changes to the "Window" structure.