I've started using OpenGL a while ago, using GLUT. You can't pass member functions to GLUT functions. (or pointers to members for that matter, though I did not explore that option really far).
I was wondering if there is a "decent" way, or what is the "most decent" way to solve this? I know you can use static member functions, but isn't there a better way?
I know there are other libraries, like SFML that are written in C++ and provide a C++ class-based interface, but I was wondering what the possibilities are concerning GLUT (freeglut to be exact).
First, GLUT is not for serious application work. It's for simple graphics demos. And for that purpose, it is fine. If you find yourself trying to do serious work in GLUT, you will find yourself spending lots of time working around its limitations. This limitation is only one of many that you will eventually encounter. GLFW, while still having this limitation (though the next version will not), is generally superior for serious application work.
Second, the "most decent" way to solve this depends on what you're doing. If you only have one window, then the correct solution is just a simple static function, which can global pointers (or functions that return global pointers) to whatever class you're interested in.
If you have multiple windows, then what you need is a global std::map that maps from GLUT's window identifiers to pointers to some object. Then you can get which window a particular function was called from and use the map to forward that call to the particular object that represents that window.
Passing member functions to glut, or any other library, is easy enough. GLUT is looking for a function pointer.
Let Controller be a class with a member function OnKeyPress that we want to send into glutKeyboardFunc. You might first be tempted to try something like
glutKeyboardFunc(&Controller::OnKeyPress);
Here, we are passing a function pointer, however this is incorrect, since you want to send the member function of that class object. In C++11 you can use the new std::bind, or if you are on an older compiler, I would recommend boost::bind. Either way the syntax is around the same.
using namespace std::placeholders; // for the _1, _2 placeholders
glutKeyboardFunc(std::bind(&Controller::OnKeyPress, &GLInput, _1, _2, _3));
From the documentation it looks like glutKeyboardFunc requires 3 parameters. First we fix the first argument memory address of your object, since its a member function, and then supply 3 placeholders.
For those new to std::bind, it feels odd, but for anyone who has done object oriented code in C, its obvious. The function is really just a C function, and needs the "this" pointer to the class. The bind would not be necessary if the callback was a simple function.
Related
So here's the situation: I'm using C++, SDL and GLConsole in conjunction. I have a class, SDLGame, which has the Init(), Loop(), Render() etc - essentially, it holds the logic for my game class.
GLConsole is a nice library so far - it lets me define CVars and such, even inside my SDL class. However, when defining commands, I have to specify a ConsoleFunc, which is typedef'd as
typedef bool (*ConsoleFunc)( std::vector<std::string> *args);
Simple enough. However, like I said, my functions are all in my class, and I know I can't pass pointer-to-class-functions as pointer-to-function arguments. I can't define static functions or make functions outside my class because some of these ConsoleFuncs must access class data members to be useful. I'd like to keep it OOP, since - well, OOP is nice.
Well, I actually have this problem "solved" - but it's extremely ugly. I just have an instance of SDLGame declared as an extern variable, and use that in my ConsoleFuncs/main class.
So, the question is: Is there a way to do this that isn't stupid and dumb like the way I am doing it? (Alternatively: is there a console library like GLConsole that supports SDL and can do what I'm describing?)
If the only interface you have is that function pointer, then you're screwed.
A member function needs a this pointer to be called, and if you have no way of passing that, you're out of luck (I guess the std::vector<std::string>* args pointer is what you get passed from the library).
In other words, even though that library uses C++ containers, it's not a good C++ library, because it relies on free functions for callbacks. A good C++ library would use boost::function or something similar, or would at the very least let you pass a void* user_data pointer that gets passed through to your callback. If you had that, you could pass the this pointer of your class, cast it back inside the callback, and call the appropriate member function.
So here's the situation: I'm using C++, SDL and GLConsole in conjunction. I have a class, SDLGame, which has the Init(), Loop(), Render() etc - essentially, it holds the logic for my game class.
GLConsole is a nice library so far - it lets me define CVars and such, even inside my SDL class. However, when defining commands, I have to specify a ConsoleFunc, which is typedef'd as
typedef bool (*ConsoleFunc)( std::vector<std::string> *args);
Simple enough. However, like I said, my functions are all in my class, and I know I can't pass pointer-to-class-functions as pointer-to-function arguments. I can't define static functions or make functions outside my class because some of these ConsoleFuncs must access class data members to be useful. I'd like to keep it OOP, since - well, OOP is nice.
Well, I actually have this problem "solved" - but it's extremely ugly. I just have an instance of SDLGame declared as an extern variable, and use that in my ConsoleFuncs/main class.
So, the question is: Is there a way to do this that isn't stupid and dumb like the way I am doing it? (Alternatively: is there a console library like GLConsole that supports SDL and can do what I'm describing?)
If the only interface you have is that function pointer, then you're screwed.
A member function needs a this pointer to be called, and if you have no way of passing that, you're out of luck (I guess the std::vector<std::string>* args pointer is what you get passed from the library).
In other words, even though that library uses C++ containers, it's not a good C++ library, because it relies on free functions for callbacks. A good C++ library would use boost::function or something similar, or would at the very least let you pass a void* user_data pointer that gets passed through to your callback. If you had that, you could pass the this pointer of your class, cast it back inside the callback, and call the appropriate member function.
I wrapped the SDL functions in C++ classes in order to make it simple for me to use them without complicating the code, but I wonder if it is good practice to do so, as, as far as I know, SDL is written for C, no?
Also:
The Core class I made is the one which initializes the screen, so is there any way I can make the screen I made in this class (the main screen) accessible in other classes? (I know I can just pass the pointer, I just don't like this way because it causes a lot of problems when the logic gets complicated.)
SDL is written in C, not just for C. It's perfectly fine to write a C++ wrapper round it if you want to.
As far as the screen goes, you make it available to other classes in exactly the same sort of way you'd make anything else available - you either pass the pointer around (sensible) or use some sort of global access point for it (usually discouraged).
It seems to me that I saw something weird being done in a boost library and it ended up being exactly what I'm trying to do now. Can't find it though...
I want to create a macro that takes a signature and turns it into a function pointer:
void f(int,int) {}
...
void (*x)(int,int) = WHAT( (f(int,int)) );
x(2,4); // calls f()
I especially need this to work with member function pointers so that WHAT takes two params:
WHAT(ClassType, (f(int,int)); // results in static_cast<void (ClassType::*)(int,int)>(&ClassType::f)
It's not absolutely necessary in order to solve my problem, but it would make things a touch nicer.
This question has nothing, per-se, to do with function pointers. What needs to be done is to use the preprocessor to take "f(int,int)" and turn it into two different parts:
'f'
'(int,int)'
Why:
I've solved the problem brought up here: Generating Qt Q_OBJECT classes pragmatically
I've started a series of articles explaining how to do it:
http://crazyeddiecpp.blogspot.com/2011/01/quest-for-sane-signals-in-qt-step-1.html
http://crazyeddiecpp.blogspot.com/2011/01/quest-for-sane-signals-in-qt-step-2.html
The signature must be evaluated from, and match exactly, the "signal" that the user is attempting to connect with. Qt users are used to expressing this as SIGNAL(fun(param,param)), so something like connect_static(SIGINFO(object,fun(param,param)), [](int,int){}) wouldn't feel too strange.
In order to construct the signature I need to be able to pull it out of the arguments supplied. There's enough information to get the member function address (using C++0x's decltype) and fetch the signature in order to generate the appropriate wrapper but I can't see how to get it out. The closest I can come up with is SIGINFO(object, fun, (param,param)), which is probably good enough but I figured I'd ask here before considering it impossible to get the exact syntax I'd prefer.
What are you trying to do is impossible using standard preprocessor, unfortunately. There are a couple of reasons:
It is impossible to split parameters passed to a macro using custom character. They have to be comma delimited. Otherwise that could solve your problem instantly.
You cannot use preprocessor to define something that is not an identifier. Otherwise you could use double expansion where ( and ) is defined as , and split arguments on that as if it was passed as f, int, int,, then process it as variadic arguments.
Function pointer definition in C++ does not allow you to deduce the name given to defined type, unfortunately.
Going even further, even if you manage to create a function pointer, the code won't work for methods because in order to invoke a method, you need to have two pointers - pointer to the method and to the class instance. This means you have to have some wrapper around this stuff.
That is why QT is using its own tools like moc to generate glue code.
The closes thing you might have seen in Boost is probably Signals, Bind and Lambda libraries. It is ironic that those libraries are much more powerful than what you are trying to achieve, but at the same time they won’t allow you to achieve it the way you want it. For example, even if you could do what you want with the syntax you want, you won’t be able to “connect” a slot to a “signal” if signal has a different signature. At the same time, libraries from Boost I mentioned above totally allow that. For example, if your “slot” expects more parameters than “signal” provides, you can bind other objects to be passed when “slot” is invoked. Those libraries can also suppress extra parameters if “slot” does not expect them.
I’d say the best way from C++ prospective as for today is to use Boost Signal approach to implement event handling in GUI libraries. QT doesn’t use it for a number of reasons. First, it started in like 90-s when C++ was not that fancy. Plus, they have to parse your code in order to work with “slots” and “signals” in graphic designer.
It seems for me than instead of using macros or even worse – non-standard tools on top of C++ to generate code, and using the following:
void (*x)(int,int) = WHAT( (f(int,int)) );
It would be much better to do something like this:
void f (int x, int y, int z);
boost::function<void (int, int)> x = boost::bind (&f, _1, _2, 3);
x (1, 2);
Above will work for both functions and methods.
What is mean by delegates in c++, does sort function in c/c++ which takes a compare function/functor as last parameter is a form of delegate?
"delegate" is not really a part of the C++ terminology. In C# it's something like a glorified function pointer which can store the address of an object as well to invoke member functions. You can certainly write something like this in C++ as a small library feature. Or even more generic: Combine boost::bind<> with boost::function<>.
In C++ we use the term "function object". A function object is anything (including function pointers) that is "callable" via the function call operator().
std::sort takes a "predicate" which is a special function object that doesn't modify its arguments and returns a boolean value.
Callback functions in C++ can be (loosely) referred as a form of delegates ( though delegate term is not used for this). The callback functions use Pointers to Functions to pass them as parameters to other functions.
But delegates in C# is more advanced compared to callback functions in C++.
To delegate work means to share the work load with others. In real life, if you were to delegate your task, ie if you are a manager, you would be sharing your work expecting others to complete a task without you having to know how.
The concept is the same in C++ and any other languages having the capability of delegates. In C you could see this as a delegate:
int calculate(int (*func)(int c), int a, int b)
Because you are expected to send a pointer, to another function which will compute some work for you. I recently wrote a blog post on function pointers in Python and C, check it out, you might find it helpfull. This might not be the "traditional" way to delegate work in C or C++, but then again, the termonoligy says i am a bit right.
Delegation is mostly used as a way to pass functions to functionality embedded in a class (pimpl, aggregation, private inheritance). They are mainly (inlined) functions of one line, calling functions of member-classes. As far as I know, it has nothing to do with C#'s delegates.
In this sense, a function-pointer as used in qsort is not a delegate, but a callback in which framework modules can be extended by user-software as in the Hollywood principle.
Delegate: An object that acts like a multi-function pointer with a subscription system. It really simplifies the use of static or 'object' member function pointers for callback notifications and event handling.
This link explains Delegates in a lucid manner or you may also refer to the MSDN link.