I'm working at a module that use a callback system that wasn't implemented very nice. The clients are registering with an ID and will be called back with a variable (or two, or none). The problem is that for almost every ID is a different variable. (Ex: Id1 -> char*, Id2 -> int). This is achieved by passing variable via a pointer. So callback looks like
typedef void (*NotifFunctionPtr)(void* ctx, const void* option);
There are many problems with this approach like, and I want to replace this with a (type) safe and modern way of handling this. However this isn't as simple as it looks like, I have some ideas (like boost::function or replacing the void* with a struct that encapsulate type and ptr) but i think maybe there is a better idea, so I was wondering what is the modern way of settings a typesafe callback in C++.
Edit: Another idea is registering an callback with a type T via a template function that calls back with the same type T. Is this viable or implemented in a library somewere ?
Your problem is not that of callbacks, but rather that you want to treat all callbacks as the same type, when they are not (the signatures are different). So either you do the nasty C void* trick or if you want to use a type-safe approach you will have to pay for it, and provide different methods to register the different callback types --which IMHO is the right way.
Once you have solved that, you can use the signals or signals2 libraries or else implement your own wheel using function as a base (to avoid having to rewrite the type erasure).
boost::function is just the right choice here. You gain the type-safety of function objects without having to change the code much.
If you already looked into boost. Why not use the signals or signals2 library.
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.
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.
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.
I'm in the process of writing a kind of runtime system/interpreter, and one of things that I need to be able to do is call c/c++ functions located in external libraries.
On linux I'm using the dlfcn.h functions to open a library, and call a function located within. The problem is that, when using dlsysm() the function pointer returned need to be cast to an appropriate type before being called so that the function arguments and return type are know, however if I’m calling some arbitrary function in a library then obviously I will not know this prototype at compile time.
So what I’m asking is, is there a way to call a dynamically loaded function and pass it arguments, and retrieve it’s return value without knowing it’s prototype?
So far I’ve come to the conclusion there is not easy way to do this, but some workarounds that I’ve found are:
Ensure all the functions I want to load have the same prototype, and provide some sort mechanism for these functions to retrieve parameters and return values. This is what I am doing currently.
Use inline asm to push the parameters onto the stack, and to read the return value. I really want to steer clear of doing this if possible!
If anyone has any ideas then it would be much appreciated.
Edit:
I have now found exactly what I was looking for:
http://sourceware.org/libffi/
"A Portable Foreign Function Interface Library"
(Although I’ll admit I could have been clearer in the original question!)
What you are asking for is if C/C++ supports reflection for functions (i.e. getting information about their type at runtime). Sadly the answer is no.
You will have to make the functions conform to a standard contract (as you said you were doing), or start implementing mechanics for trying to call functions at runtime without knowing their arguments.
Since having no knowledge of a function makes it impossible to call it, I assume your interpreter/"runtime system" at least has some user input or similar it can use to deduce that it's trying to call a function that will look like something taking those arguments and returning something not entirely unexpected. That lookup is hard to implement in itself, even with reflection and a decent runtime type system to work with. Mix in calling conventions, linkage styles, and platforms, and things get nasty real soon.
Stick to your plan, enforce a well-defined contract for the functions you load dynamically, and hopefully make due with that.
Can you add a dispatch function to the external libraries, e.g. one that takes a function name and N (optional) parameters of some sort of variant type and returns a variant? That way the dispatch function prototype is known. The dispatch function then does a lookup (or a switch) on the function name and calls the corresponding function.
Obviously it becomes a maintenance problem if there are a lot of functions.
I believe the ruby FFI library achieves what you are asking. It can call functions
in external dynamically linked libraries without specifically linking them in.
http://wiki.github.com/ffi/ffi/
You probably can't use it directly in your scripting language but perhapps the ideas are portable.
--
Brad Phelan
http://xtargets.heroku.com
I'm in the process of writing a kind of runtime system/interpreter, and one of things that I need to be able to do is call c/c++ functions located in external libraries.
You can probably check for examples how Tcl and Python do that. If you are familiar with Perl, you can also check the Perl XS.
General approach is to require extra gateway library sitting between your interpreter and the target C library. From my experience with Perl XS main reasons are the memory management/garbage collection and the C data types which are hard/impossible to map directly on to the interpreter's language.
So what I’m asking is, is there a way to call a dynamically loaded function and pass it arguments, and retrieve it’s return value without knowing it’s prototype?
No known to me.
Ensure all the functions I want to load have the same prototype, and provide some sort mechanism for these functions to retrieve parameters and return values. This is what I am doing currently.
This is what in my project other team is doing too. They have standardized API for external plug-ins on something like that:
typedef std::list< std::string > string_list_t;
string_list_t func1(string_list_t stdin, string_list_t &stderr);
Common tasks for the plug-ins is to perform transformation or mapping or expansion of the input, often using RDBMS.
Previous versions of the interface grew over time unmaintainable causing problems to both customers, products developers and 3rd party plug-in developers. Frivolous use of the std::string is allowed by the fact that the plug-ins are called relatively seldom (and still the overhead is peanuts compared to the SQL used all over the place). The argument stdin is populated with input depending on the plug-in type. Plug-in call considered failed if inside output parameter stderr any string starts with 'E:' ('W:' is for warnings, rest is silently ignored thus can be used for plug-in development/debugging).
The dlsym is used only once on function with predefined name to fetch from the shared library array with the function table (function public name, type, pointer, etc).
My solution is that you can define a generic proxy function which will convert the dynamic function to a uniform prototype, something like this:
#include <string>
#include <functional>
using result = std::function<std::string(std::string)>;
template <class F>
result proxy(F func) {
// some type-traits technologies based on func type
}
In user-defined file, you must add define to do the convert:
double foo(double a) { /*...*/ }
auto local_foo = proxy(foo);
In your runtime system/interpreter, you can use dlsym to define a foo-function. It is the user-defined function foo's responsibility to do calculation.
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.