I have a Java function which can take variable number of parameters and in JNI I am receiving all the parameters in jobjectArray. But the problem is all the parameters available in String type,but originally thy are of different datatype. So in c/c++ converting them to their original type is not possible. So if i could call some other java method which will make these conversions easy for me.is it possible in to call a java method from native code in JNI?
Please help me out. I am really struck at it from a long time. Thanks in advance.
Here's a succinct example: http://journals.ecs.soton.ac.uk/java/tutorial/native1.1/implementing/method.html.
Fundamentally you need to look up native objects representing the class and method you want to call, format your arguments, call the appropriate JNIEnv->CallXXX method, and process the resulting value.
we can pass the String as a parameter from native code when you callback the java method using env->NewStringUTF(actual parameter).
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I'm just looking for solution how to pass object from C to lua callback as function argument, is it even possible? I cannot find any referece. just trying something like this:
luabind::call_function(State, "Callback_name", object);
but luabind is not a option in this case because it is obsolete and additionally some features just don't work that's probably because of boost as I read but don't know exactly, thats why I switched to toLua++ but still cannot find any solution for it. I'm just trying to achieve to access data from C in lua callback ex:
function LUA_Callback(object1, object2)
print(object1.name)
print(object2.size)
end
Is there any way to solve it or library(without boost) to do this?
That depends on what you mean by "push object". It's trivial to push numbers, strings, C functions, tables, etc. -- in other words, Lua objects -- using API routines like lua_pushstring, lua_pushnumber, etc.
If you're talking about pushing C++ objects, you need to create and push userdata. If you want them to behave like objects from Lua, then you'll need the appropriate metatable/methods.
I am just going through a problem that I haven't before in C/C++, and I have no idea how to solve it. Reflection. I need to call a function or method by a string that was given by the user. Not just this, I also need to give the function or method some parameters and get its result if any.
Imagine the user has typed printSomething.
I need to evaluate "printSomething"(paramA, paramB). Of course, the function or method T printSomething() is defined.
How is the best way I can do it?
Use a structure mapping from strings to pointers to functions or methods (member functions).
C++ doesn't provide such a structure; you will have to build it yourself, passing in the name-strings and the pointers. Conversion of parameters and return values to and from strings also needs to be implemented. The language has no conventions or ideas about how this is to be done, so you must specify it.
I've got an implemented function MyFunc(int, double, string) in my project. How can I call this function with necessary parameters having its string representation, for example
std::string str = "MyFunc(2, 3.12, \"Lemon Juice\")";
And what about the standard functions, for example, how can I call time() having std::string "time()"?
Ok, here's more detailed task.
I've got a map <string, Data>
Data is a class-wrapper with many childs. When I call Data.GetValue() method, it returns a std::string, depending of child class inner data. One of Data childs must return the string representation of int, another - the string representation of double, and the last one - the string representation of current date and time. Here's the problem - I don't know how to call the standard function ctime() for getting information from one of Data childs.
You cannot in general execute code in C++ whose source is contained in a string. Probably the most compelling motivation for this is so that a C++ implementation is not required to have a C++ compiler on the machine that the code runs on, although there is more to it than just that.
You may be able to do it for specific cases. That could be:
Platform-specific, for example:
Wrap the source up in some boilerplate, then either call the compiler externally, link against it, or embed it in your program. This gives you a program or library.
Run that new program externally, or dynamically link against that library.
Input-specific, for example:
You could parse a function call to a particular known function, with literals as arguments, relatively easily without needing a complete C++ compiler. Get the argument values out into variables and call the function. But if you're going to do that, you could specify a more easily parsable format for the string, than one that looks like a C++ function call.
It sounds as though in your task you have a string that is one of:
a representation of an int
a representation of a double
a representation of a date and time.
You don't say what you want to do with this string, but whatever that is you probably need to (a) examine the string to find out which of the three it is, and then (b) do something appropriate to that format. Better, you could give the derived class the responsibility of returning the same representation no matter which of the three GetValue() returns. For example, if what you really want is seconds since 1970, add a GetSecondsSinceEpoc function, and implement it differently in each class.
As mentioned by others, C++ in itself is not able to do that. However external frameworks can help you.
ROOT (used at CERN) provides reflection for C++ along with an interpreter. You will be able to execute/interpret a method call or a macro written in C++ from within your code.
You can not do that using C++.
I'd like some advice on how to check for the correctness of the parameters I receive.
The checking is going to be done in C++, so if there's a good solution using Boost.Python (preferably) or the C API, please tell me about that. Otherwise, tell me what attributes the object should have to ensure that it meets the criteria.
So...
How do you check that an object is a function?
How do you check that an object is a bound method?
How do you check that an object is a class object?
How do you check that a class object is a child of another class?
When in doubt just work out how you would get the required effect by calling the usual Python builtins and translate it to C/C++. I'll just answer for Python, for C you would look up the global such as 'callable' and then call it like any other Python function.
Why would you care about it being a function rather than any other sort of callable? If you want you can find out if it is callable by using the builtin callable(f) but of course that won't tell you which arguments you need to pass when calling it. The best thing here is usually just to call it and see what happens.
isinstance(f, types.MethodType) but that won't help if it's a method of a builtin. Since there's no difference in how you call a function or a bound method you probably just want to check if it is callable as above.
isinstance(someclass, type) Note that this will include builtin types.
issubclass(someclass, baseclass)
I have two unconventional recommendations for you:
1) Don't check. The Python culture is to simply use objects as you need to, and if it doesn't work, then an exception will occur. Checking ahead of time adds overhead, and potentially limits how people can use your code because you're checking more strictly than you need to.
2) Don't check in C++. When combining Python and C (or C++), I recommend only doing things in C++ that need to be done there. Everything else should be done in Python. So check your parameters in a Python wrapper function, and then call an unchecked C++ entry point.
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.