I need a kind of variable in c++ to save all objects and ... like "Object" in c# anyway I want to pass every kind of vars in it . (unknown variables )
thanks.
Check out boost::any and boost::variant from the Boost library.
That said, usually a need to circumvent the type system is an indication of something wrong. Instead of using one of the aforementioned solutions, I recommend thinking hard about what constraints put you in this direction.
Related
I am tryintg to cast const FB::variant& sample into SampleJS* in C++.
like this:
SampleJS* info = sample.cast<SampleJS*>();
i do not know what is going wrong here.
this gives me error of:
boost::exception_detail::clone_impl<boost::exception_detail::error_info_injector<boost::bad_any_cast> >
Thank you in advance.
You need to understand that FB::variant just stores whatever type is put into it. Your code will absolutely work... if what is inside happens to be a MouseInfoJS*. However, you'd have to go through a lot of work to put something like that inside, since the FB::variant class is designed to make it difficult, but not impossible, to store inside it types that it doesn't know.
So basically, what you're trying to do probably doesn't make any sense, so you can't do it.
Depending on what type MouseInfoJS is, it might sorta make sense. Does MouseInfoJS inherit from JSAPI? (or JSAPIAuto?) If so, then it would somewhat make sense to try what you're doing, except that you'd never have a MouseInfoJS*, you'd have a MouseInfoJSPtr, which would be a typedef for boost::shared_ptr, since it would then be partially owned by the page and thus dangerous to store a raw pointer there.
Even in this case, which you'd need to use .convert_cast<MouseInfoJSPtr>, not .cast, it won't work on most modern browsers because they wrap the NPObject returned by FireBreath in another object which doesn't allow us to get the original object back; I believe this is a security feature. For more information, see A firebreath JSAPI will not be recognized in firefox
In C# you can write the below and if the type is correct it just works. Is there something like that which exist in C++?
object o = anything;
...
var anything2=(Anything)o;
Maybe boost::any is what you are looking for? It is not quite the same but might be applicable for your particular scenario
Avoid using object use interface or templates instead. Which is the reason you need something like that?? In case if you need to store a group of objects in the same list (for example) or something like that then all of your objects probably has something common. So all of them should implement an interface and your list will be like ( std::list< IMyObject* > ).
If you want a type that is a pointer to anything, then that would be void*.
The difference is that in C#, you can safely convert (almost) anything into a reference. In C++, it's not that simple and if you have something that's not a pointer, you can't just convert it to void* and expect it to work.
But, I try to avoid using object in C# whenever possible. And the same applies to void* in C++. Try to use the type system, not work around it.
I am trying to understand the programming of Siemens scanner using C++ and given that my C++ skills are limited, I am having problems in understanding many parts of the code provided by the vendor.
Problem 1
For instance, the code uses reference (rMrProt) to object MrProt and notations (such as the use of use of (). and ()[].) are very confusing to me.
For instance:
ImageSamples = rMrProt.kSpace().baseResolution()
ImageSize = rMrProt.sliceSeries()[0].readoutFOV()
Some explanation of these statements would be appreciated.
All information regarding object MrProt are in “MrProt.h”, “MrProt.dll”, “MrProt.lib”. All these files have been shared at:
https://docs.google.com/open?id=0B0Ah9soYnrlIYWZkNDU2M2EtYTNmNC00YTc5LTllMzItYzIyMWU4M2ZhY2Fi
Problem 2
Also, I have been trying to read MrProt.dll and MrProt.lib without any success. Only now, I came to know of dumpbin. Any help would be appreciated.
Problem 3
Another confusion that I have is related to some part of MrProt.h itself. There is a statement in MrProt.h:
class __IMP_EXP MrProt: public MrProtocolData::MrProtDataDelegate
{
typedef MrProtocolData::MrProtDataDelegate BasicImplementation;
public:
MrProt();
MrProt(const MrProt& rSource);
…
….
}
Here, __IMP_EXP, I guess that it’s some compiler specific stuff.. some decoration etc. But, I still have no idea what to make of this.
Problem 1.
rMrProt.sliceSeries()[0].readoutFOV()
means
Take rMrProt's sliceSeries member and call that. Apparently, it returns an array-like object, something that can be indexed.
From the result, take the first element ([0]). That's some kind of object.
On that element/object, call readoutFOV.
Problem 2. You're not really supposed to read binary files. There should be documentation with them.
1)
ImageSamples = rMrProt.kSpace().baseResolution()
This is just method chaining. You call the method kSpace() on rMrPrto which returns an object, and you call baseResolution() on that object.
2) Those are binary files. What would you expect to see? To read them you'd have to be an expert in asm or at least know some low-level concepts.
3) __IMP_EXP is a common type of directive that tells the compiler that the class is either exported or imported.
It expands to _declspec(dllimport) or _declspec(dllexport), depending on whether the definition of the class is in the current module or another module.
identifier() is a method/function call
identifier[i] returns the i'th element in an array.
identifier()[i] returns the i'th element of the array returned by identifier()
I can only help on problem 1:
if the return value of rMrProt.kSpace() is a struct. instead of saving it to a struct and then access it's member you can directly access a member of his with rMrProt.kSpace().MemberName
same for rMrProt.sliceSeries() which I guess is returning an array. so rMrProt.sliceSeries()[0] will access the first value in the returning array.
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.
I'd like to generate a number of objects (in C++) based on the amount/number the user enters.
Now I've somewhere heard that it has to be done using pointer tricks, creating a pointer to an array of the Object type required, and then dynamically increasing the size of array ( at runtime ).
Isn't there a workaround of directly using names like
Object1, Object2..... ObjectX
instead of having
Classname *Object[]
and then using the array index to get the object ?
In either case, it'd be great if someone could clarify on the issue.
Thanks !
If you want dynamically-sized array, then use std::vector. You won't be able to resize a built-in array.
If you want to be able to get an object by string name, then you should use std::map, it has an indexer:
std::map<string, Classname> myMap;
myMap["Object1"] = Classname();
Classname newClassname = myMap["Object1"];
So far no-one has explained why your thinking is flawed. C++ is a compiled language, and it goes to great lengths to turn the source program into efficient machine code. For this reason, the names you give variables are available to the program only at compile time, when you turn it from source into an executable file. Afterwards, when you want to create objects dynamically, those kinds of information are no longer available. The program only knows about the machine addresses where operands to machine instructions are located.
No, there isn't. Moreover, you don't need to; use std::vector.
When I began programming 9 years ago I asked myself the same question. The answer is: you can't.
You can indeed use an array and resize it dynamically, however using an stl vector is much easier (once you learn how to use it).
You can not do that because C++ doesn't have an "environment" (reflection) where variables (and metadata) can reside. Moreover, in C++ all variable names are vanished when the code is compiled.
A way to achieve the effect you want is to use a Map where the keys are strings.