Is it possible to separate class definition and implementation in MetaTrader 4, as in C++ .h and .cpp files?All MetaTrader examples have a class definition and implementation in the same file.I would like to give header and *.ex4 compiled files, as in C++ I would give header and lib/dll files.
I found this post that explains how to do.
Related
I know that .m files are where the implementation is and .h files have the method signatures, etc. When one wants to use a certain class in his class, then he imports the .h file. Preprocessor replaces the import .h file with the content of the .h file. What I don't understand is how come access to implementation become available from just preprocessor bringing the .h content? What is the runtime mechanism that allows this?
Importing the .h file isn't actually what does that, so you're correct to be confused!
When a program is compiled, each file is compiled to an "object file", and those are all linked together into an executable program. It's this linking step that provides access to the implementation.
Similarly, any libraries you use need to be linked against (Xcode's project templates do this for you for Foundation, UIKit/AppKit, and other common libraries). This type of linkage is done partially at compile time, then finished dynamically when your app launches, so that it gets the version of the libraries included with the OS instead of the version you compiled with.
Importing the header simply lets the compiler know what things are in the linked library so that it can compile code that references them. If you look up the functionality you use dynamically instead of letting the compiler do it (via dlopen, dlsym, NSClassFromString, NSSelectorFromString, etc...), then you can use linked code without importing its header.
I'm just learning objective-c after a fair amount of experience with C#. One of the things I sorely miss is the ability to write extension methods in a separate project that I could reference in all of my projects. Here's some naive c#:
public static bool IsShortString(this string s) {
return s.length <= 3;
}
In Visual Studio, I could just add a reference, an using, and bam myString.IsShortString() would be a, rather useless, method.
So I think I want to write a static library, but I'm not sure where I'm going from there.
One additional question, if I do write this static library, will I be able to use all of the methods throughout various files in the library using one #import directive, or will I have to import each header individually?
What you are looking for is called Category, and it allows you to add some additional methods to existing classes. Check the reference http://developer.apple.com/library/ios/#documentation/cocoa/conceptual/objectivec/chapters/occategories.html
You can create your own toolkit which is a static library containing categories you made. Common practice is to create one header file containing imports for all the headers in the lib, so when using it, you just do
#import "libName.h"
Also, when creating a static library containing categories it is important to include -all_load and -ObjC linker flags to your project.
The closest thing in objective-c is categories.
This is also a good tutorial on categories.
Where can i find a concrete document or a dos and donts documentation on using C++ with Objective-C?
Apple seems to have removed that document from their website and i am all puzzled with collating bits of information from blogs and questions posted here.
Anyone can guide about the same.
When do we use .mm file, while mixing syntax or while using an object in .m file which belongs to a C++ class ?
While passing objects between functions belonging to two different language like passing objective-c object to a function in cpp file is it necessary to collect it in void * or can I use (ObjectiveC inteface)*?
You need to use Objective-C++ whenever you are either #include/#importing or directly writing both Objective-C and C++ code in the same file. It's usually obvious with explicit code; the #includes are often less so, and you need to take care to avoid "leaking" one of the languages into too much of the other. Your example of #importing a C++ header file is clear-cut: you can only do that with Objective-C++. Note that if your Cplusplus was a struct type, you could forward-declare it as such instead of #importing a C++ header.
If you do this in a header, it will "poison" the header to only work in that mode. You'll need to actively avoid this, or your whole project will soon end up with only .mm files. I have documented some techniques in this article and previously, in this earlier article. For newer versions of Objective-C, you can also add ivars to classes in category extensions. This means you can define C++-typed ivars in your .mm file, not the header, which allows .m files to #import it.
For your second question (Please only ask one question at a time): the id type is defined in the objc.h header file in terms of C and thus will work in both Objective-C and C++. Likewise, the Objective-C runtime API is exposed in terms of C functions, which will work from C++, too. If you actually want to send messages and access properties on Objective-C objects with the canonical syntax from C++ code, you'll need to switch that file to Objective-C++.
Use .mm files when you have a c++ syntax in your code or when including file(s) which contain c++ code.
Mixing C++ with objective-c may be a bit confusing but if you think pointer-wise than it is not a big deal. Treat C++ object instance methods as you would in C++ and the same goes for objective c objects.
I am working on a project that requires a third party library implemented in C++. I have successfully added library to my xcode project, but the problem is that the classes in library contains namespaces and when I try to access methods via namespaces, the XCode generates an error that: "utils undeclared". "utils" is the namespace I am trying to use.
My question is that is there a way to use C++ namespaces in ObjectiveC?
The code I am using to call the method is:
utils::method();
I have tried renaming my ObjectiveC ".m" file to ".mm" file, but the problem remains the same.
We are using C++ libraries in Objective-C and have no problem using C++ namespaces. As Mustafa has indicated, you need to change the Objective-C file extension to .mm to get XCode to recognize the file as Objective-C++. Then you just need to #include (not #import) the C++ headers containing the C++ namespace declarations - this is as you would normally do for 'normal' C++.
I have an Objective C Class called Donald, I also have a C++ class called Donald in a static library that I would like to use in the same project. They both have a header file called Donald.h. Is there a way to do this?
You can include both header files by specifying a bit more of the path e.g.
#import "staticlibraryheaders/Donald.h"
#import "Donald.h"
However, you might find that the code won't compile since you are declaring two types both called Donald. If the compiler sees:
Donald* duck;
How does it know to type duck as a pointer to an instance of the C++ class or the Objective-C class? You might be able to fix that if the C++ class is in a C++ namespace. However, that hits the limit of my C++ knowledge.