NSClassFromString returns nil

Question

Why does NSClassFromString return nil ? As per the definition it has to return class name. How should I take care to rectify this problem? I need to instantiate a class from string and call the method, which is in the class, using the instance created.

This is how my code looks like:

id myclass = [[NSClassFromString(@"Class_from_String") alloc] init];
[myclass method_from_class];

But the method_from_class function is not being called, control is not going into it. And my code is error free. Any idea how to solve this in Objective-C?

Solution 1

The Documentation for the function says:

Return Value The class object named by aClassName, or nil if no class by that name is currently loaded. If aClassName is nil, returns nil.

An example of how this should be properly used is as follows:

Class dictionaryClass = NSClassFromString(@"NSMutableDictionary");
id object = [[dictionaryClass alloc] init];
[object setObject:@"Foo" forKey:@"Bar"];

Solution 2

If you are trying to instantiate a class from a static library, you must add the "-ObjC" flag to the "Other Linker Flags" build setting.

Solution 3

This happened to me when I add an external file to the Xcode project. Adding the .m file to Build Phases > Compile Sources solve the problem.

Solution 4

It is possible that your class is not getting linked if this is the only reference to it.

I had a factory method to instantiate various types of subclass. The factory had a switch statement that went to the appropriate subclass and alloc'd and init'ed it. I noticed that all of the alloc/init statements were exactly the same, except for the name of the class. So I was able to eliminate the entire switch block using the NSClassFromString() function.

I ran into the same problem - the return was nil. This was because the class was not used elsewhere in the program, so it wasn't getting linked, so it could not be found at runtime.

You can solve this by including the following statement:

[MyClass class];

That defeats the whole purpose of what I was trying to accomplish, but it might be all you need.

Solution 5

You also need to make sure the class you are trying to instantiate is included in the project. If you added it later, you made need to click the checkbox next to the Target you are building.

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Comments

[Brandon Bodnar]

Look at stackoverflow.com/questions/1634644/…

[Stefan]

My code is error-free, that's a nice one. :)

[suse]

:) :) but without serving my purpose :(

[suse]

But the value in the string @"Class_from_String" has the value of the class name. because i've printed it just above ,before instantiating the class from string.Then how come it is taking as nil?

[suse]

hey 1've written 3rd line as: [object TestMethod]; TestMethod is the method in same class, but stil the control is not going into the method defination.

[suse]

wat should i do. plz tel me some alternative.

[suse]

hi Dave.. do u have any idea, for alternative methos for NSClassFromString in Objective-C?My main purpose is to create an instance of class from string and to call a method using the instance created.

[Stefan]

@suse: Either your string isn't getting loaded correctly or there is no class with that name known to the runtime, in which case you have to load it first.

[NeilJiang]

add this flag to the static lib or to the main program which uses the static lib?

[zerodog]

Worked for me setting it in the main program.

[SpacyRicochet]

@GeorgSchölly What could cause a class not to be loaded to the runtime? I'm having the same issue.

[Stefan]

@SpacyRicochet: Open a new question where you describe why you think your class should get loaded. Then we might be able to figure out why it doesn't work.

[newacct]

"By the way, is method_from_class an instance method or a class method ?" Well, if it wasn't an instance method, his code should crash.

[matm]

@Dave: In my opinion, when no class can be loaded with a given name, return value should be Nil ((Class)0 - literal null value for classes), not nil ((id)0 - literal null value for objects). Is the documentation not precise here or am I missing something?

[Dave DeLong]

@delirus they're the same thing, so it doesn't really matter. Nil may be more semantically correct, but in the end it's all just 0.

[matm]

@Dave: semantical correctness was my only concern. Thanks for feedback.

[defactodeity]

You are awesome! Saved my life! :) Thank you very much.

[David Manpearl]

This answer worked for me. I added the new class while a different target was selected. I had to select: Target > Build Phases > Compile Sources > '+' to get the class really added to the project.

[SpacyRicochet]

Interestingly enough, I came upon this post again due to another issue. And again, @GeorgSchölly's comment helped! Add -ObjC to the linker flags did the trick.

[Harrison Xi]

This also happened to me when I added .m file but forgot to write @implementation