how to forbid the basic init method in a NSObject

I want to force user to use my own init method (for example -(id)initWithString:(NSString*)foo;) and not the basic [[myObject alloc]init];.

how can I do that?

The accepted answer is incorrect - you CAN do this, and it's very easy, you just have to be a bit explicit. Here's an example:

You have a class named "DontAllowInit" which you want to prevent people init'ing:

@implementation DontAllowInit

- (id)init
{
    if( [self class] == [DontAllowInit class])
    {
        NSAssert(false, @"You cannot init this class directly. Instead, use a subclass e.g. AcceptableSubclass");

        self = nil; // as per @uranusjr's answer, should assign to self before returning
    }
    else
        self = [super init];

    return nil;
}

Explanation:

1. When you call [super init], the class that was alloc'd was the SUBCLASS.
2. "self" is the instance - i.e. the thing that was init'd
3. "[self class]" is the class that was instantiated - which will be SUBCLASS when the SUBCLASS is calling [super init], or will be the SUPERCLASS when the SUPERCLASS is being called with plain [[SuperClass alloc] init]
4. So, when the superclass receives an "init" call, it just needs to check whether the alloc'd class is the same as its own class

Works perfectly. NB: I don't recommend this technique for "normal apps" because usually you INSTEAD want to use a Protocol.

HOWEVER ... when writing Libraries ... this technique is VERY valuable: you frequently want to "save (other developers) from themselves", and its easy to NSAssert and tell them "Oops! you tried to alloc/init the wrong class! Try class X instead...".