Return Obj-C blocks in C++ code

Question

I'm currently porting some classes from the Apple iOS Foundation Framework to C++ and i'm expecting some issues. I'm trying to port this Obj-C method from the NSExpression @class :

- (id, NSArray *, NSMutableDictionary *)expressionBlock

So in my sfExpression class, I have the following code (when deleting others methods ^^)

class sfExpression : public sfObject {
public:
    id (^ExpressionBlock())(id, NSArray*, NSMutableDictionary*);

private:
    NSExpression* _Expression;
};

And when I'm trying to implement this function like this :

id (^sfExpression::ExpressionBlock())(id, NSArray*, NSMutableDictionary*) {
    return [_Expression expressionBlock];
}

It doesn't work... Any ideas ?

I've tried many things but... without success :'(

EDIT : The code is right. Just switch to LLVM Compiler instead of GCC

Joshua Weinberg · Accepted Answer · 2011-10-08 15:32:36Z

3

Edit: Moral of the story, don't use GCC when dealing with blocks.

Here is a full example as far as I can see, this works in my tests.

class Foo
{
public:
    Foo(NSExpression *_exp) : expression([_exp retain])
    {

    }

    ~Foo()
    {
        [expression release];
    }

    id (^ExpressionBlock())(id, NSArray*, NSMutableDictionary*);
private:
    NSExpression *expression;
};

id (^Foo::ExpressionBlock())(id, NSArray*, NSMutableDictionary*)
{
    return [expression expressionBlock];
}


int main()
{
    NSAutoreleasePool *pool = [NSAutoreleasePool new];
    Foo a([NSExpression expressionForBlock:^id(id evaluatedObject, NSArray *expressions, NSMutableDictionary *context) {
        return evaluatedObject;
    } arguments:[NSArray arrayWithObject:@"Test"]]);
    NSLog(@"%@", a.ExpressionBlock()(@"Hello", nil, nil));
    [pool drain];
}

edited Oct 8, 2011 at 15:32

answered Oct 8, 2011 at 15:05

Joshua Weinberg

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3 Comments

abidon Over a year ago

Compiler return : error: cannot convert 'void (^)(objc_object*, NSArray*, NSMutableDictionary*)' to 'objc_object* (^)(objc_object*, NSArray*, NSMutableDictionary*)' in return

abidon Over a year ago

It works only with LLVM Compiler 2.0. LLVM GCC 4.2 & GCC 4.2 return the error i've copied 3mins ago. Thanks a lot.

Joshua Weinberg Over a year ago

Let me guess, your'e using GCC? You should switch to using LLVM if your'e dealing with blocks. GCC doesn't support them properly

millimoose · Accepted Answer · 2011-10-08 15:04:42Z

0

/usr/include/Block.h manipulates ObjC blocks using opaque void pointers, so that's probably what you should use to pass them around to C++ code if you can't compile it with a compiler supporting blocks. Also take note of the Block_copy and Block_release functions in that header. There's also an article on memory management of ObjC blocks.

answered Oct 8, 2011 at 15:04

millimoose

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1 Comment

abidon Over a year ago

I have to use void pointers ? Pointing on the block ? Can you make an exemple because i'm confused... I already read the article but there is somethings that i've don't understood. (Hope you understand my poor english... I'm a french guy...)

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