Property type '' is incompatible with type '' inherited from ''

Question

I created a BaseRequest Class and it has a property params, which is an id that conforms to the <BaseParams> protocol.

Next I created a DiagramRequest Class inherited from BaseRequest and it has a property whose type is DiagramParam, which is class that conforms to the <BaseParams> protocol.

I think it is safe for compiling, but Xcode still warns that: Property type 'DiagramParams *' is incompatible with type 'id<BaseParams>' inherited from 'BaseRequest'

I wonder why.

Simplified Demo is here:

BaseRequest.h

@interface BaseRequest

@property (nonatomic,retain) id<BaseParams> params;

@end


@protocol BaseParams <NSObject>

- (NSMutableDictionary *)getParamsDict;

@end

DiagramRequest.h

@interface DiagramRequest : BaseRequest

//warning: Property type 'DiagramParams *' is incompatible with type 'id<BaseParams>' inherited from 'BaseRequest'
@property (nonatomic,retain) DiagramParams *params;

@end


@interface DiagramParams : NSObject <BaseParams>

@property (nonatomic) int id;
@property (nonatomic,retain) NSString *city;

- (NSMutableDictionary *)getParamsDict;

@end

Answer 1

The warning might be removed by placing the DiagramParams's interface definition before DiagramRequest's.

See my answer here.. https://stackoverflow.com/a/14632135/1347502 which removes the warning for a slightly simpler setup.