gistfile1.m

#import <Foundation/Foundation.h>

NSDictionary *TestD(void) {
    return @{
        @"string" : @"abc",
        @"number" : @42,
        @"dictionary" : @{
            @"string" : @"abcdef",
            @"array" : @[ @"a", @2 ]
        },
        @"data" : [@"hello" dataUsingEncoding: NSUTF8StringEncoding],
        @"date" : [NSDate dateWithTimeIntervalSince1970: 42],
    };
}

typedef __unsafe_unretained NSDictionary *JSONDictionary;
typedef __unsafe_unretained NSArray *JSONArray;
typedef __unsafe_unretained NSString *JSONString;

void JSONDecodeF(void *target, id inJSON, NSString *fieldsString, void(^Error)(NSString *fieldName, Class desiredType, Class actualType)) {
    char *cursor = target;
    
    NSArray *fieldsStrings = [fieldsString componentsSeparatedByString: @";"];
    for(NSString *fieldString in fieldsStrings) {
        NSString *trimmed = [fieldString stringByTrimmingCharactersInSet: [NSCharacterSet whitespaceAndNewlineCharacterSet]];
        NSArray *split = [trimmed componentsSeparatedByCharactersInSet: [NSCharacterSet whitespaceAndNewlineCharacterSet]];
        NSString *typeName = [split firstObject];
        NSString *fieldName = [split lastObject];
        
        if([typeName length] == 0)
            continue;
        
        NSDictionary *typeMap = @{
            @"JSONString": [NSString class],
            @"JSONDictionary": [NSDictionary class],
            @"JSONArray": [NSArray class],
            @"double": [NSNumber class]
        };
        Class class = typeMap[typeName];
        if(class == Nil) {
            NSLog(@"Unknown JSON type name %@", typeName);
            abort();
        }
        
        __autoreleasing id value = inJSON[fieldName];
        if(![value isKindOfClass: class]) {
            Error(fieldName, class, [value classForCoder]);
        }
        
        if([typeName isEqual: @"double"]) {
            double primitiveValue = [value doubleValue];
            memcpy(cursor, &primitiveValue, sizeof(primitiveValue));
            cursor += sizeof(primitiveValue);
        } else {
            void *voidValue = (__bridge void *)value;
            memcpy(cursor, &voidValue, sizeof(value));
            cursor += sizeof(value);
        }
    }
}

#define JSONDecode(varname, inJSON, fields) \
    struct { fields } varname; JSONDecodeF(&varname, inJSON, @#fields, ^(NSString *fieldName, Class desiredType, Class actualType){ NSLog(@"Error: %@ is %@ but expected %@", fieldName, actualType, desiredType); })

int main(int argc, char **argv) {
    @autoreleasepool {
        NSDictionary *dict = TestD();
        
        
        JSONDecode(decoded, dict, 
            JSONString string;
            double number;
            JSONDictionary dictionary;
        );
        
        NSLog(@"%@ %f", decoded.string, decoded.number);
        
        JSONDecode(innerDecoded, decoded.dictionary,
            JSONString string;
            JSONArray array;
        );
        NSLog(@"%@ %@", innerDecoded.string, innerDecoded.array);
    }
}