NSMutableDictionary+ValueHolding.h

#import <Foundation/Foundation.h>

@interface NSMutableDictionary (ValueHolding)

- (void)setDouble:(double)number forKey:(NSString *)key;
- (double)doubleForKey:(NSString *)key;

- (void)setCGFloat:(CGFloat)number forKey:(NSString *)key;
- (CGFloat)CGFloatForKey:(NSString *)key;

@end

NSMutableDictionary+ValueHolding.m

#import "NSDictionary+ValueHolding.h"

@implementation NSMutableDictionary (ValueHolding)

- (void)setDouble:(double)number forKey:(NSString *)key;
{
    [self setValue:[NSNumber numberWithDouble:number] forKey:key];
}

- (double)doubleForKey:(NSString *)key;
{
    return [[self objectForKey:key] doubleValue];
}

- (void)setCGFloat:(CGFloat)number forKey:(NSString *)key;
{
    [self setValue:[NSNumber numberWithDouble:number] forKey:key];
}

- (CGFloat)CGFloatForKey:(NSString *)key;
{
    return [[self objectForKey:key] doubleValue];
}

@end

Usage.m

    NSMutableDictionary *valueHolder = [NSMutableDictionary dictionary];
    
    double number = 123.45678;
    
    [valueHolder setDouble:number forKey:@"doubleVal"];
    [valueHolder setCGFloat:number forKey:@"floatVal"];
    
    NSLog(@"Value holder: %@", valueHolder);
    
    number = [valueHolder doubleForKey:@"doubleVal"];
    
    [valueHolder setDouble:number forKey:@"doubleVal"];
    [valueHolder setCGFloat:number forKey:@"floatVal"];
    
    NSLog(@"Value holder: %@", valueHolder);
    
    [valueHolder setCGFloat:[valueHolder doubleForKey:@"doubleVal"] forKey:@"floatVal"];
    [valueHolder setDouble:[valueHolder doubleForKey:@"doubleVal"] forKey:@"doubleVal"];
    
    NSLog(@"Value holder: %@", valueHolder);
    
    [valueHolder setCGFloat:[valueHolder CGFloatForKey:@"floatVal"] forKey:@"floatVal"];
    [valueHolder setDouble:[valueHolder CGFloatForKey:@"floatVal"] forKey:@"doubleVal"];
    
    NSLog(@"Value holder: %@", valueHolder);

xxoutput

2014-04-08 16:21:23.078 TypeTest[8699:60b] Value holder: {
    doubleVal = "123.45678";
    floatVal = "123.45678";
}
2014-04-08 16:21:23.080 TypeTest[8699:60b] Value holder: {
    doubleVal = "123.45678";
    floatVal = "123.45678";
}
2014-04-08 16:21:23.081 TypeTest[8699:60b] Value holder: {
    doubleVal = "123.45678";
    floatVal = "123.45678";
}
2014-04-08 16:21:23.082 TypeTest[8699:60b] Value holder: {
    doubleVal = "123.45678";
    floatVal = "123.45678";
}