HTSemanticVersion.h

//
//  HTSemanticVersion.h
//
//  Created by 최건우 on 13. 7. 25..
//  Copyright (c) 2013년 Hardtack. All rights reserved.
//

#import "HTVersion.h"

@interface HTSemanticVersion : HTVersion

@property (nonatomic) NSInteger majorVersion;
@property (nonatomic) NSInteger minorVersion;
@property (nonatomic) NSInteger patchVersion;

@end

HTSemanticVersion.m

//
//  HTSemanticVersion.m
//
//  Created by 최건우 on 13. 7. 25..
//  Copyright (c) 2013년 Hardtack. All rights reserved.
//

#import "HTSemanticVersion.h"

@implementation HTSemanticVersion

#pragma mark - Initializers

- (id)initWithString:(NSString *)string {
    self = [super initWithString:string];
    if (self) {
        NSArray *components = [string componentsSeparatedByString:@"."];
        if ([components count] > 3) { // Invalid string
            return nil;
        }
        NSInteger versions[3] = {0,};
        NSUInteger i = 0;
        for (NSString *component in components) {
            NSScanner *scanner = [NSScanner scannerWithString:component];
            NSInteger v;
            if (![scanner scanInteger:&v]) {
                return nil;
            }
            versions[i] = v;
            i++;
        }
        self.majorVersion = versions[0];
        self.minorVersion = versions[1];
        self.patchVersion = versions[2];
    }
    return self;
}

- (NSString *)description {
    return [NSString stringWithFormat:@"%d.%d.%d", self.majorVersion, self.minorVersion, self.patchVersion];
}

- (NSComparisonResult)compare:(HTVersion *)version {
    if (![version isKindOfClass:[self class]]) {
        return [super compare:version];
    }
    HTSemanticVersion *v = (HTSemanticVersion *)version;
    NSComparisonResult r = NSIntegerCompare(self.majorVersion, v.majorVersion);
    if (r == NSOrderedSame) {
        r = NSIntegerCompare(self.minorVersion, v.minorVersion);
    }
    if (r == NSOrderedSame) {
        r = NSIntegerCompare(self.patchVersion, v.patchVersion);
    }
    return r;
}

@end

HTVersion.h

//
//  HTVersion.h
//
//  Created by 최건우 on 13. 7. 25..
//  Copyright (c) 2013년 Hardtack. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface HTVersion : NSObject

@property (strong, nonatomic) NSString *string;

+ (instancetype)versionWithNumber:(NSNumber *)number;
+ (instancetype)versionWithString:(NSString *)string;

- (id)initWithString:(NSString *)string;

// Default implementation is not reliable. You must override it.
- (NSComparisonResult)compare:(HTVersion *)version;

@end

NSComparisonResult NSUIntegerCompare(NSUInteger left, NSUInteger right);
NSComparisonResult NSIntegerCompare(NSInteger left, NSInteger right);

HTVersion.m

//
//  HTVersion.m
//
//  Created by 최건우 on 13. 7. 25..
//  Copyright (c) 2013년 Hardtack. All rights reserved.
//

#import "HTVersion.h"

@implementation HTVersion

#pragma mark - Private methods

#pragma mark - Class methods

+ (instancetype)versionWithString:(NSString *)string {
    return [[[self class] alloc] initWithString:string];
}

+ (instancetype)versionWithNumber:(NSNumber *)number {
    return [self versionWithString:[number description]];
}

#pragma mark - Initializers

- (id)init {
    return [self initWithString:nil];
}

- (id)initWithString:(NSString *)string {
    self = [super init];
    if (self) {
        self.string = string;
    }
    return self;
}

#pragma mark - Public methods

- (NSComparisonResult)compare:(HTVersion *)version {
    return [self.string isEqualToString:version.string] ? NSOrderedSame : NSOrderedDescending;
}

@end

NSComparisonResult NSUIntegerCompare(NSUInteger left, NSUInteger right) {
    if (left < right) {
        return NSOrderedAscending;
    } else if (left > right) {
        return NSOrderedDescending;
    } else {
        return NSOrderedSame;
    }
}

NSComparisonResult NSIntegerCompare(NSInteger left, NSInteger right) {
    if (left < right) {
        return NSOrderedAscending;
    } else if (left > right) {
        return NSOrderedDescending;
    } else {
        return NSOrderedSame;
    }
}

main.m

int main(int argc, char *argv[]) {
    HTVersion *systemVersion = [[HTSemanticVersion alloc] initWithString:[[UIDevice currentDevice] systemVersion]];
    if ([systemVersion compare:[[HTSemanticVersion alloc] initWithString:@"7.0"]] == NSOrderedDescending) {
        ...
    }
}