change_osx_wallpaper.mm

#include <iostream>

auto main() -> int
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    
    NSURL *basePath = [NSURL URLWithString:@"path/to/directory"];
    
    NSFileManager *fm = [NSFileManager defaultManager];
    NSArray<NSString *> *files = [fm contentsOfDirectoryAtPath:[basePath absoluteString] error:nil];
    
    if (0 < [files count])
    {
        NSWorkspace *ws = [NSWorkspace sharedWorkspace];
        NSURL *imageURL = [NSURL fileURLWithPath:[NSString stringWithFormat:@"%@/%@", [basePath absoluteString], files[0]] isDirectory:NO];
        
        NSError *error;
        BOOL success = [ws setDesktopImageURL:imageURL forScreen:[NSScreen mainScreen] options:nil error:&error];
        
        if (!success)
        {
            std::cout << [[error localizedDescription] UTF8String] << std::endl;
        }
    }
    
    [pool release];
    
    return 0;
}