How to programmatically use iOS voice synthesizers? (text to speech)

Starting from iOS 7, Apple provides this API.

Objective-C

#import <AVFoundation/AVFoundation.h>
…
AVSpeechUtterance *utterance = [AVSpeechUtterance 
                            speechUtteranceWithString:@"Hello World!"];
AVSpeechSynthesizer *synth = [[AVSpeechSynthesizer alloc] init];
[synth speakUtterance:utterance];

Swift

import AVFoundation
…
let utterance = AVSpeechUtterance(string: "Hello World!")
let synth = AVSpeechSynthesizer()
synth.speakUtterance(utterance)