It reveals the errors:
1>first.obj : error LNK2001: unresolved exterior image
“public: virtual void __thiscall first::onKeyPressed(enum cocos2d::EventKeyboard::KeyCode,class cocos2d::Event *)” (?onKeyPressed@first@@UAEXW4KeyCode@EventKeyboard@cocos2d@@PAVEvent@4@@Z)
1>first.obj : error LNK2001: unresolved exterior image
“public: virtual void __thiscall first::onKeyReleased(enum cocos2d::EventKeyboard::KeyCode,class cocos2d::Event *)” (?onKeyReleased@first@@UAEXW4KeyCode@EventKeyboard@cocos2d@@PAVEvent@4@@Z)
1>D:cocos2d-x-3.17testsgame1proj.win32Debug.win32game1.exe : deadly error LNK1120: 2 unresolved exterior instructions

How to resolve?

Implement the lacking strategies.

I’ve declared these features within the first.h file:

	void onKeyPressed(cocos2d::EventKeyboard::KeyCode keyCode, cocos2d::Event* occasion);
	void onKeyReleased(cocos2d::EventKeyboard::KeyCode keyCode, cocos2d::Event* occasion);

I’ve carried out these features in first.cpp file,

	auto listener = EventListenerKeyboard::create();
	listener->onKeyPressed = [=](cocos2d::EventKeyboard::KeyCode keyCode, Event* occasion) {
		keys[keyCode] = true;
	};
	listener->onKeyReleased = [=](cocos2d::EventKeyboard::KeyCode keyCode, Event* occasion) {
		keys[keyCode] = false;
	};
	_eventDispatcher->addEventListenerWithSceneGraphPriority(listener, this);

It nonetheless reveals these errors.Why?

You didn’t implement the strategies. What you probably did is use is lambda expressions to create nameless features.

This is a lambda expression:

listener->onKeyPressed = [=](cocos2d::EventKeyboard::KeyCode keyCode, Event* occasion) {
		keys[keyCode] = true;
	};

So, you’ve got 2 choices.

Option 1 – Get rid of the declarations within the header file, since you’re not implementing them as class member features.

Option 2 – Don’t use lambda expressions, and implement the features.

Either works.