[C++] explicit & converting constructor

Recently I saw some code use the explicit keyword in the constructor. To explain the explicit, see the sample code here.

class Enemy {
public:
    Enemy(int hp) : hp(hp) {} // implicit constructor
    Enemy(int hp, int damage) : hp(hp), damage(damage) {}
    int hp;
    int damage;
};

class Player {
public:
    explicit Player(int hp) : hp(hp) {} // explicit constructor
    explicit Player(int hp, int damage) : hp(hp), damage(damage) {}
    int hp;
    int damage;
};

int main() {
    Enemy a1 = 1000;               // OK　copy-initialization selects Player(int hp)
    Enemy a2 = { 1000, 200 };      // OK
    Enemy a3 = Enemy(1000);        // OK
    Enemy a4 = Enemy(1000, 200);   // OK
    Enemy a5 = (Enemy)1;           // OK

    Player b1 = 1000;              // error: no viable conversion from 'int' to 'Player'
    Player b2 = { 1000, 200 };     // error: chosen constructor is explicit in copy-initialization
    Player b3 = Player(1000);      // OK
    Player b4 = Player(1000, 200); // OK
    Player b5 = (Player)1000;      // OK
    return 0;
}

### Explanation

When use the implicit constructor, it equals to create a temporary instance and copy the temporary instance to the final instance.

For the explicit constructor, you must do something like this:

Player b1(1000);

You could find more information from the c++ reference page.