Implicitly-Defined Default Constructors

In C++, a default constructor is a constructor that can be called with no arguments. The compiler will implicitly define a default constructor for a class if:

1. The class has no user-defined constructors at all.
2. All data members of the class have a default constructor (or are built-in types).

For example:

1class Player {
2  int health_;
3  std::string name_;
4};
5
6Player p1; // OK, implicit default constructor

However, if we define any constructor for the class, the compiler will no longer provide an implicit default constructor:

1class Player {
2 public:
3  Player(int health) : health_{health} {}
4
5 private:
6  int health_;
7  std::string name_;
8};
9
10Player p1;       // Error, no default constructor
11Player p2{100};  // OK, using defined constructor

In this case, if we still want a default constructor, we need to define it explicitly:

1class Player {
2 public:
3  Player() = default;  
4  Player(int health) : health_{health} {}
5
6 private:
7  int health_;
8  std::string name_;
9};
10
11// OK, using defaulted default constructor
12Player p1;

So remember: if you define any constructors for a class, and you want a default constructor, you need to define it explicitly (either by writing it or by using = default).