Default Constructor

A constructor is a special member function that runs when an object is created. It has the same name as the class and no return type. A default constructor is a constructor that can be called with no arguments:

class Foo {
public:
  Foo() : val(0) {}
  int val;
};

Foo obj;
std::cout << obj.val << "\n";  // 0

If a class declares no constructors, the compiler generates a default constructor that does nothing. Members of fundamental types are left uninitialised:

class Foo {
public:
  int val;
};

Foo obj;  // OK: val is uninitialised.

Declaring any other constructor stops the compiler from generating the default constructor:

class Foo {
public:
  Foo(int v) : val(v) {}
  int val;
};

Foo a(1);  // OK.
Foo b;     // ERROR.

A constructor where every parameter has a default argument is also a default constructor:

class Foo {
public:
  Foo(int v = 0) : val(v) {}
  int val;
};

Foo a;     // val = 0
Foo b(1);  // val = 1

Foo obj();  // Function named obj that returns a Foo.

A default constructor that does nothing can be requested explicitly with = default, even when other constructors are declared:

class Foo {
public:
  Foo() = default;
  Foo(int v) : val(v) {}
  int val;
};

Foo a;     // val is uninitialised.
Foo b(1);  // val = 1

Braces can be used to call the default constructor without declaring a function:

Foo obj{};