Petty C++ Initialization

In C++, initialization or initializers are expressions that set the initial state of variables or objects, often appearing in contexts such as object creation, member initialization, array assignment, etc.

Direct Initialization#

Direct initialization can use () or {} to initialize variables:

int a(10);
int b{10};

// Member initialization expression list is a form of direct initialization
  MyClass(int a, int b): x(a), y(b){
    // do something..
  }

Direct initialization can invoke explicit constructors.

Copy Initialization#

Copy initialization uses the assignment operator =, unlike direct initialization, it cannot invoke explicit constructors.

int a = 10;
shared_ptr<int> p = new int(10); // Error

List Initialization#

List initialization is a new feature introduced after C++11, allowing the use of braces {} to initialize variables of any type, preventing narrowing conversions (implicit conversion from one type to another causing loss of precision, etc.: int x{4.2}).

MyClass myClass = {1,2,3};
// or
MyClass myClass = new MyClass{1,2,3};
std::vector<int> v = {1,2,3};

Aggregate (struct) Initialization#

An aggregate is a class or struct that has no user-defined constructors, no private or protected non-static data members, no base classes, and no virtual functions.

For aggregate types, braces can be used to initialize their members, for example:

struct Point {int x; int y;}; 
Point p = {1, 2};

Default Member Initializer#

Default member initializer is a new feature introduced in C++11, allowing direct specification of default values for member variables in class definitions.

class MyClass{
  int x = 4;
  int y = 2;
  public:
    MyClass() = default; // Initialize using default constructor and default member initializer
    // do something ..
}