Member Initializer Lists

Member Initializer Lists

Instead of assigning members inside the constructor body, you can initialize them in a member initializer list - a colon after the parentheses, followed by each member and its starting value:

class Car {
public:
  string brand;
  int year;
  Car() : brand("Toyota"), year(2022) {
    // the body can stay empty
  }
};

• The colon comes right after the constructor's parentheses.
• Each entry is member(value).
• Entries are separated by commas.

Why it matters

Without the list you do two jobs: the member is first default-constructed, then assigned. With the list the member is constructed directly with the value. Fewer steps - which is why it is often more efficient, especially for strings and class-type members.

const members need it

A const member cannot be assigned after construction, so it must be initialized in the list:

class Box {
public:
  const int size;
  Box(int s) : size(s) { }
};

TL;DR

• Syntax: Car() : brand("Toyota"), year(2022) { }.
• Members are constructed directly instead of default-then-assign.
• That direct building is more efficient.
• const (and reference) members must be initialized there.