These are the member initialiser lists not only used for the syntax also for the performance of not creating other non wanted object

[Butcher3Years]

Member Initializer Lists in C++ – Why They Matter

Member initializer lists are the preferred and often only correct way to initialize class members and base classes in a constructor.

Syntax

class Entity {
private:
    std::string name;
    int         score;
    Example     component;   // some other class

public:
    // Best practice: use initializer list
    Entity(const std::string& n, int s)
        : name(n)              // direct construction
        , score(s)
        , component(42)        // calls Example(int)
    {
        // constructor body – only for logic that can't go in init list
    }
};

Key Advantages

Direct initialization (not assignment) → avoids double construction
→ No unnecessary default constructor call + assignment
Required in these cases (you cannot do it in the body):
const members
Reference members
Members without default constructor
Base class constructors with arguments

More efficient – especially for types that are expensive to default-construct + assign
(e.g. std::string, std::vector, custom classes with allocations)
Guaranteed order of initialization: follows declaration order in the class (not the order in the list)
Cleaner code – constructor body stays focused on logic, not setup