How do you define a constructor with an initializer list in C++?

After the constructor signature, list members with : member(value) before the body — e.g. Vehicle::Vehicle(int n) : _nwheels(n) {}.

ClassName::ClassName(int x)
    : _member(x)   // initializer list
{
    // constructor body (can be empty)
}

Example:

Vehicle::Vehicle(int nwheels)
    : _nwheels(nwheels)
{
}

Why use initializer lists?

• More efficient — members are initialized directly, instead of being default-constructed and then assigned in the body
• Required for const members and references (they can only be initialized, never assigned)
• Required for member objects that have no default constructor

Multiple members are comma-separated:

Point::Point(int x, int y)
    : _x(x), _y(y)
{
}

Go deeper:

• Constructors and member initializer lists — cppreference — the full rules, including initialization order.