C++ member initializer lists

struct S {
    int n;
    S(int); // constructor declaration
    S() : n(7) {} // constructor definition.
                  // ": n(7)" is the initializer list
                  // ": n(7) {}" is the function body
};
S::S(int x) : n{x} {} // constructor definition. ": n{x}" is the initializer list
int main()
{
    S s; // calls S::S()
    S s2(10); // calls S::S(int)
}

Constructors and member initializer lists

Constructor is a special non-static member function of a class that is used to initialize objects of its class type. In the definition of a constructor of a class, member initializer list specifies the initializers for direct and virtual bases and non-static data members.

https://en.cppreference.com/w/cpp/language/constructor