Query about member initialization

while I was doing hackerrank c++ exercises I stumbled upon this code in the discussions section:

class BadLengthException : public std::runtime_error
{
public:
    BadLengthException(int length) : std::runtime_error{std::to_string(length)}
    { }
};

I don't really understand what is going on after the member initializer part, this part to be exact:

std::runtime_error{std::to_string(length)}

Can someone explain what this line of code does to me? I have never seen such a use of member initialization. I am used to seeing:

Foo(int num) : bar(num) {};

So please explain it as clearly as possible. Thank you for your time!

You are inheriting from the standard exception class std::runtime_error.

In this code:

 class BadLengthException : public std::runtime_error
 {
  public:
      BadLengthException(int length) std::runtime_error{std::to_string(length)}
      { }
};

You are defining a new exception class in terms of std::runtime_error.

std::runtime_error takes a string message as input, which you can print with runtime_error_object.what() in a catch block. So, that is why the length variable is being converted to a std::string. You can read more about that here.

Lastly:

Foo(int num) : bar(num) {};

This is constructor list initializer syntax. That is used to initialize member variables of a class. You can read more about that here.