In C/C++, you can initialise a struct without giving values for any of the fields:
struct Point {
float x;
float y;
float z;
};
Deploy key is a SSH key set in your repo to grant client read-only (as well as r/w, if you want) access to your repo.
As the name says, its primary function is to be used in the deploy process in replace of username/password, where only read access is needed. Therefore keep the repo safe from the attack, in case the server side is fallen.