default for constructor

The space is allocated at the beginning, and the constructor will not run until it runs to that line

default：

default (aka default)

default constructor：

A constructor with no parameters written by yourself or given by the compiler

When defining an array with two parameters:

The first parameter is initialized, the second one is not, it will tell the compiler to make a constructor with no parameters, and then it will report an error (the compiler said no):

The most terrible thing is that it will report various prompts for you to complete other answers, and will not get to the point.