struct use notes in C + + _c language

1.c++ structure variables can be omitted in the declaration of struct, in C this is not possible, examples are as follows

#include <iostream>
#include <string>
using namespace std;

struct test{
  int num;
  string name;
};

int main (void)
{
  Test T;
  T.num=1;
  T.name= "Jack";
  cout<<t.num<< "" <<t.name<<endl;    
}

The 2.c++ struct declaration can be declared in the main () function, it is also possible to invoke the entire program before the main () function, which is also the most commonly used, and, internally, only within a function, that is, the concept of a local variable that can be used within a declared function. As follows

int main (void)
{
  struct test{
    int num;
  };
  
  Test Hello;//right
  
}

void T ()
{
  Test T;//wrong
}