What is the purpose of this c++ declaration?

Question

Hi, I got a C++ struct:

   struct Student{

         Student(){

         }
   };

And the enum:

   enum studenttype_t { levelA = 0, levelB = 1, levelC = 2 };

Then somewhere else it is declared:

   vector<Student *> student;

Could it be used as follows:

   student[levelA] = new Student();

If so, what does the student[levelA] mean? is it 0 in this case? and what is the function of the last statement, i.e. student[levelA] = new Student();? thanks

Answer 1

That's exactly what student[LevelA] means. A vector is like an array, but expandable. All that

enum studenttype_t { levelA = 0, levelB = 1, levelC = 2 };
vector<Student *> student(5); // need a size, else defaults to zero.
student[levelA] = new Student();

is doing is creating such a vector (of Student pointers) and creating the first one (index 0).

Answer 2

levelA is the constant, 0. student is a vector, and student[levelA] is the zeroth element of that vector. The elements of the vector are of type Student*, i.e. pointers to Student objects.

student[levelA] = new Student();

instantiates student[0] with a new Student class.

Answer 3

Yes, levelA is 0. C++ allows a silent conversion from enum value to int (unlike C#, which requires a explicit cast).

student[levelA] = new Student(); means allocate memory in the heap, create a Student object in that memory, and store the address of the new object into the first element of the student vector. However, there is not necessary an element 0 in the vector -- when it was created, it had length 0 -- so that is actually undefined behavior.

student.push_back(new Student()); says add the new object to the next (presently the first) element of the vector.