How do I generate objects in std::vector and without copy?

Question

There is a student class

class Student
{
public:
    inline static int current_id_max = 0;
    int id = 0;
    string name;
public:
    Student()
    {
        id = (++current_id_max);
        cout << "Student constructor\n";
    }
    Student(const string& _name)
    {
        name = _name;
        id = (++current_id_max);
        cout << "Student constructor: " << _name << endl;
    }
    Student(const Student& o)
    {
        name = o.name;
        id = (++current_id_max);
        cout << "Student constructor copy: " << name << endl;
    }
    ~Student() { cout << "Student destructor: " << name << endl; }
};

I want to create 5 students with parameters into a vector,

std::vector<Student> school = 
    { Student("Tom"), Student("Mike"),Student("Zhang"), Student("Wang"), Student("Li")};

There will be 5 Student constructor: name and 5 Student constructor copy: name.

What can I do to avoid the useless copying?

Answer 1

I suggest the following:

1. Create an empty vector.
2. Reserve the number of elements that you would like to have in the vector.
3. Use std::vector::emplace_back to add elements to it.

A complete example:

#include <iostream>
#include <vector>
#include <string>

class Student
{
   public:
      inline static int current_id_max = 0;
      int id = 0;
      std::string name;
   public:
      Student()
      {
         id = (++current_id_max);
         std::cout << "Student constructor\n";
      }
      Student(const std::string& _name)
      {
         name = _name;
         id = (++current_id_max);
         std::cout << "Student constructor: " << _name << std::endl;
      }
      Student(const Student& o)
      {
         name = o.name;
         id = (++current_id_max);
         std::cout << "Student constructor copy: " << name << std::endl;
      }
      ~Student() { std::cout << "Student destructor: " << name << std::endl; }
};

int main()
{
   std::vector<Student> school;
   school.reserve(5);
   school.emplace_back("Tom");
   school.emplace_back("Mike");
   school.emplace_back("Zhang");
   school.emplace_back("Wang");
   school.emplace_back("Li");
}

Output in my test:

Student constructor: Tom
Student constructor: Mike
Student constructor: Zhang
Student constructor: Wang
Student constructor: Li
Student destructor: Tom
Student destructor: Mike
Student destructor: Zhang
Student destructor: Wang
Student destructor: Li