如何使用 std::vector 动态实例化具有参数化构造函数的类？

Question

I want to dynamically instantiate the object using vector, However, I don't know how to pass the parameter to the constructor.

#include <iostream>
#include <cstring>
#include <vector>
using namespace std;

class Player {
    
    public:
    Player(int id, string name)
     : p_id(id)
     , p_name(name)
                                                                                     
  {
  }
  
   public:
      int p_id;
      string p_name;
   };

int main() {

vector<Player>  player;      **//How to instantiate the object with parametrized constructor** 
    
player.resize(sizeof(player));
  
return 0;
   
}

Answer 1

I assume, you may want to look toward this:

class Player {
public:
    Player() = default;
    Player(int id, string name): p_id(id), p_name(name) {}
private:
    int p_id;
    string p_name;
};

int main() {
    vector<Player> players;
    // construct a Player from an int and a string 
    players.emplace_back(33, "Foo");
}

Here, we use a std::vector member functionemplace_back() , which constructs an object inside (at the back) the vector.

---

There are other thing(s) to note in your code:

players.resize(sizeof(players)); // Is nonsensical

You should check the documentation on std::vector::resize and the documentation on sizeof() . In short, we can use resize(n) to change the amount of elements in the container.

We can either increase the number of elements , by providing such n that is greater than the current size() . In which case, additional default-inserted elements are appended.

Or decrease the number of elements in the container by providing smaller n . In which case, the content of the container is reduced to its first n elements, removing those beyond (and destroying them).

Note, that this function alters the actual contents of a container:

players.emplace_back(11, "Foo");
players.emplace_back(22, "Bar");
players.emplace_back(33, "Buz");

players.resize(2); // Now, players.size() == 2
// ...and there are no Buz among the players

That's why passing the sizeof(players) to resize() is meaningless, especially when we consider that the sizeof() returns a number of bytes .

If you want to have a vector with some number of pre-constructed Player objects in it, you should use vector's fill constructor :

// Ten objects of class Player
vector<Player> players(10, Player(0, ""));
// Ten default constructed objects of type Player
vector<Player> players(10);  // Player has to have a default constructor