C++ Allocate Vector Size in class variable without default initialization

Question

So I have a class (HostObject) that has a vector of complex classes (object1) inside it. Such as the pseudocode depicted below

#DEFINE ARRAY_SIZE 10
class HostOb         


        

Answer 1

You could do this by calling reserve() in default constructor.

#DEFINE ARRAY_SIZE 10
class HostObject {
  //other member variables
  vector<Object1> vec; //vec(ARRAY_SIZE);  // not here

  //default constructor
  HostObject() {
    //leave vector default constructed
    vec.reserve(ARRAY_SIZE);               // but here
  }

  //my custom constructor                  // no need to change, since push_back will increase size automatically
  HostObject(double parmeter1, double parameter2, doubleparameter3) {

    //some code doing some calculations 

    for (int i = 0; i <ARRAY_SIZE; i++) {

      vec.push_back(Object1(double p1, int p2, double p3, intp4));

    }
  }
}

Also consider using static unsigned array_size = 10; as a member of class HostObject, so that you can change the size dynamically every time creating an object of HostObject.

HostObject ho1;  // array size is 10
HostObject::array_size = 20;
HostObject ho2;  // array size is 20

Answer 2

There is no constructor for std::vector for for reserving capacity. The only way you can reserve capacity is to use reserve member function.

#DEFINE ARRAY_SIZE 10
class HostObject {
    //other member variables
    vector<Object1> vec;

    //default constructor
    HostObject(){
        vec.reserve(ARRAY_SIZE);
    }

    //my custom constructor
    HostObject(double parmeter1, double parameter2, doubleparameter3) {
        vec.reserve(ARRAY_SIZE);
        //some code doing some calculations 

        for (int i = 0; i <ARRAY_SIZE; i++){

            vec.push_back(Object1( double p1, int p2, double p3, intp4));

        }
    }
}

Read more about std::vector