I would like to be able to initialize a class member in one of N ways (in this examples N=2) based on a condition that is recieved via the class constructor as in the code that follows, but the MainObject's initialization seems to be only local to the (container) class' constructor. I wonder what are the best-practices for this particular pattern.

// in ContainerObject.hpp

class ContainerObject {
public:
    MainClass MainObject;
    ContainerObject(int type);
}



// in ContainerObject.cpp

ContainerObject::ContainerObject(int type);{
    if (type == 0){
        MainObject("something", 1, 2);
    } else if (type == 1){
        MainObject(4, "another thing", "yet another thing");
    }
}

I have so-far thought about

1. putting the main object in the heap
2. defining N class constructors and call the appropiate one recursively inside the "main"/"first" class constructor.

please note that the "0" and "1" initialization is only an example and it could be drastically different.

EDIT1: added ";" required for compiling EDIT2: Changed the original

//...
if (type == 0){
    MainObject(0);
} else if (type == 1){
    MainObject(1);
}
//...

for the current one

//...
if (type == 0){
    MainObject("something", 1, 2);
} else if (type == 1){
    MainObject(4, "another thing", "yet another thing");
}
//...

as it was called a duplicate by being misinterpreted for a case that could be solved by adding the following.

//...
ContainerObject(int type): MainObject(type);
//...

CodePudding user response：

I am interpreting the question as "how to execute non-trivial logic before/during the member initialization list".

A good way to go about that is to delegate the work of converting the constructor parameter of the outer object into the child object to a utility function:

// in ContainerObject.cpp

#include <stdexcept> // for std::invalid_argument

// Anonymous namespace since main_factory() is only needed in this TU.
namespace {

MainClass main_factory(int type) {
  if (type == 0) {
    return MainClass("something", 1, 2);
  } else if (type == 1) {
    return MainClass(4, "another thing", "yet another thing");
  }

  // N.B. This is one of the scenarios where exceptions are indisputably
  // the best way to do error handling.
  throw std::invalid_argument("invalid type for MainClass");
}

}

ContainerObject::ContainerObject(int type) 
  : MainObject(main_factory(type)) {}

CodePudding user response：

A constructor is always called automatically whenever an object is created. You can never call it by yourself anywhere.