How to create member objects by class constructor?

Question

In my example I created a class Person which has a member object: struct data. This object contains data about person. Each time a Person-Object is created, also the data-object shall be initialized.

Observation: When adding object initializer to code (1) at class constructor I get failure message:
incomplete type is not allowedC/C++(70)

class person {
public:
  struct data;
  person() { /* (1) */
    person::data myPersonData;
  }

private:
};

So here is how I practice it now:

No struct object initialization myPersonData in class person constructor (class_person.hpp)
Create person object in main.cpp
Create myPersonData in main.cpp (I would like to save this initialization and put it to class contructor)

The whole example looks like this:

// class_person.hpp
    
    #include <iostream>
    #include <string>
    
    class person {
    public:
      struct data;
    
    private:
    };
    
    struct person::data {
      std::string name = "John";
      int age = 42;
      int weight = 75;
    };

_

// main.cpp

#include <iostream>

#include "class_person.hpp"

void outputPersonData(person::data myPerson) {
  std::cout << myPerson.name << "\n";
  std::cout << myPerson.age << "years\n";
  std::cout << myPerson.weight << "kg\n";
};

int main() {
  person John;
  person::data myPersonData;
  outputPersonData(myPersonData);
  getchar();
  return 0;
}

struct data; just declares person::data; the struct is lacking a definition and is thus incomplete — Sebastian Hoffmann
– Sebastian Hoffmann, Commented Oct 22, 2020 at 9:32

super · Accepted Answer · 2020-10-22 09:36:53Z

You should put the definition of data inside the definition of person if you want a member of it. Something like this.

#include <string>
#include <iostream>

class person {
public:
    struct data {
        std::string name = "John";
        int age = 42;
        int weight = 75;
    };
    // This is just the definition the the class. We need the class
    // to have a definition if we want to use a value of it in person

    person() = default;
    // Default constructors, give us a default constructed personData
    // that uses the default values from the definition

    person(data d) : personData(std::move(d)) {}
    // Constructor that takes a personData object and uses it to
    // initialize our member. std::move is to avoid uneccesary copying

    void outputPersonData() const {
        std::cout << personData.name << "\n";
        std::cout << personData.age << "years\n";
        std::cout << personData.weight << "kg\n";
    }

    data personData;
    // This is the actual data member, now person contains
    // a member named personData of type person::data
};

int main() {
    person john;
    person mike({"Mike", 47, 82});

    john.outputPersonData();
    mike.outputPersonData();
}

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