Thread: Initializing array of class objects via loop

Most of the book examples I've seen re initializing array of class objects have been 'by hand', e.g.

Code:

 Dog dogs[2] = {Dog("Frisky", 2), Dog("Tabby", 3);

where the data members are names and ages of the dogs for example, etc.

I'm trying to initialize an array of similar class objects by looping the default constructor through the array but getting weird results. Though my array takes only 3 elements, the code is not reading the first 3 inputs but if I keep adding data it saves the 4th, 5th and 6th entries and prints them out. Any helps re where I'm going wrong would be much appreciated. Thanks

Code:

#include<iostream>using namespace std;


const int MEMB = 3; 


class Dog{
    private:
        string m_name;
        int m_age;
    public:
        Dog(){
            cout<<"Enter dog's name: ";
            string name;
            cin>>name;
            m_name = name;
            cout<<"Enter dog's age: ";
            int age;
            cin>>age;
            m_age = age; 
            
        }
        void showDog(){
            cout<<"The dog's name is: "<<m_name<<endl;
            cout<<"The dog's age is: "<<m_age<<" years"<<endl; 
        }
};


int main(){
    
    Dog list[MEMB];
    for (int i = 0; i < MEMB; i++){
        list[i]=Dog();
        cout<<endl; 
    }
    cout<<endl;
    for (int i = 0; i < MEMB; i++){
        list[i].showDog();
        cout<<endl; 
    }
 




}

Well you try to call your constructor 6 times, remember when you defined the array the default constructor is called. You really shouldn't be doing all that work in the constructor, you should just default initialize the variables. Then in your loop call a "set" member function to set the new values.

Jim

Originally Posted by jimblumberg

remember when you defined the array the default constructor is called

Jim

Indeed, I didn't. Many thanks, works perfectly now. For those interested, here's the code as per Jim's suggestion:

Code:

#include<iostream>
using namespace std;


const int MEMB = 3; 


class Dog{
	private:
		string m_name;
		int m_age;
	public:
		Dog():m_name("doggy"), m_age(0){} 
		void setDog(); 
		void showDog(); 
};


int main(){
	
	Dog list[MEMB];
	for (int i = 0; i < MEMB; i++){
		list[i].setDog();
		cout<<endl; 
	}
	cout<<endl;
	for (int i = 0; i < MEMB; i++){
		list[i].showDog();
		cout<<endl; 
	}
}


void Dog::setDog(){
	cout<<"Enter dog's name: ";
			string name;
			cin>>name;
			m_name = name;
	cout<<"Enter dog's age: ";
			int age;
			cin>>age;
			m_age = age; 
}
void Dog::showDog(){
	cout<<"The dog's name is: "<<m_name<<endl;
	cout<<"The dog's age is: "<<m_age<<" years"<<endl; 
}

I know this is nitpicky but I also recommend getting array from C-style arrays and use std::array when you can as it gives you a lot more flexibility and power.

I'd also recommend getting away from raw arrays, but I would recommend you switch to std::vector instead of std::array.

Jim

Thanks all, I've taken on board your kind suggestions and re-worked the above using vector and iterator, this was useful learning.

For those still interested, code below:

Code:

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


const int MEMB = 3; 


class Dog{
	private:
		string m_name;
		int m_age;
	public:
		Dog():m_name("doggy"), m_age(0){} 
		void setDog(); 
		void showDog(); 
};


int main(){
	
	vector<Dog> list (MEMB); 
	for (vector<Dog> ::iterator it = list.begin(); it != list.end(); ++it){
		(*it).setDog();
		cout<<endl; 
	}
	cout<<endl;
	for (vector<Dog>::iterator it = list.begin(); it != list.end(); ++it){
		(*it).showDog();
		cout<<endl; 
	}
}


void Dog::setDog(){
	cout<<"Enter dog's name: ";
			string name;
			cin>>name;
			m_name = name;
	cout<<"Enter dog's age: ";
			int age;
			cin>>age;
			m_age = age; 
}
void Dog::showDog(){
	cout<<"The dog's name is: "<<m_name<<endl;
	cout<<"The dog's age is: "<<m_age<<" years"<<endl; 
}

Okay, but you may want to start using range based loops as well.

Code:

    vector<Dog> list (MEMB); // Also note that the size of your vector doesn't need to be a constant. 
    // Instead of:
    for (vector<Dog> ::iterator it = list.begin(); it != list.end(); ++it){
        (*it).setDog();
        cout<<endl; 
    }

    for(auto& itr : list)
        itr.setDog();

And perhaps overload the extraction operator to print the data as well.


Jim

Actually, range-based for-loops were the entire reason I suggested std::array XD

But as for array vs vector...

(This is more for the OP than anyone else)

Only use array if and only if the size is known at compile-time and you know for a fact that the array won't overflow your stack size and you don't mind each element being default constructed.

Actually, range-based for-loops were the entire reason I suggested std::array XD

You can use range-based for() loops with std::vector, std::array, or even raw arrays that have been statically allocated in the same scope as the loop.

The reason I suggest std::vector over std::array is because std::vectors don't need a compile time constant for their sizes and std::vectors are easier to use when writing reusable code.

Jim

Originally Posted by jimblumberg

You can use range-based for() loops with std::vector, std::array, or even raw arrays that have been statically allocated in the same scope as the loop.

The reason I suggest std::vector over std::array is because std::vectors don't need a compile time constant for their sizes and std::vectors are easier to use when writing reusable code.

Jim

Can you really use range-based syntax with a raw array? I thought it was just syntactic sugar for the `begin` and `end` methods.

I don't also see why a vector would be better when writing reusable code. If you can write your algorithms in terms of a vector, you know that you can implement all your algorithms in terms of _at most_ RandomAccessIterator which is pretty flexible. Because of the iterator-based approach, actual choice of container is relatively irrelevant.

Plus, std::array is on the stack and that is an advantage.

Hello again, if the size of the vector was not known at compile time, rather if I was told to keep entering Dog objects into the vector until a blank string is entered for a Dog's name, how should I code it?

My pseudo-code / thinking is as follows:

1. declare (a) vector of Dog objects, dog_list, (b) generic dog object, d, (c) iterator it;
2. push_back the vector with the generic dog object d;
3. use the iterator and the "set" member function to set the latest entered generic dog's data members as desired;
4. check if the name of the latest dog is blank; if not, continue, if blank, break;

Based on this, I have the following code that compiles but aborts after the first dog's details have been entered. Any suggestions would be very welcome. Thanks

Code:

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


class Dog{
	private:
		string m_name;
		int m_age;
	public:
		Dog():m_name("doggy"), m_age(0){} 
		void setDog(); 
		void showDog(); 
		string nameDog() { return m_name;}
};


int main(){
	
	Dog d; 
	vector<Dog> dog_list; 
	vector<Dog> ::iterator it;
	
	do{
		dog_list.push_back(d);
			for (it = dog_list.begin();  ; ++it){
				(*it).setDog();
			}
			cout<<endl;
	}
	while ((*it).nameDog() != " ");
	 
	cout<<endl;
	
	for (vector<Dog>::iterator it = dog_list.begin(); it != dog_list.end(); ++it){
		(*it).showDog();
		cout<<endl; 
	}
}


void Dog::setDog(){
	cout<<"Enter dog's name: ";
			string name;
			cin>>name;
			m_name = name;
	cout<<"Enter dog's age: ";
			int age;
			cin>>age;
			m_age = age; 
}
void Dog::showDog(){
	cout<<"The dog's name is: "<<m_name<<endl;
	cout<<"The dog's age is: "<<m_age<<" years"<<endl; 
}

Hello again, if the size of the vector was not known at compile time, rather if I was told to keep entering Dog objects into the vector until a blank string is entered for a Dog's name, how should I code it?

I'd actually recommend using `std::list` or just using `std::vector` with the `push_back` method. `push_back` will automatically reallocate your vector. This is typically pretty slow though so I think a list might actually be a more suitable container.