Auto Increment static variables

[musicman]

I am trying to figure out using static variables to auto increment each new instance of a class. Here is what I have so far:

People.h

#ifndef PEOPLE_H
#define PEOPLE_H

#include <string>
using namespace std;

class People
{
private:
	int id;
	string name;
	int age;
	string instrument;
public:
	People(string n, int a, string i);
	void setName(string newName);
	void setAge(int newAge);
	void setInstrument(string newInstrument);

	string getName();
	int getAge();
	string getInstrument();
	int getID();

	void printAll();
};
#endif

People.cpp

#include "People.h"
#include <iostream>

using namespace std;

People::People(string n, int a, string i)
{
	name = n;
	age = a;
	instrument = i;
	id++;
}

void People::setName(string newName)
{
	name = newName;
}

void People::setAge(int newAge)
{
	age = newAge;
}

void People::setInstrument(string newInstrument)
{
	instrument = newInstrument;
}

string People::getName()
{
	return name;
}

int People::getAge()
{
	return age;
}

string People::getInstrument()
{
	return instrument;
}

int People::getID()
{
	return id;
}

void People::printAll()
{
	cout << id << name << " is " << age << " years old and plays the " << instrument << endl;
}

Main.cpp

#include "People.h"
#include <iostream>

using namespace std;

int main()
{
	People luke("Luke", 16, "Piano");
	People alex("Alex", 17, "Piano");
	People ewan("Ewan", 19, "Saxophone");

	luke.printAll();
	alex.printAll();
	ewan.printAll();

	cin.get();
	return 0;
}

To be fair I found other examples for static variables but I am probably lost in the separate files setup I try to use. Any hints how/where I should be using the static variables?

[manasij7479]

Make a simple test case first for not getting confused.

In this case, making the id static would do what you want.

[Salem]

You're almost there!

In people.h, you declare the class variable as
static int id;

In people.cpp, you just add this line.
int People::id = 0;


Then the results look like this.
$ g++ foo.cpp
$ ./a.out
3Luke is 16 years old and plays the Piano
3Alex is 17 years old and plays the Piano
3Ewan is 19 years old and plays the Saxophone

[hengshan]

if you want to get a class's member , you can declare a static variable.

[whiteflags]

No hengshan, static member data is shared among all of the objects. It is specifically defined wherever (in this case) int People::id = 0; is, and that memory is what each People object uses for the member id.

If you want to "get a class's member," you should use the class interface; basically, the public member function that returns the member.

[grumpy]

You're almost there!

In people.h, you declare the class variable as
static int id;

In people.cpp, you just add this line.
int People::id = 0;


Then the results look like this.
$ g++ foo.cpp
$ ./a.out
3Luke is 16 years old and plays the Piano
3Alex is 17 years old and plays the Piano
3Ewan is 19 years old and plays the Saxophone

Close but, assuming the aim is that every object of type People has a unique id number ....

// in People.h

class People
{
private:
	int id;
        static int id_counter;
	string name;
	int age;
	string instrument;
public:
	People(string n, int a, string i);

        // other stuff

	int getID();

	void printAll();
};

and in the .cpp file.

int People::id_counter = 0;

//   and somewhere in EVERY accessible constructor

    id = id_counter++;

Note that comment about what is needed in EVERY constructor. It will be necessary to implement a copy constructor and assignment operator to ensure both id and static_id are maintained appropriately (or, alternatively, declare the copy constructor and assignment operator as private, and not implement them).

All bets are off in the above if the People class is used from multiple threads though.

[hk_mp5kpdw]

Avoid putting a using namespace std statement in your header file, you already have one in the CPP file that takes care of that code. For the objects in the header itself you should explicitly qualify those with std:: as needed.