Question

how can one create a class with a constructor that counts the number of people who are alive,if i have 10 people it should count the quantity of people who are alive,people should be objects in this example,lets say i have a class man like this below

class man{

     private:
     string name;

   man(string name=""){
     cout<<"there 10 people alive"<<endl;

~man(){}
};
int main(){


}

i am getting confused on how to go about it,i really need a simple example i want to use the set and get methods

Answer 1

The sane version of this would be to store your man objects in a collection, say a std::vector. From this std::vector, you could use size on the vector to fetch the number of items contained in it.

Otherwise, go with a solution that doesn’t make sense in this context, which would be to store a static variable that is increased in the constructor and decreased in the destructor.

By the way, your man class has a few mistakes, the constructor is private and you’re missing brackets… here’s a simple version of what you’re looking for:

class man
{
private:
    std::string mName;
public:
    man(std::string name="") : mName(name)
    {
    }

    const std::string& GetName() const
    {
        return mName;
    }
};

int main(int argc, char *argv[])
{
    man Bob = man("Bob");
    man Jimmy = man("Jimmy");

    std::vector<man> men; // This will copy the man object, might want to consider using pointers
    men.push_back(Bob);
    men.push_back(Jimmy);

    std::cout << "There is " << men.size() << " men." << std::endl;

    std::cout << "Known men are:" << std::endl;
    for(std::vector<man>::const_iterator itor = men.begin(); itor != men.end(); ++itor)
    {
        std::cout << itor->GetName() << std::endl;
    }

    // You could also iterate using something like for(unsigned int i = 0; i < men.size(); ++i)
}

Edit:

Lets add how to handle it if you’re going with a static variable… see the following scenario (I collapsed much of the brackets just to make it shorter)

See the following code:

class man
{
private:
    static unsigned int sCount;
    std::string mName;
public:
    man(std::string name="") : mName(name) { ++sCount; }
    ~man() { --sCount; }
    const std::string& GetName() const { return mName; }
    static unsigned int Count() { return sCount; }
};

unsigned int man::sCount = 0;

int main(int argc, char *argv[])
{
    man Bob = man("Bob");
    man Jimmy = man("Jimmy");
    std::cout << "There is " << man::Count() << " men" << std::endl;
    std::vector<man> men; // This will copy the man object, might want to consider using pointers
    men.push_back(Bob);
    men.push_back(Jimmy);
    std::cout << "There is " << man::Count() << " men" << std::endl;
}

The output to that will be

There is 2 men
There is 1 men

Wait… what?!? Where did we lose a man? Well, this happened in the vector resizing, since we didn’t define a copy constructor, the compiler did it for us, but the compiler wasn’t aware that we wanted to increment sCount, so when the vector resized, new objects were created, old ones destructed, and sCount didn’t get updated properly.

By changing our man class to:

class man
{
private:
    static unsigned int sCount;
    std::string mName;
public:
    man(std::string name="") : mName(name) { ++sCount; }
    man(const man& in_man) : mName(in_man.mName) { ++sCount; }
    ~man() { --sCount; }
    const std::string& GetName() const { return mName; }
    static unsigned int Count() { return sCount; }
};

We now have:

There is 2 men
There is 4 men

But why?!?. There are 2 man objects on the stack, Bob and Jimmy. Then there’s 2 copy of these these objects in std::vector<man> men because the vector contains “objects of type man“.

If we changed this to pointers:

std::vector<man*> men; 
men.push_back(&Bob);
men.push_back(&Jimmy);

We now have the following output, which is what we’re looking for:

There is 2 men
There is 2 men

Hope this clears things up for you!