如何制作指向动态分配对象的指针向量？

Question

I want to create a vector of pointers that each point to their own Martian object but i can't figure out how to arrange it. I'm currently getting the error

Non-const lvalue reference to type 'vector' cannot bind to a value of unrelated type 'martianDatabase'

but the error constantly changes with every change i make. I've watched a ton of tutorials the last two days trying to figure this out but I'm still stuck.

struct Martian
{
    Martian(string fname, string lname, string ssid);
    string fname, lname, ssid;
    ~Martian();
};


class martianDatabase
{
    vector<Martian*> database;
    martianDatabase();
    void addMartian(vector <Martian*> &database, int &iterator, string f,  string l, string id);
    int iterator = 0;
};

Martian::Martian(string f, string l, string id)
{
    fname = f;
    lname = l;
    ssid = id;
}

void martianDatabase::addMartian(vector <Martian*> &database, int &i, string f, string l, string id)
{
    Martian* m = new Martian(f, l, id);
    database[i].push_back(m);
    i++;
}

Answer 1

vector<Martian*> database;

Your database is a std::vector of pointers to Martian objects. That's what this declaration states.

database[i].push_back(m);

Since database is a vector, database[i] would be the i th value in this vector. Since this vector is a vector of Martian * , therefore database[i] is some Martian * value in this vector.

Obviously, you understand that if you have Martian * , a pointer to a Martian, it's not a class, and you can't push_back () anything on it. It's a pointer. A plain, garden variety pointer. You can't push_back() anything to a pointer. You can't begin() or end() it, etc...

That's what the compiler is telling you, with it's error message.

And, as far as your "How can i make a vector of pointers" question, you already did it:

vector<Martian *> database;

That's a vector of pointers. Now, whether it's a vector of pointers to dynamically-allocated objects, or not, that's no longer relevant. The vector doesn't care where the objects it's pointing to come from. A pointer to a dynamically-allocated object is exactly the same as a pointer to some object in a static scope.

Answer 2

There are several things wrong with your code:

• You shouldn't pass your vector<Martian*> as an argument to addMartian , instead you can just access it through the this pointer.
• There is no need for your int& iterator , as all you are trying to do is add your Martian to the end of a vector
• Your code database[i].push_back(m); gets the i'th element of the vector database , which is a Martian& , and then trys to call push_back(m) on it, which is not possible, as there is no such function declared for type Martian , what you probably wanted is database.push_back(m) , which insert m at the back of the database vector

Consider the following alternative:

class martianDatabase
{
    vector<Martian*> database;
    martianDatabase();
    void addMartian(string f,  string l, string id);
};

void martianDatabase::addMartian(string f, string l, string id)
{
    this->database.push_back(new Martian(f, l, id));
}

Though not realy a problem, it is potentially better to directly initialise member in your constructor (where possible) rather then copy assign them, ie use the code:

Martian::Martian(string f, string l, string id) : fname(f), lname(l), ssid(id) { }