Creating an array of classes not working

Question

I'm trying to create an array of classes using a vector, but I think I'm getting the syntax wrong from instantiating the array. The error I'm getting is:

error: request for member 'setX' in objects[0], which is of non-class type 'std::vector'

#include <iostream>
#include <vector>

using std::cout;

class A {
    public:
        void setX(int a) { x = a; }
        int getX() { return x; }
    private:
        int x;
};

int main() {

    std::vector<A> *objects[1];

    objects[0].setX(5);
    objects[1].setX(6);

    cout << "object[0].getX() = " << objects[0].getX() << "\nobject[1].getX() = " << objects[1].getX() << std::endl;
}

By the way... By declaring the size with a 1... This will make the size 1... Not 2. It looks like you were thinking in vb — poy
– poy, Commented Aug 18, 2012 at 1:27

Grozz · Accepted Answer · 2012-08-18 01:11:38Z

5

std::vector<A> objects; // declare a vector of objects of type A
objects.push_back(A()); // add one object of type A to that vector
objects[0].setX(5); // call method on the first element of the vector

edited Aug 18, 2012 at 1:11

answered Aug 18, 2012 at 1:05

Grozz

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3 Comments

Cubic Over a year ago

The answer could use a bit more explainatory text, but it's definitely correct so I don't really see what the downvotes are for.

template boy Over a year ago

@Grozz A is a class, not a function...So how is it being called in objects.push_back( A() )??

derpface Over a year ago

A() is the default constructor for an object of type A. It's a nameless temporary being passed into vector::push_back().

Sergey Kalinichenko · Accepted Answer · 2012-08-18 01:06:31Z

3

With an asterisk and a square brackets, you are declaring an array of pointers to vectors instead of a vector. With std::vector<T> you do not need square brackets or an asterisk:

std::vector<A> objects(2); // 2 is the number of elements; Valid indexes are 0..1, 2 is excluded

objects[0].setX(5); // This will work
objects[1].setX(6);

The reason the compiler thought that you were trying to call setX on a vector is that the square bracket operator, overloaded by the vector, is also a valid operator on an array or a pointer.

answered Aug 18, 2012 at 1:06

Sergey Kalinichenko

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Comments

Jason · Accepted Answer · 2012-08-18 01:10:52Z

1

An array and std::vector are two completely different container types. An array is actually a fixed-size block of memory, where-as a std:vector object is a dynamic sequential container type, meaning it can be dynamically "grown" and "shrunk" at run-time, and the object itself manages the memory allocation of the objects it owns. Their apparent similarities are that both can access members in O(1) complexity and can use the bracket syntax for accessing members.

What you want is something like the following:

int main() 
{

    //make a call to the std::vector<T> cstor to create a vector that contains
    //two objects of type A
    std::vector<A> objects(2);

    //you can now access those objects in the std::vector through bracket-syntax

    objects[0].setX(5);
    objects[1].setX(6);

    cout << "object[0].getX() = " << objects[0].getX() << "\nobject[1].getX() = " << objects[1].getX() << std::endl;

    return 0;
}

edited Aug 18, 2012 at 1:10

answered Aug 18, 2012 at 1:05

Jason

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2 Comments

poy Over a year ago

if you however do not require your data structure to have a dynamic size, then use an array. Indexing will be faster. Especially if the size is set at compile time

Bo Persson Over a year ago

The potential difference between indexing an array and a vector will be in the nanosecond range, and most often not important. Using a vector will save you from all kinds of other problems.

Baiyan Huang · Accepted Answer · 2012-08-19 01:24:49Z

-1

here what you did is define an array with 1 element of type std::vector*, you may want to read more about vector and array first.

The correct way to define it is:

std::vector<A> objects(2);

or using pointers if that is what you intend to

std::vector<A*> objects(2);
objects[0] = new A();
objects[1] = new A();

edited Aug 19, 2012 at 1:24

answered Aug 18, 2012 at 1:13

Baiyan Huang

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4 Comments

Jason Over a year ago

That won't work ... you can't assign using operator= an A* to a std::vector<A*> object ... you have to insert it using push_back. You also end up with an extra layer of indirection, making this approach very inefficient...

Baiyan Huang Over a year ago

@Jason of course I can, push_back is to add new element at the end, but I already have 2 elements in the vector, I just what to assign it a value.

Jason Over a year ago

Okay, if you think this works, then please explain why your code won't compile: ideone.com/Svx0O

Baiyan Huang Over a year ago

@Jason sorry, my fault. there is a typo in my code: objects[2] should be objects() -> That is how we call vector's constructor. updated.

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