Calling member functions of an object stored in a vector

Question

This may sound like a newbie question. How do I call a member function of an object stored in a vector ? Supposedly, I have this class:

class A {
public:
void foo() {
std::cout << "Hello World"; }
};

Then I store some objects in a vector:

std::vector<A*> objects;
A* b;
A* c;
A* d;

objects.push_back(b);
objects.push_back(c);
objects.push_back(d);

Now I want to create a loop, in which every object stored in the vector would call it's own foo() function. How should I do this ? At first I thought I could just do something like this:

objects[2].foo();

But it seems that I can't do it like this.

Solution 1

std::vector::operator[] returns a reference to the object in std::vector. As your objects stores a pointer to A, you should call it:

objects[2]->foo();

Now I want to create a loop, in which every object stored in the vector would call it's own foo() function. How should I do this

The easiest loop is:

for (int i=0; i<objects.size(); ++i)
{
   objects[i]->foo();
}

use C++11 for loop:

for(auto ep : objects)
{
    ep->foo();
}

Or for_each(need to write a small lambda though)

for_each(objects.begin(), objects.end(), [](A* ep){ep->foo(); });

Side note: In practice you'd better store value in STL container.

std::vector<A> objects;
for(auto e : objects)
{
    ep.foo();    // call member function by "."
}

If you want to store pointer with STL container, better use smart pointer, e.g.

#include <memory>
std::vector<std::unique_ptr<A> > objects;

objects.push_back(new A());
objects.push_back(new A());
objects.clear(); // all dynamically allocated pointer elements will be de-allocated automatically

Solution 2

You can do this:

std::vector<A> objects;
A b;
A c;
A d;

objects.push_back(b);
objects.push_back(c);
objects.push_back(d);

objects[2].foo();

Please be a bit more specific about the exact error. I suspect maybe the whole problem was trying to reference a pointer to an object with "." instead of "->".

But yes, in general:

1) You can save an object, or a pointer to an object, in any STL container

2) You can call any public method of that object upon accessing it from the container.

Solution 3

You have created pointers to class A and not instances. So, you ought to access the method foo with pointer syntax. i.e

    for(int i = 0; i < 3; i++) {
      objects[i]->foo();
    }

Solution 4

In C++98 and C++03 (Since the other answers already tell you how to do it in the new standard) you can use std::for_each and the std::mem_fun adapter:

std::for_each(objects.begin(), objects.end(), std::mem_fun(&A::foo));

The std::for_each applies a function or functor to each of the elements in the range.

The std::mem_fun adapter takes a pointer to a function in its constructor and a pointer to an object as its (first) argument for operator().

This will result in the std::for_each invoking the foo() member function on each element on the array. If your vector had values on it instead of pointer you can use std::mem_fun_ref instead.

As for the error you are getting, remember that your vector is a collection of pointers so you should invoke the functions as follows:

objects[index]->foo();

View more solutions