Std::vector not changing data inside class - c++

Question

I'm doing some stuff with vectors and I'm trying to write a program that can loop through and perform operations on objects inside a vector. I've read a lot about minimal code so I've programmed this little application to illustrate the problem.
The output of this program is:
Element 1 is equal to 2
Element 2 is equal to 4
Element 1 is equal to 2
Element 2 is equal to 4
Press any key to continue...
The output should be:
Element 1 is equal to 2
Element 2 is equal to 4
Element 1 is equal to 7
Element 2 is equal to 9
Press any key to continue...
Why am I not achieving the desired output?
#include <vector>
#include <iostream>
// Some object
class A {
public:
A(int data)
: data(data)
{}
int data;
};
class B {
public:
// A function that adds a new 'A' object to the vector in class b
void push_back_to_vector(A &element);
// A function that changes the data of the objects stored in vector_of_a
void add_to_vector();
// A vector to hold objects of A
std::vector<A> vector_of_a;
};
void B::push_back_to_vector(A &element) {
vector_of_a.push_back(element);
}
void B::add_to_vector() {
for (size_t i = 0; i < vector_of_a.size(); i++) {
// Add five to all the elements in the vector
vector_of_a[i].data += 5;
}
}
int main() {
A element_1(2);
A element_2(4);
B b;
b.push_back_to_vector(element_1);
b.push_back_to_vector(element_2);
std::cout << "Element 1 is equal to " << element_1.data << std::endl;
std::cout << "Element 2 is equal to " << element_2.data << std::endl;
// This should change element_1.data to 7 and element_2.data to 9
b.add_to_vector();
std::cout << "Element 1 is equal to " << element_1.data << std::endl;
std::cout << "Element 2 is equal to " << element_2.data << std::endl;
system("pause");
return 0;
}

Answer 1

The push_back copies new elements. If you want to do what you seem to want to do, you need to store pointers to A, not just A.
Basically, your code is just this:
A element_1(2);
A element_2(4);
std::cout << "Element 1 is equal to " << element_1.data << std::endl;
std::cout << "Element 2 is equal to " << element_2.data << std::endl;
std::cout << "Element 1 is equal to " << element_1.data << std::endl;
std::cout << "Element 2 is equal to " << element_2.data << std::endl;
system("pause");
return 0;
Everything related to B doesn't work. Try this:
class B {
public:
// A function that adds a new 'A' object to the vector in class b
void push_back_to_vector(A *element);
// A function that changes the data of the objects stored in vector_of_a
void add_to_vector();
// A vector to hold objects of A
std::vector<*A> vector_of_a;
};
void B::push_back_to_vector(A* element) {
vector_of_a.push_back(element);
}
void B::add_to_vector() {
for (size_t i = 0; i < vector_of_a.size(); i++) {
// Add five to all the elements in the vector
vector_of_a[i]->data += 5;
}
}
And pass pointers:
b.push_back_to_vector(&element_1);
b.push_back_to_vector(&element_2);
Of course, be aware of the lifetime of the objects on the stack that you are pointing to in B.