How to create a vector of class objects in C++? - c++

Question

I am trying to create a simple stack using vector in C++.
Here is the code:
#include <vector>
class Site
{
public:
int i; // site position i (x-axis)
int s; // site state
vector<Site> neighbors;
Site(void);
Site(int ii, int ss);
void AddNeighbor(Site &site);
};
Site::Site()
{
i = -1;
s = -1;
vector<Site> neighbors;
}
Site::Site(int ii, int ss)
{
i = ii;
s = ss;
}
void Site::AddNeighbor(Site &site)
{
neighbors.push_back(site);
}
void testStack()
{
int tot = 600;
vector<Site> myStack();
int i = 0;
while (i < tot)
{
Site site(i, 1);
myStack.push_back(site);
i++;
}
i = 0;
while (i < tot)
{
Site *site = myStack.back();
myStack.pop_back();
cout << site->i << site->s << endl;
i++;
}
}
Compiler errors:
ising_wolff.cpp: In function ‘void testStack()’:
ising_wolff.cpp:373:17: error: request for member ‘push_back’ in
‘myStack’, which is of non-class type ‘std::vector()’
myStack.push_back(site);
^ ising_wolff.cpp:380:30: error: request for member ‘back’ in ‘myStack’, which is of non-class type ‘std::vector()’
Site *site = myStack.back();
^ ising_wolff.cpp:381:17: error: request for member ‘pop_back’ in ‘myStack’, which is of non-class type
‘std::vector()’
myStack.pop_back();
What do these errors mean?
Here are some sites I have looked at:
1) Creating objects while adding them into vectors
2) push_back causing errors in C
3) how to create vectors of class object

Answer 1

How to create a vector of class objects in C++?
Start with something simpler so you can get the hang of it.
First, create a vector of primitive ints:
#include <vector>
#include <iostream>
using namespace std;
int main(){
vector<int> sites(5);
sites.push_back(5);
for(int x = 0; x < sites.size(); x++){
cout << sites[x];
}
cout << endl;
return 0;
}
Compiling it:
g++ -o test test.cpp
Running it:
./test
000005
Create a vector of class objects in a similar way as above:
#include <iostream>
#include <vector>
using namespace std;
class Site {
public:
int i;
};
int main() {
vector<Site> listofsites;
Site *s1 = new Site;
s1->i = 7;
Site *s2 = new Site;
s2->i = 9;
listofsites.push_back(*s1);
listofsites.push_back(*s2);
vector<Site>::iterator it;
for (it = listofsites.begin(); it != listofsites.end(); ++it) {
cout << it->i;
}
return 0;
}
Which should print:
79

Answer 2

vector<Site> myStack();
This is actually a function declaration. The function is called myStack and it returns a vector<Site>. What you actually want is:
vector<Site> myStack;
The type of neighbours at the moment will store copies of the objects, not references. If you really want to store references, I recommend using a std::reference_wrapper (rather than using pointers):
vector<reference_wrapper<Site>> neighbors;

Answer 3

vector<Site> myStack();
This is wrong. Lose the ().
You're declaring a function, not a vector.
Just write:
vector<Site> myStack;

Answer 4

You could use:
vector<Site> myStack;
myStack.resize(100); //will create 100 <Site> objects