Need to reference and update value from nested class C++ - c++

Question

Bear with me, I'm new to C++. I'm trying to update a value which is stored in a vector, but I'm getting this error:
non-const lvalue reference to type 'Node'
I'm using a simple wrapper around std::vector so I can share methods like contains and others (similar to how the ArrayList is in Java).
#include <vector>
using namespace std;
template <class T> class NewFrames {
public:
// truncated ...
bool contains(T data) {
for(int i = 0; i < this->vec->size(); i++) {
if(this->vec->at(i) == data) {
return true;
}
}
return false;
}
int indexOf(T data) {
for(int i = 0; i < this->vec->size(); i++) {
if(this->vec->at(i) == data) {
return i;
}
}
return -1;
}
T get(int index) {
if(index > this->vec->size()) {
throw std::out_of_range("Cannot get index that exceeds the capacity");
}
return this->vec->at(index);
}
private:
vector<T> *vec;
};
#endif // A2_NEWFRAMES_H
The class which utilizes this wrapper is defined as follows:
#include "Page.h"
#include "NewFrames.h"
class Algo {
private:
typedef struct Node {
unsigned reference:1;
int data;
unsigned long _time;
Node() { }
Node(int data) {
this->data = data;
this->reference = 0;
this->_time = (unsigned long) time(NULL);
}
} Node;
unsigned _faults;
Page page;
NewFrames<Node> *frames;
};
I'm at a point where I need to reference one of the Node objects inside of the vector, but I need to be able to change reference to a different value. From what I've found on SO, I need to do this:
const Node &n = this->frames->get(this->frames->indexOf(data));
I've tried just using:
Node n = this->frames->get(this->frames->indexOf(data));
n.reference = 1;
and then viewing the data in the debugger, but the value is not updated when I check later on. Consider this:
const int data = this->page.pages[i];
const bool contains = this->frames->contains(Node(data));
Node node = this->frames->get(index);
for(unsigned i = 0; i < this->page.pages.size(); i++) {
if(node == NULL && !contains) {
// add node
} else if(contains) {
Node n = this->frames->get(this->frames->indexOf(data));
if(n.reference == 0) {
n.reference = 1;
} else {
n.reference = 0;
}
} else {
// do other stuff
}
}
With subsequent passes of the loop, the node with that particular data value is somehow different.
But if I attempt to change n.reference, I'll get an error because const is preventing the object from changing. Is there a way I can get this node so I can change it? I'm coming from the friendly Java world where something like this would work, but I want to know/understand why this doesn't work in C++.

Answer 1

Node n = this->frames->get(this->frames->indexOf(data));
n.reference = 1;
This copies the Node from frames and stores the copy as the object n. Modifying the copy does not change the original node.
The simplest "fix" is to use a reference. That means changing the return type of get from T to T&, and changing the previous two lines to
Node& n = this->frames->get(this->frames->indexOf(data));
n.reference = 1;
That should get the code to work. But there is so much indirection in the code that there are likely to be other problems that haven't shown up yet. As #nwp said in a comment, using vector<T> instead of vector<T>* will save you many headaches.
And while I'm giving style advice, get rid of those this->s; they're just noise. And simplify the belt-and-suspenders validity checks: when you loop from 0 to vec.size() you don't need to check that the index is okay when you access the element; change vec.at(i) to vec[i]. And in get, note that vec.at(index) will throw an exception if index is out of bounds, so you can either skip the initial range check or keep the check (after fixing it so that it checks the actual range) and, again, use vec[index] instead of vec.at(index).