Problems with passing different data types - c++

Question

I am working on my own graph project in C++ at the moment and I have come along a problem which I'm sure it's easy to solve but I can't seem to get it right. I am building a template graph class with a vertex class declared inside of it as such:
template <typename T>
class graph{
public:
class vertex{
public:
bool visited; //used for paths. True if vertex has been visited
//vertex constructor
vertex(const T& d = T{}, int i = 0): data(d), visited(false), id(i){
std::cout << "Just created a vertex using its constructor :)\n";}
//vertex move constructor
vertex(T&& d, int i = 0): data(std::move(d)), visited(false), id(i){}
//returns vertex ID
int returnID() const{ return id; };
//returns data in vertex
T& operator*(){ return retrieve(); }
//returns const reference to data in vertex
const T& operator*() const{ return retrieve(); }
//returns list of adjacent vertices
std::list<vertex>& getList() const{ return adjacent; }
//adds vertex to current vertex's adjacency list
void addToList(const vertex& add){
if(!isAdjacent(add))
adjacent.push_back(add);
}
//returns true if vertices are adjacent
bool isAdjacent(const vertex& add){
return (find(begin(adjacent), end(adjacent), add) == adjacent.end());
}
//overloaded equal operator for vertex class
bool operator==(const vertex& add){
if(data == *add && id == add.returnID()) return true;
return false;
}
private:
T data; //vertex stores data of any type
std::list<vertex> adjacent; //list of adjacent vertices
int id;
T& retrieve() const{ return data; }
];
];
I keep getting this error:
g++ -std=c++11 -Wno-reorder -Wall -pedantic -o executable.x main.cpp
In file included from main.cpp:2:0:
graph.h: In instantiation of ‘T& graph<T>::vertex::retrieve() const [with T = int]’:
graph.h:70:47: required from ‘const T& graph<T>::vertex::operator*() const [with T = int]’
graph.h:84:17: required from ‘bool graph<T>::vertex::operator==(const graph<T>::vertex&) [with T = int]’
/usr/lib/gcc/x86_64-pc-linux-gnu/4.8.4/include/g++-v4/bits/stl_algo.h:139:46: required from ‘_InputIterator std::__find(_InputIterator, _InputIterator, const _Tp&, std::input_iterator_tag) [with _InputIterator = std::_List_iterator<graph<int>::vertex>; _Tp = graph<int>::vertex]’
/usr/lib/gcc/x86_64-pc-linux-gnu/4.8.4/include/g++-v4/bits/stl_algo.h:4441:45: required from ‘_IIter std::find(_IIter, _IIter, const _Tp&) [with _IIter = std::_List_iterator<graph<int>::vertex>; _Tp = graph<int>::vertex]’
graph.h:80:54: required from ‘bool graph<T>::vertex::isAdjacent(const graph<T>::vertex&) [with T = int]’
graph.h:75:24: required from ‘void graph<T>::vertex::addToList(const graph<T>::vertex&) [with T = int]’
graph.hpp:25:4: required from ‘bool graph<T>::addEdge(graph<T>::vertex&, graph<T>::vertex&) [with T = int]’
main.cpp:12:18: required from here
graph.h:92:31: error: invalid initialization of reference of type ‘int&’ from expression of type ‘const int’
T& retrieve() const{ return data; }
^
graph.h: In member function ‘T& graph<T>::vertex::retrieve() const [with T = int]’:
graph.h:92:37: warning: control reaches end of non-void function [-Wreturn-type]
T& retrieve() const{ return data; }
And to sum up the problem it seems to have an issue with my private T& retrieve() function. It keeps saying invalid initialization of type 'int&' from expression with type 'const int' referring to the data it is returning, which is private member data of type T (instantiated in main as an int). Any help would be greatly appreciated! Thanks

Answer 1

T data;
...
T& retrieve() const{ return data; }
The const modifier makes this method a const method. Which means that, essentially, its this is a pointer to a const class instance. Since data was not declared with a mutable modifier, data is a const value, and your compiler is complaining because it can't return a reference to a mutable value, because data is a constant value.
Either change the method to:
const T& retrieve() const{ return data; }
Or, I don't see why a reference here is needed, just:
T retrieve() const{ return data; }
P.S. You should try to make sense of these compiler errors, before giving up so quickly. It's true that C++ compilation errors are legendary for their typical obtuseness, but in this case you should be able to figure out what the compiler is telling you, after staring at those error messages, for a few minutes.