All elements in a c++ vector point to the same element - c++

Question

I'm a complete beginner in c++ and everything has been going on well until now. I'm new to the idea of pointers (I'm from python), and I have this weird error.
So basically, I created this "SearchNode" class, and found below is one of it's methods "getChildren" which should return a vector of other SearchNode instances, representing the possible cells to which a Knight (chessboard) could travel from it's current state. (BFS)
That said, when I finish pushing into my vector, all the elements suddenly point to 1st element only. Could someone help me out here?
PS: it's a similar problem to c++ push_back doesn't work as it is supposed ... but unlike Angela (who's was writing her own compiler), I'm a total beginner in c++. Your help with be greatly appreciated.
UPDATE
I got rid of the int*, and used array for my state. I could now successfully search the graph (therefore the states are ok) and find the shortest path, but I couldn't seem to reconstruct the path.
To test, I started at {0,0} and could find {4,4}, but the path, according to the getPath method was {4,4}, {3,6}, {3,6}, {3,6} ... (infinite loop of {3,6}). Is there something wrong with my parent pointers, or my getPath function? Thanks for your support in advance.
//Search class
class SearchNode
{
public:
//Variables
SearchNode *m_parent;
array<int,2> m_state; //I don't understand typedef's yet, will use them when I'm clearer with them :)
//Normal Constructor
SearchNode(array<int,2>& state_, SearchNode *parent_=nullptr) :
m_state(state_),
m_parent(parent_)
{}
//Method to get Next reachable states. Returns instances of SearchNode.
vector<SearchNode> getChildren()
{
int legalMoves[8][2] = {{1,2},{1,-2},{-1,2},{-1,-2},{2,1},{2,-1},{-2,1},{-2,-1}};
vector<SearchNode> children;
children.reserve(8);
for(int i=0; i<8; i++)
{
int x = (m_state[0] + legalMoves[i][0]);
int y = (m_state[1] + legalMoves[i][1]);
if( (x>-1) and (x<9) and (y<9) and (y>-1)) // Within the bounds of the board
{
array<int,2> childState = {x,y};
SearchNode childNode = SearchNode(childState,this);
children.push_back(childNode);
}
}
return children;
}
void getPath()
{
cout<<"\nPath: ";
cout<< this->print();
SearchNode current = *this;
unsigned int counter = 1;
while((current.m_parent!=nullptr) and counter< 10)
{
counter++;
cout<< (current.m_parent)->print();
current = *(current.m_parent);
}
cout << (current.m_parent)->print();
}
string print()
{
stringstream out;
out << "{" << this->m_state[0] << "," << this->m_state[1] << "} ";
return out.str();
}
};

Answer 1

Lots of mistakes and errors, I strongly suggest you turn up the warning level in your compiler so you can get more information. With GCC/G++/Clang, try "-Wall" or "-Wextra", as moshbear points out.
Your nodes never get assigned the "parent" value, you're creating a "shadow" local variable called "parent" and assigning that. To avoid common errors like this, use a prefix or postfix for member variable names to separate them from local names, e.g. "m_parent" or "_parent".
You don't assign default values in your constructor, you explicitly leave the values uninitialized.
SearchNode()
{
//do nothing
}
and then you introduce this garbage data in your pointer-based constructor, what you probably want is
SearchNode() : parent(NULL), state(NULL) {}
Your copy constructor is a disaster. You need to read up on and understand pointers and local variables.
//Start Node constructor. Still looking for an equivalent for null.
SearchNode(int *state)
{
int genericStartState[2] = {-1,-1};
SearchNode blankParent = SearchNode();
SearchNode genericStart = SearchNode(genericStartState,&blankParent);
this->parent = &genericStart;
this->state=state;
}
Firstly, "blankParent" here is a local variable containing random data because of your current copy constructor. Secondly, you're taking the address of it - of a private, local variable, which is about to stop existing when you hit the "}" at the end of the routine.
"genericStartState" is also about to go out of scope.
And aside from that, I don't think you want or need this particular constructor.
But fundamentally, the bug in your subject, is because you do the same thing in your assignment loop -- you use a temporary, local array to store the new values, and then pass a pointer to that to your constructor. Since you are taking the address, it will be the same every loop.
int childState[2] = { x, y };
SearchNode childNode = SearchNode(childState,this);
This is why all of your nodes have the same state - because they all point to the same memory location (edit: as pointed out by DyP, that side-effect isn't something you can count on, just an artefact of ordering in this case).
It might be easier for you to use simple array of ints rather than a pointer in your node structure.
Here's how the constructor side of things might look, if your compiler is VisualStudio 2012 or G++ 4.8 or Clang 4.2.
class SearchNode
{
public:
typedef std::array<int, 2> State;
private:
// I use the 'm_' convention for members, 'g_' for globals, 's_' for statics.
SearchNode* m_parent;
State m_state;
public:
//////////
// Default ctor.
SearchNode()
: m_parent(nullptr) // C++11 constant meaning pointer with value 0
, m_state({-1, -1}) // preferred but requires recent C++11 features
{
//m_state[0] = m_state[1] = -1; // had to do this instead for gcc 4.7.3
}
//////////
// Normal ctor
// I use the '_'-postfix convention for parameter names.
SearchNode(SearchNode* parent_, const State& state_)
: m_parent(parent_)
, m_state(state_)
{
}
//////////
// Copy constructor.
// We could do this, but it's the default behavior anyway.
/*
SearchNode(const SearchNode& rhs)
: m_parent(rhs.m_parent)
, m_state(rhs.m_state)
{
}
*/
// Current C++11 compilers let us be explicit and do this:
//SearchNode(const SearchNode& rhs) = default;
// But it's the default behavior so we don't have to do this one at all.
};
The latest C++11 language changes (MSVC > 2012, GCC >= 4.8, Clang >= 4.1) would allow you to replace the first two constructors with
// Kill two birds with one stone and have default parameters on our normal ctor,
// replacing both the default and normal ctor with one function.
SearchNode(SearchNode* parent_ = nullptr, const State& state_ = { -1, -1 }))
: m_parent(parent_)
, m_state(state_)
{
}
If you had a fully C++1y compatible compiler, you could boil all that down to:
class SearchNode
{
public:
typedef std::array<int, 2> State;
private:
// I use the 'm_' convention for members, 'g_' for globals, 's_' for statics.
SearchNode* m_parent = nullptr; // c++1y keyword to replace 'NULL'
State m_state = { -1, -1 };
public:
SearchNode() = default;
SearchNode(const State& rhs_) = default; // not strictly required.
SearchNode(SearchNode* parent_, const State& state_)
: m_parent(parent_), m_state(state_)
{}
};