problem with static vector of a class pointer inside the class itself.

euklid

Hello, I am trying to build a class Matrix which contains a member

vector<Matrix*> vecMat

i.e. the vector of class pointers to the class itself; I want this vector as static so that in each element of the vector(i.e. the class) contains only one copy of this vector. However it is not working. If I do not make this member as static it works fine.

Does any one have idea why is that? why can't I make the vecMat as static.
Any suggestions on this program would be very appreciable.

#include<iostream>
#include<vector>
using namespace std;

struct Matrix{
int  nrow;
int ncol;
vector< vector<int > > mat; 
public:
	Matrix(int row, int col): nrow(row),ncol(col)
	{
		mat.resize(ncol);
		for(unsigned int i=0;i<mat.size();++i)
			mat[i].resize(nrow);
	}
	static vector<Matrix*>  matVec;	
          /* if matVec is not static then this code is working */
	~Matrix();
	static void fillUpVectorElement( int x , int y, vector<Matrix*> & pmatVec);

};

Matrix::~Matrix()
{
		for (unsigned int i=0;i<matVec.size();++i)
			delete matVec[i];
			//matVec[i]=NULL;
	}
void Matrix:: fillUpVectorElement(int x , int y, vector<Matrix*> & pmatVec)
{
	Matrix* pMatrix=new Matrix (x, y);
	//....  /*fill up matrix element*/
	pmatVec.push_back(pMatrix);

}

int main (int argc, char**argv){ 
	Matrix MAT (3, 4);

return 0;}

SeppJ

Static class members must be defined somewhere. A declaration is not enough.

class foo
{
 static int bar;  // This ist just a declaration that a variable foo::bar exists
};

int foo::bar;  // It must be defined somewhere. For example in some foo.cpp

BUT: This sounds like an awful, awful design choice. Why would you ever make elements of a matrix static? What happens if you have two matrices?

The error:

main.cpp:(.text+0x1d): undefined reference to `Matrix::matVec'

The solution:
You need to define the static data. Place

vector<Matrix*> Matrix::matVec;

outside the class Definition inside your .cpp-file (to satisfy ODR this must not be in the .h)

euklid

SeppJ schrieb:

Static class members must be defined somewhere. A declaration is not enough.
class foo
{
 static int bar;  // This ist just a declaration that a variable foo::bar exists
};
int foo::bar;  // It must be defined somewhere. For example in some foo.cpp
BUT: This sounds like an awful, awful design choice. Why would you ever make elements of a matrix static? What happens if you have two matrices?

thank you SeppJ, each class object of class Type Matrix contains a vector of Matrix pointer. I need only one vector of class objects but not two or more than two vector of class matrix. but this is a vector of pointer to class obj.so, probably it wont be a problem.

SeppJ

But why are you deleting all of those Pointers on every destructor-call? While your program is technically correct it still makes no sense. Whatever you want to achieve here, I am sure you are doing it wrong.