c++ "no appropriate default constructor available" error using template class data member

Question

I made a template class Grid(where i said in the header file that the default for T is float), i quoted a part of the source file:

#include"Grid.h"

template <class T>
Grid<T>::Grid(unsigned int rows=1, unsigned int columns=1)
:Rows(rows),Columns(columns)
{
reset();
}

template<class T>
Grid<T>::~Grid(){}

template <class T>
void Grid<T>::reset()
{
vector<T> vec(Rows * Columns,T());
matrix = vec;
}

And the other member functions can read/change a value of matrix or cout it.

Grid.h:

template<typename T=float> class Grid{

public:
        Grid(unsigned int, unsigned int);
        ~Grid();
        T getValue(unsigned int, unsigned int);
        void setValue(unsigned int, unsigned int, T);
        void reset();
        void write();

private:
        unsigned int Rows;
        unsigned int Columns;
        vector<T> matrix;
};

I found on the internet that in order to use a template class I needed to #include Grid.cpp as well as Grid.h, and doing this I can use the clas Grid and its member functions in my main(). I also put a preprocessor wrapper arround Grid.cpp.

Now, when I try to build a new class PDEProblem, without inheritance but using members from type Grid I get errors:

    Error   2   error C2512: 'Grid<>' : no appropriate default constructor available      c:\users\...  15  

    Error   3   error C2512: 'Grid<T>' : no appropriate default constructor available   c:\users\...    15  
4   IntelliSense: no default constructor exists for class "Grid<float>" c:\Users\...    15

PDEProblem.h:

#include"grid.h"
#include"grid.cpp"

class PDEProblem: Grid<>
{
public:
PDEProblem(unsigned int,unsigned int);
~PDEProblem();
//some more other data members

private:
Grid<char> gridFlags;
Grid<> grid;
unsigned int Rows;
unsigned int Columns;
void conPot(unsigned int, unsigned int);
void conFlag(unsigned int, unsigned int);
};

PDEProblem.cpp:

#include"grid.h"
#include"grid.cpp"
#include "PDEProblem.h"

PDEProblem::PDEProblem(unsigned int rows=1,unsigned int columns=1)
    :Rows(rows), Columns(columns)
{
    conPot(rows, columns);
    conFlag(rows,columns);
}

PDEProblem::~PDEProblem(){}

void PDEProblem::conPot(unsigned int rows, unsigned int columns)
{
    grid=Grid<>(rows,columns);
}

void PDEProblem::conFlag(unsigned int rows, unsigned int columns)
 {gridFlags=Grid<char>(rows,columns);
    // some stuff with a few if and for loops which sets some elements of gridFlags to 1 and the others to 0
}

How can I fix this? It seems to me that I have defaults for everything relevant? Thank you

Answer 1

With my compiler (Visual Studio 2010) and your code, I can make your error go away by moving the default parameter values from the function definition to the function prototype. Specifically:

Grid.h

template<typename T=float> class Grid{

public:
    Grid(unsigned int rows = 1, unsigned int columns = 1);
...
};

Grid.cpp

template <class T>
Grid<T>::Grid(unsigned int rows, unsigned int columns)
:Rows(rows),Columns(columns)
{
reset();
}

Answer 2

Your issue is that your main class inherits from Grid and at the same time contains two other instances of Grid. Except for the bad design, you don't have any explicit constructors for your two Grid instances, which is why you have the error. Putting default values is not the proper way to go.