C++: expected identifier before numeric constant

Question

I'm trying to write a small program using MTL, but I'm getting the mentioned error when I try to make a MTL Matrix a member of a class.

#include <boost/numeric/mtl/mtl.hpp>

class myClass
{
private:
    mtl::dense2D<double> Ke(6,6);
};

However, there is no problem with the same statement in main():

#include <boost/numeric/mtl/mtl.hpp>

int main(int argc, char** argv)
{
    mtl::dense2D<double> Ke(6,6);
    return 0;    
}

I'm very new to C++, and I don't think this is really related to the MTL, but that's where the error occurred for me.

Answer 1

You need to do that in the constructor's initialiser list.

class myClass {
    mtl::dense2D<double> Ke;
public:
    myClass() : Ke(mtl::dense2D<double>(6, 6)) { }
};

Answer 2

Because when you declare

mtl::dense2D<double> Ke;

you're only supposed to declare it, not create it yet. This is the constructor's job in C++:

class myClass
{
public:
    myClass() // constructor
        : Ke(6, 6) // here we use the constructor initializer
    {
    }
private:
    mtl::dense2D<double> Ke; // declaration
};

Answer 3

You can't initialize variable within the class scope, you need to do it in a constructor. Change this:

class myClass
{
private:
    mtl::dense2D<double> Ke(6,6);
};

to this --

class myClass
{
public:
    myClass() : Ke(6,6) { }
private:
    mtl::dense2D<double> Ke;
};