C++ - Default arguments cannot be added to an out-of-line definition of a member of a class template

Question

I have an iOS app that makes use of a C++ class from MPL called 'square.h' and whenever I build the app Xcode throws this error -

Default arguments cannot be added to an out-of-line definition of a member of a class template

Now, my understanding of C++ syntax is very little, so what does this error mean?

The problematic line is below -

template <class T> 
TSquare<T>::TSquare (T size_, T x_, T y_, T scale_=0)
{
    size = size_;
    x = x_;
    y = y_;
    scale = scale_;
}

Answer 1

Default arguments are not part of the definition, only declaration:

template <class T>
class TSquare
{
public:
    TSquare (T size_, T x_, T y_, T scale_ = 0);
};

// out-of-line definition of a member of a class template:
template <class T>
TSquare<T>::TSquare (T size_, T x_, T y_, T scale_ /* = 0 */)
{

}

or:

template <class T>
class TSquare
{
public:
    // in-line definition of a member of a class template
    TSquare (T size_, T x_, T y_, T scale_ = 0)
    {

    }
};