[Boost-users] [type_traits] How can I test if a class has a public constructor?

From: Steven van Dijk (SDijk_at_[hidden])
Date: 2006-05-22 10:29:16

Hi,

I want to prevent non-factory creation of my classes. All classes have
constructors without arguments. The constructor of a class should be
'protected' and a special creator template class is used to derive from
the class and give the factory access to it. Example:

class A : public Base
{
protected:
        A();
public:
        void DoSomething();
};

template < class T >
class Creator : public T
{
public:
        static Base* Create()
        {
                return new Creator<T>;
        }
};

...

class Factory
{
        static A* GetAObject()
        {
             return Creator<A>::Create(); 
        }
};

The example is somewhat contrived to omit non-essential details. 

Anyway, what I would like to do is get a compiler error when I register
a class with the factory that has a publicly accessible constructor. I
tried to do this with boost::has_trivial_constructor< T > like this:

class A : public Base
{
public:
        A() {} 
};

template < bool B >
class CGiveError
{
public:
        static void error() {}
};

template <>
class CGiveError<true>
{
public:
        static void error() { assert( false ); }
};

void
RegisterAll()
{
        CGiveError< boost::has_trivial_constructor< A >::value
>::error();
}

However, this doesn't work. I guess has_trivial_constructor just checks
if the constructor takes arguments or not.

Is there some way to achieve what I want? That is, issue an error if the
constructor is public?

Thanks,

Steven