wrong number of template arguments (3, should be 4) - c++

Question

I tried to use property on c++ to use it instead of too many setter and getter function in data class have lots of member variable.
there are two property class. first one has fixed setter and getter function by default set, get. second one support using custom setter and getter function of its class. below is the code
template <class T>
class Property
{
T data;
public:
// access with function call syntax
Property() : data() { }
T operator()() const
{
return data;
}
T operator()( T const & value)
{
data = value;
return data;
}
// access with get()/set() syntax
T get() const
{
return data;
}
T set( T const & value )
{
data = value;
return data;
}
// access with '=' sign
operator T() const
{
return data;
}
T operator = ( T const & value )
{
data = value;
return data;
}
typedef T value_type; // might be useful for template deductions
};
// a read-write property which invokes user-defined functions
template <class T, class Object, T(Object::*real_getter)(), T(Object::*real_setter)(T const &) >
class RWProperty
{
Object * my_object;
public:
// this function must be called by the containing class, normally in a
// constructor, to initialize the RWProperty so it knows where its
// real implementation code can be found
void operator () ( Object * obj )
{
my_object = obj;
}
// function call syntax
T operator()() const
{
return (my_object->*real_getter)();
}
T operator()( T const & value )
{
return (my_object->*real_setter)( value );
}
// get/set syntax
T get() const
{
return (my_object->*real_getter)();
}
T set( T const & value )
{
return (my_object->*real_setter)( value );
}
// access with '=' sign
operator T() const
{
return (my_object->*real_getter)();
}
T operator = ( T const & value )
{
return (my_object->*real_setter)( value );
}
typedef T value_type; // might be useful for template deductions
};
and i'm testing these properties in OptionSet class before putting it into project code
#include <QString>
class OptionSet
{
public:
explicit OptionSet() {}
Property<QString> m_MeshMode;
RWProperty<uint, OptionSet, &getNumberOfVbo, &setNumberOfVbo> uNumberOfVbo;
// this causes problems
protected:
private:
Property<uint> m_uNumberOfVbo;
uint setNumberOfVbo(const uint& rVboCount)
{
// something to do here
return m_uNumberOfVbo(rVboCount);
}
uint getNumberOfVbo() const
{
return m_uNumberOfVbo();
}
};
but in use RWProperty, even i passed 4 arguments of template like member type, class type has setter and getter function, getter function pointer, setter function pointer in order, it says
"wrong number of template arguments (3, should be 4) :
RWProperty <uint, OptionSet, &getNumberOfVbo, &setNumberOfVbo>
uNumberOfVbo"
"provided for 'template<class T, class Object,
T(Object::*real_getter)(), T(Object::*real_setter)(const T&)> class
RWProperty : class RWProperty"
I guess i'm doing something wrong to pass arguments in template.
is there anyone knows what happened?

Answer 1

There are three mistakes in your code:
Firstly, to get the address of a member function, you need to include the class name:
RWProperty<uint, OptionSet
, &OptionSet::getNumberOfVbo
// ~~~~~~~~~~~^
, &OptionSet::setNumberOfVbo> uNumberOfVbo;
// ~~~~~~~~~~~^
Secondly, to form a pointer to const qualified member function, you need to append const keyword to the declaration of that pointer:
T (Object::*real_getter)() const
// ~~~~^ to match 'uint getNumberOfVbo() const'
Lastly, OptionSet inside OptionSet itself is an incomplete type. You can't refer to its member unless the point of declaration of that member comes first. This basically means you need to reorder your declarations within OptionSet so that setNumberOfVbo and getNumberOfVbo comes before you declare uNumberOfVbo data member:
class OptionSet
{
//...
uint setNumberOfVbo(const uint& rVboCount) { /*...*/ }
uint getNumberOfVbo() const { /*...*/ }
// Ok, now they are known and can be found in the class scope
//...
RWProperty<uint, OptionSet
, &OptionSet::getNumberOfVbo
, &OptionSet::setNumberOfVbo> uNumberOfVbo;
};