Question

I’ve got the following lines of code:

p_diffuse = ShaderProperty<Vector4>(Vector4(1,1,1,1));
addProperty(&p_diffuse, "diffuse");

p_shininess = ShaderProperty<float>(10.0f);
addProperty(&p_shininess, "shininess");

the addProperty function is implemented as follows:

template <class A_Type> 
void IShader<A_Type>::addProperty( ShaderProperty<A_Type>* shaderProperty, 
                                   std::string propertyName )
{
  m_shaderProperties[propertyName] = shaderProperty;
}

now i get a strange compiler error on the last line of the first chunk of code. addProperty works fine in the first case, but in the second (when trying to add p_shininess) i get:

error C2664: 'IShader<A_Type>::addProperty': cannot convert parameter 1 from 'ShaderProperty<A_Type> *' to 'ShaderProperty<A_Type> *'

Huh!?
a hint of the problem could be the following: if I go to the project settings and set in the C++ general tab “check for 64-bit compatibility problems” from “no” to “yes(/Wp64)” then the error reads slightly different:

error C2664: 'IShader<A_Type>::addProperty': cannot convert parameter 1 from 'ShaderProperty<A_Type> *__w64 ' to 'ShaderProperty<A_Type> *'

what’s going on?? what is __w64??

edit: class definition of IShader:

template <class A_Type> class IShader {

public:
virtual ~IShader(void) {};
virtual A_Type shade(IntersectionData* iData, Scene* scene) = 0;

protected:

ShaderProperty<A_Type>* getProperty(std::string propertyName);
void addProperty(ShaderProperty<A_Type>* shaderProperty, std::string propertyName);

private:
std::map<std::string, ShaderProperty<A_Type>*> m_shaderProperties;
};

Answer 1

float != Vector4. Your whole class (IShader), is templated on A_Type, not just the addProperty method. /Wp64 has nothing to do with anything. The solution to this problem will need more context, you may want to define addProperty to be a template member function instead of IShader (or in addition to) being templated.

Again this will be hard to get right without knowing exactly what you are doing, but I suspect what you want is a heterogeneous collection of properties. To do this safely you’ll need to employ some runtime checking.

 class ISharderProperty {
    public:
    virtual ~IProperty() {}
 };

 template<typename ShadeType>
 class IShader;

 template <typename T>
 class ShaderProperty : public IShaderProperty {
    IShader<T> *m_shader;
    ...
  };

 template<typename ShadeType>
 class IShader { 
    ShadeType shade(...) = 0;
    protected:
      map<string, IShaderProperty*> m_shaderProperties;
      template<typename T>
      void addProperty(ShaderProperty<T>* prop, string name) {
         m_shaderProperties[name] = prop;
      }
      template<typename T>
      void getProperty(const string& name, ShaderProperty<T>** outProp) {
         map<string, IShaderProperty*>::iterator i = m_shaderProperties.find(name);
         *outProp = NULL;
         if( i != m_shaderProperties.end() ) {
             *outProp = dynamic_cast<ShaderProperty<T>*>( *i );
         }
      }
 };

You’ll have to use getProperty like

ShaderProperty<float> *x;
ashader.getProperty("floatprop", &x);
if( x ) {
    ...
}

Alternatively, getProperty could directly return the value, but then you’ll need to mention T twice, e.g.

ShaderProperty<float> *x = ashader.getProperty<float>("floatprop");

if( x ) { ... }

You’ll note I use dynamic_cast and check for NULL. If you have some other mechanism for mapping property names to property types you can use that instead and static_cast. There is some runtime overhead associated with dynamic_cast.