std::map containing a struct of four doubles as key

[miteshpandey]

Hello all,

I have a struct of four doubles and I want to use it as a key to std::map

struct MatPropVals
{
   double ambient;
   double diffuse;
   double specular;
   double emission;
}

How do I write the comparing function. I know I have to be careful about comparing doubles. Can somebody please help me. Before this I have only use std::map with simple types.

I don't want to insert a new MatPropVals object if it has already been inserted with all the four double values being equal. std::map will take care of this. But how do I write the comparing function.

I know that there is a FAQ about comparing doubles (epsilon thing). Does it apply in my case as well. I will take care of this but just need help on the comparing function.

Thanks in advance

[Philip Nicoletti]

The simplest method is to provide operator < for the class.

struct MatPropVals
{
   double ambient;
   double diffuse;
   double specular;
   double emission;

   bool operator < (const MatPropVals & rhs) const
   {
       if (ambient < rhs.ambient) return true;
       if (ambient > rhs.ambient) return false;

       if (diffuse < rhs.diffuse) return true;
       if (diffuse > rhs.diffuse) return false;

       if (specular < rhs.specular) return true;
       if (specular > rhs.specular) return false;

       return emission < rhs.emission;
   }
};

You could probably add the "epsilon thing" to the above code. I
would have to think about how to implement that.

[miteshpandey]

Thanks Philip,

But I think I cannot use it

Actually my struct is like this:

#pragma pack( push, 1 )
struct MatPropVals
{
   double ambient;
   double diffuse;
   double specular;
   double emission;
}
#pragma pack( pop )

In order to read the material property values I directly pass the object's address. It's a COM function I don't have control over it.

Could you please give me a compare function example?

[Philip Nicoletti]

struct MyCompare
{
    bool operator() (const MatPropVals & lhs , const MatPropVals & rhs) const
    {
       if (lhs.ambient < rhs.ambient) return true;
       if (lhs.ambient > rhs.ambient) return false;

       if (lhs.diffuse < rhs.diffuse) return true;
       if (lhs.diffuse > rhs.diffuse) return false;

       if (lhs.specular < rhs.specular) return true;
       if (lhs.specular > rhs.specular) return false;

       return lhs.emission < rhs.emission;
    }
};

[miteshpandey]

Thanks a lot.

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