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calling a function in a std::binary_function, for std::list sort functor. gaaah :P

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Hello! Im trying to sort my render list after the objects z-position. However, it seems it is not possible to call functoins from the arguments you send into a class that inherits from std::binary_function. Anyone know how to do this? Here is what i want to do, i have a class, gameobject. Which have a member, int zpos. But i want to call the member getZPos() instead of the variable. Because i do not want it to be public. Anyhow, this works:
//-----------------------------------------------------------------------------

// Name: SortByZPos

// Desc: 

//-----------------------------------------------------------------------------

struct SortByZPos : public std::binary_function<const Gameobject*, const Gameobject*, bool>
{ 
	result_type operator()( first_argument_type a, second_argument_type b ) 
	{
		return a->zpos > b->zpos;   
	}
};
This dosnt work: ( the return a->getZPos() > b->getZPos(); part)
//-----------------------------------------------------------------------------

// Name: SortByZPos

// Desc: 

//-----------------------------------------------------------------------------

struct SortByZPos : public std::binary_function<const Gameobject*, const Gameobject*, bool>
{ 
	result_type operator()( first_argument_type a, second_argument_type b ) 
	{
		return a->getZPos() > b->getZPos();   
	}
};
The error i get is:
error C2662: ''Gameobject::getZPos'' : cannot convert ''this'' pointer from ''const Gameobject'' to ''Gameobject &
And i cant see how to solve this.. is it some wierd syntax issue or is it just not possible? Thanks in advance!

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You have to make the getZPos() member constant:

float GameObject::getZPos() const

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