How do I use GLM Vector Relational Functions?

I'm trying to create an std::set of GLM vectors (glm::vec3 specifically). Since C++ does not know how to perform < operation on the vectors, I must pass in a Compare funciton.

I can write my own by creating a structure like so:

struct compareVec
{
    bool operator() (const glm::vec3& lhs, const glm::vec3& rhs) const
    {
        return lhs.x < rhs.x && lhs.y < rhs.y && lhs.z < rhs.z;
    }
};
std::set< glm::vec3, compareVec > myset;

However, I'm sure that GLM includes their own vector comparison functions.

I found the following resource, but I'm unsure of how to use it: https://glm.g-truc.net/0.9.4/api/a00137.html

How can I pass one of these comparison functions to my set?

Solution

Ok, almost there! The glm::lessThan returns a vector type, not a bool. This is why your comparator does not work. You could use glm::all on it to get a bool. From Documentation of glm::all

bool glm::all ( vecType< bool > const & v )

Returns true if all components of x are true.

Template Parameters vecType Boolean vector types.

If this makes sense for you, you have to decide for yourself, even if I recommend against it as this, as far as I understand, will lead to the following issue:

Consider:
lhs = (1,2,3)
rhs = (0,1,4)

Than:

lhs < rhs ==> false, since lhs.x and lhs.y are larger than the corresponding components of rhs
rhs < lhs ==> false, since rhs.z component is larger than lhs.z

Since neither vector can be ordered to be less, this implies that they are equal. I doubt this is the behavior you have in mind (I was already warning you about this).

If you still decide to use it, here is a minimal working example tested on MSVC2010:

#include <set>
#include <glm/vec3.hpp>
#include <glm/detail/func_vector_relational.hpp>

struct compareVec
{
    bool operator() (const glm::vec3& lhs, const glm::vec3& rhs) const
    {
        return glm::all(glm::lessThan(lhs, rhs));
    }
};

int main()
{

    std::set<glm::vec3, compareVec> myset;

    return 0;
}

Maybe this helps.