Question

I am looking over this piece of code, and am having trouble with the syntax:

struct Instance
{
    typedef glm::vec3(*OffsetFunc)(float);

    OffsetFunc CalcOffset;

    glm::mat4 ConstructMatrix(float fElapsedTime)
    {
        glm::mat4 theMat(1.0f);

        theMat[3] = glm::vec4(CalcOffset(fElapsedTime), 1.0f);

        return theMat;
    }
};

It is C++ code, related to OpenGL, but my question is not about OpenGL. The glm::vec3, vec4, mat4 are just vectors of dimension 3 and 4, and mat4 is a 4×4 square matrix. The glm library has overloaded operators so lines like:

theMat[3] = glm::vec4(CalcOffset(fElapsedTime), 1.0f);

work as you might expect, filling up the 4th column of theMat with a 4-d vector that is comprised of 1.0f and that cast or typedef or function call, I’m not exactly sure what that is, and that is my question.

What does typedef glm::vec3(*OffsetFunc)(float); and CalcOffset(fElapsedTime) mean?

I tried reading here: http://en.wikipedia.org/wiki/Typedef#Using_typedef_with_type_casts , but that explanation doesn’t really seem to make sense to me.

Edit: I know how typedefs work when it simply involves providing another alias for a type.

Edit 2: I thought that this might be doing something with a pointer to a function because it kind of looks like a pointer and has float in there like its a function parameter type, and it seems like this is the case, but I am still not sure what this kind of code precisely does.

Answer 1

typedef glm::vec3(*OffsetFunc)(float);

defines OffsetFunc to be an alias for the type glm::vec3(*)(float), i.e. pointer to function taking float and returning glm::vec3.

CalcOffset(fElapsedTime)

calls the function pointed to by CalcOffset on fElapsedTime.