Struct initialisation with function pointer

Question

I found the following C++ code with syntax I have never before seen. Does anybody care to elaborate how this workss?

Function 1

glm::vec3 BottomCircleOffset(float fElapsedTime)
{
    return glm::vec3(.0f,.0f,.0f);
}

Function 2

glm::vec3 OvalOffset(float fElapsedTime)
{
    return glm::vec3(.1f, .1f, .1f);
}

Instance Struct

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;
    }
};

Until now this is all fine. I understand OffsetFunc is a typedef for a function pointer taking a float as argument and returning a glm::vec3. I also understand CalcOffset is a variable to such a function.

The code then goes to create an array of Instance types like so:

Instance g_instanceList[] =
{
    {StationaryOffset},
    {OvalOffset},
    {BottomCircleOffset},
};

This is syntax I have never come across before:

• How are we initialising an Instance type by simply putting the name of a function?
• The Instance struct not even having a constructor which takes a function pointer, how does it know to initialise CalcOffset to this value?

Answer 1

Instance is an aggregate - it contains public data members, but no constructors, destructors or other complications, so can (in many ways) be treated like a struct in C.

In particular, it can be aggregate-initialised, by specifying a list of values for the members enclosed in braces. A simple example:

struct s {int a,b,c;};
s example = {1,2,3};    // initialises a=1, b=2, c=3

Arrays can also be aggregate-initialised:

int a[3] = {1,2,3};

and, if the array members are aggregates, then each of them can be aggregate-initialised in turn:

s a[3] = {{1,2,3},{4,5,6},{7,8,9}};

Your example is the same as this: each Instance in the array is aggregate-initialised, using the provided function pointer to initialise the CalcOffset member.

Answer 2

This is a regular C-style initialization of a struct. Recall that you can initialize an array of structs like this:

struct x {
    int a;
};
x data[] = {{1}, {2}, {3}};

Your code uses the same syntax, but with a function pointer in place of an int.