C++ ::  assigning To A Member From Struct Obtained By Overloaded Operator

I'm trying to assign a value to a member of a struct that I called via an overloaded [] operator. I have the following code for the struct:

typedef struct {
float r, g, b, a;
float operator [](int pos) {
switch (pos) {

[Code] ....

And what I wish to do is

MyStruct a;
a[0] = 0.5;

Is it possible with a struct? How to express this to search engines so I haven't been able to find anything about it. If this is not possible with a struct, is there a way to define something that can do all the following things:

SomeStruct test = {0.5, 0.5, 0.5, 1};
test.g = 1.0;
test[0] = 0.0; // test[0] would be equivalent to calling test.r
float somevalue = test[3]; // test[3] would be equivalent to calling test.a

I hope I've been sufficiently clear.