How to initialize nested structures in C++?

Question

Hi, I have created a couple of different structures in a program. I now have a structure with nested structures however I cannot work out how to initalize them correctly.The structures are listed below. Btw, thanks in advance for reading my post or posting an answer :-) :

      /***POINT STRUCTURE***/
struct Point{
float x;        //x coord of point
float y;        //y coord of point
};


      /***Bounding Box STRUCTURE***/
struct BoundingBox{
Point ymax, ymin, xmax, xmin;
};


      /***PLAYER STRUCTURE***/
struct Player{
vector<float> x;      //players xcoords
vector<float> y;      //players ycoords
BoundingBox box;
float red,green,blue;    //red, green, blue colour values
float r_leg, l_leg;   //velocity of players right and left legs
int poly[3];       //number of points per polygon (3 polygons)
bool up,down;      
};

I then attempt to intialse a newly created Player struct called player.

//Creates player, usings vectors copy and iterator constructors
Player player = { 
vector<float>(xcords,xcords + (sizeof(xcords) / sizeof(float)) ), //xcords of player
vector<float>(ycords,ycords + (sizeof(ycords) / sizeof(float)) ), //ycoords of playe
box.ymax = 5;      //create bounding box
box.ymin = 1;
box.xmax = 5;
box.xmin = 1;
1,1,1,       //red, green, blue
0.0f,0.0f,      //r_leg,l_leg
{4,4,4},        //number points per polygon
true,false};       //up, down

This causes several different errors, concerning box. Stating the box has no clear identifier and missing struct or syntax before '.'.

I then tried just to create a Player struct and initalise it's members as follows:

Player bob;
bob.r_leg = 1;

But this causes more errors, as the compiler thinks bob has no identifier or is missing some syntax.

I googled the problem but I did not find any articles showing me how to initalise many different members of nested structures within the (parent) structure. Any help on this subject would be greatly appreciated :-) !!!

Answer 1

You can add default values to a structure like so:

struct Point{
   Point() : x(0), y(0)
     { };

   float x;
   float y;
};

or

struct Point{
   Point() 
     { 
       x = 0;
       y = 0;
     };

   float x;
   float y;
};

For adding those values during construction, add parameters to constructors like this:

struct Point{
   Point(float x, float y) 
     { 
       this->x = x;
       this->y = y;
     };

   float x;
   float y;
};

and instantiate them with these:

Point Pos(10, 10);
Point *Pos = new Point(10, 10);

This also works for your other data structures.

Answer 2

You initialize it normally with { ... }:

Player player = { 
  vector<float>(xcords,xcords + (sizeof(xcords) / sizeof(float)) ),
  vector<float>(ycords,ycords + (sizeof(ycords) / sizeof(float)) ),
  5, 1, 5, 1, 5, 1, 5, 1, 
  1.0f,1.0f,1.0f,                         //red, green, blue
  0.0f,0.0f,                              //r_leg,l_leg
  {4,4,4},                                //number points per polygon
  true,false
};

Now, that is using "brace elision". Some compilers warn for that, even though it is completely standard, because it could confuse readers. Better you add braces, so it becomes clear what is initialized where:

Player player = { 
  vector<float>(xcords,xcords + (sizeof(xcords) / sizeof(float)) ),
  vector<float>(ycords,ycords + (sizeof(ycords) / sizeof(float)) ),
  { { 5, 1 }, { 5, 1 }, { 5, 1 }, { 5, 1 } }, 
  1.0f, 1.0f, 1.0f,               //red, green, blue
  0.0f, 0.0f,                     //r_leg,l_leg
  { 4, 4, 4 },                    //number points per polygon
  true, false
};

If you only want to initialize the x member of the points, you can do so by omitting the other initializer. Remaining elements in aggregates (arrays, structs) will be value initialized to the "right" values - so, a NULL for pointers, a false for bool, zero for ints and so on, and using the constructor for user defined types, roughly. The row initializing the points looks like this then

{ { 5 }, { 5 }, { 5 }, { 5 } },

Now you could see the danger of using brace elision. If you add some member to your struct, all the initializers are "shifted apart" their actual elements they should initialize, and they could hit other elements accidentally. So you better always use braces where appropriate.

Consider using constructors though. I've just completed your code showing how you would do it using brace enclosed initializers.

Answer 3

It looks like the bounding box should just contain floats, not Points?

That said this might work:

Player player = { 
vector<float>(xcords,xcords + (sizeof(xcords) / sizeof(float)) ), //xcords of player
vector<float>(ycords,ycords + (sizeof(ycords) / sizeof(float)) ), //ycoords of playe
{{1.0,5.0},{1.0,5.0},{1.0,5.0},{1.0,5.0}},
1,1,1,                                          //red, green, blue
0.0f,0.0f,                              //r_leg,l_leg
{4,4,4},                                //number points per polygon
true,false};

(Untested)...