Question

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 initialize them correctly. The structures are listed below.

/***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 initialise 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 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.