Setting Struct Bounding Box Values - c++

Question

I'm trying to create some bounding boxes around some objects that I have.
I create them like this:
struct boundingBox{
float top;
float bottom;
float left;
float right;
};
I'm having issues setting their values. I have this function which I was hoping would set their values:
void makeBoundingBox(boundingBox bound, float t, float b, float l, float r){
bound.top =t;
bound.bottom =b;
bound.left=l;
bound.right=r;
}
and I am trying to set them like this:
makeBoundingBox(car,b_Size,b_Size,b_Size,b_Size); //where car is a boundingBox object and b_Size is a calculated box size
When I'm using the above line, the values aren't set; I know this because I print to the console to check the values and they always come back 0. However, if within the same function I use makeBoundingBox, I set car.top = 500 manually instead, and print, the car.top is set successfully to 500 and works...
Does the makeBoundingBox method not work correctly by passing a struct object how I am expecting it too?

Answer 1

You're passing the box by value. For the routine to change the actual argument, pass it by reference. I.e.
void makeBoundingBox(boundingBox& bound, float t, float b, float l, float r){
bound.top =t;
bound.bottom =b;
bound.left=l;
bound.right=r;
}
However, instead of that, in C++11 just write
boundingBox{ 1, 1, 10, 10 }
when you need a boundingBox with those values.
In C++03 you can instead define a constructor for the boundingBox class.
Read up on that in your favorite C++ text book.
Here is an example class definition:
struct boundingBox{
float top;
float bottom;
float left;
float right;
boundingBox( float a_top, float a_bottom, float a_left, float a_right )
: top( a_top ), bottom( a_bottom )
, left( a_left ), right( a_right )
{}
};
and you can then create an instance like
boundingBox( 1, 1, 10, 10 )
As a general observation, using float instead of double can create some problems (especially with C++11 curly braces initialization), it has less precision than double, and it has no special advantages except where you need zillions of values, like a few billion.
So, use double.
That's the default floating point type in C++, e.g. it's the type of 3.14.

Answer 2

You're passing the boundingBox by value, not by reference. This means the makeBoundingBox function is operating on a local copy and changes do not propagate back to the original object.
Change your function definition as follows:
void makeBoundingBox(boundingBox& bound, float t, float b, float l, float r)

Answer 3

You're passing the struct to the function by VALUE, you need to pass by REFERENCE so:
void makeBoundingBox(boundingBox &bound, float t, float b, float l, float r){
bound.top =t;
bound.bottom =b;
bound.left=l;
bound.right=r;
}
Notice the & to pass by reference.
In your function call, do the same & in front of the variable name.