I need to call a void function in the middle of a constructors member initializer list so that other objects can be initialized correctly

Question

Here is my constructor.

SceneContainer::SceneContainer()
   :
    m_BVH(Allshapes, 0)
    {
        ParseJSONDataIntoShapeData("ShapeList.json");
    }

Here is the scene class declaration.

class SceneContainer{
public:
void ParseJSONDataIntoShapeData(std::string filename);

private:
BVHNode m_BVH;
std::vector<shape*> Allshapes;
};

So given a JSON file like this.

  {
      "scene": {
        "shape": [{
          "center": "1.0 1.0 1.0",
          "radius": 1,
          "_name": "sphere1",
          "_type": "sphere"
        },
{
          "center": "2.0 2.0 2.0",
          "radius": 1,
          "_name": "sphere2",
          "_type": "sphere"
        },
{
          "center": "3.0 3.0 3.0",
          "radius": 1,
          "_name": "sphere3",
          "_type": "sphere"
        },
{
          "center": "3.0 3.0 3.0",
          "radius": 1,
          "_name": "sphere4",
          "_type": "sphere"
        }]
      }

parseJSONDataIntoShapeData would then iterate over all shapes in the file and push_back a pointer to the shape that was created in the file. Some Pseudo code for this would look like.

for(all shapes in json file)
    Create shape pointer from shape data
    push_back shape pointer to AllShapes.

after parseJSONdata is called there would be four shapes in the Allshapes vector. However, due to how the constructor works with my original implementation, m_BVH gets initialized with an empty vector because ParseJSONData gets called after m_BVH is initialized whereas I want it to get initialized with the shape data in it.

We don't see what `m_BVH` is. We don't see what `parseJSONData` does. "_otherwise it initializes `m_BVH` with no data_" - why is that? You need to show more of your code. Preferably a [mre]. — Ted Lyngmo, Apr 24 '21 at 22:43
parseJSONData, before creating `SceneContainer` and pass reference on parsed data into constructor — user3124812, Apr 24 '21 at 22:51
I edited the post. Hopefully this is enough context to give a clearer understanding of the problem. — Henry Keskitalo, Apr 24 '21 at 23:22
@HenryKeskitalo Unfortunately no, not for me. A [mre] would be much more helpful. — Ted Lyngmo, Apr 24 '21 at 23:36
@TedLyngmo Is it not enough for me to say that parseJSONdata fills the vector m_allShapes with shape pointers and that m_BVH needs shape pointers in it at initialization? So if parseJSONdata is not called before m_BVH is initialized there will be no shapes in the vector m_allShapes? — Henry Keskitalo, Apr 25 '21 at 00:14
@HenryKeskitalo Perhaps it's enough for someone else, but I'd like to see what you're trying to do in a [mre]. — Ted Lyngmo, Apr 25 '21 at 00:29
`SceneContainer::SceneContainer(...) : m_BVH((parseJSONData("Sphere.json"), shapeList), 0) {}`. This ensures that `parseJSONData` runs before the constructor of `m_BVH` — Igor Tandetnik, Apr 25 '21 at 00:37

score 0 · Answer 1 · answered Apr 25 '21 at 00:58

So, you have some function which assigns to a member of the class. But you also have another member of the class which is going to consume the contents of the first member.

What you ought to have is a function that returns the data which is used to initialize the first member, and then you can use it as part of the constructor for the second:

class SceneContainer{
public:
  //Doesn't set data into the object.
  static std::vector<shape*> ParseJSONDataIntoShapeData(std::string filename);

  //Setting the data into the object is a separate step.
  void ResetJSONDataIntoShapeData(std::string filename)
  {
    //Free Allshapes;
    Allshapes = ParseJSONDataIntoShapeData(filename);
    m_BVH.reassign(Allshapes, 0); //Tells the m_BVH object that its data has changed.
  }

private:
  std::vector<shape*> Allshapes;
  BVHNode m_BVH;
};

SceneContainer::SceneContainer()
  : Allshapes(ParseJSONDataIntoShapeData("ShapeList.json"))
  , m_BVH(Allshapes, 0)
  {}

Note that the order of the declaration of Allshapes and m_BVH has changed. This is important, because it is the order that these objects are declared in which will determine the order of their initialization.

I need to call a void function in the middle of a constructors member initializer list so that other objects can be initialized correctly

1 Answers1