Not sure when to use exceptions in C++

Question

After many years of coding scientific software in C++, I still can't seem to get used to exceptions and I've no idea when I should use them. I know that using them for controlling program flow is a big no-no, but otherwise than that... consider the following example (excerpt from a class that represents an image mask and lets the user add areas to it as polygons):

class ImageMask
{
public:
    ImageMask() {}
    ImageMask(const Size2DI &imgSize);

    void addPolygon(const PolygonI &polygon);

protected:
    Size2DI imgSize_;
    std::vector<PolygonI> polygons_;
};

The default constructor for this class creates a useless instance, with an undefined image size. I don't want the user to be able to add polygons to such an object. But I'm not sure how to handle that situation. When the size is undefined, and addPolygon() is called, should I:

1. Silently return,
2. assert(imgSize_.valid) to detect violations in code using this class and fix them before a release,
3. throw an exception?

Most of the time I go either with 1) or 2) (depending on my mood), because it seems to me exceptions are costly, messy and simply overkill for such a simple scenario. Some insight please?

Answer 1

The general rule is that you throw an exception when you cannot perform the desired operation. So in your case, yes, it does make sense to throw an exception when addPolygon is called and the size is undefined or inconsistent.

Silently returning is almost always the wrong thing to do. assert is not a good error-handling technique (it is more of a design/documentation technique).

However, in your case a redesign of the interface to make an error condition impossible or unlikely may be better. For example, something like this:

class ImageMask
{
public:
    // Constructor requires collection of polygons and size.
    // Neither can be changed after construction.
    ImageMask(std::vector<PolygonI>& polygons, size_t size);
}

or like this

class ImageMask
{
public:
    class Builder
    {
    public:
        Builder();
        void addPolygon();
    };

    ImageMask(const Builder& builder);
}

// used like this
ImageMask::Builder builder;
builder.addPolygon(polyA);
builder.addPolygon(polyB);
ImageMask mask(builder);