Two phase construction
The first thing to do is to break your object’s construction into two phases:
Place the object into a state where it can be legally destructed without doing any initialisation which could throw an exception (i.e. everything done in phase 1 is
constexpr). This phase usually involves initialising member variables to various default values, most often using default member initialisers. Most standard C++ library objects and containers have
constexprconstructors, and thus can be initialised during phase 1. If you need to initialise a member variable without a
std::optional<T>is the usual workaround.
Do the remainder of the construction, the parts which could fail. Because phase 1 placed the object into a legally destructible state, it is usually the case that one can bail out during phase 2 and the destructor will clean things up correctly.
The phase 1 construction will be placed into a private
constructor so nobody external can call it. The phase 2 construction will be placed into a static
member initialisation function which returns a
result with either
the successfully constructed object, or the cause of any failure to
construct the object.
Finally, as a phase 3,
some simple metaprogramming will implement a
free function which will construct any object
T by calling its
static initialisation function with
Args... and returning the
result returned. This isn’t as nice as calling
but it’s not too bad in practice. And more importantly, it lets you
write generic code which can construct any unknown object which
fails via returning
result, completely avoiding C++ exception throws.