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Two phase construction

The first thing to do is to break your object’s construction into two phases:

  1. 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 constexpr constructors, and thus can be initialised during phase 1. If you need to initialise a member variable without a constexpr constructor, std::optional<T> is the usual workaround.

  2. 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 constexpr 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 make<T>{Args...}() 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 T(Args...) directly, 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.

Last revised: February 08, 2019 at 22:18:08 UTC

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