Best alternative to std::optional to return an optional value from a method? (using C++98/C++11/C++14)

Question

Obviously, std::optional is the best choice to return an optional value from a function if one uses C++17 or boost (see also GOTW #90)

std::optional<double> possiblyFailingCalculation()

But what and why would be the best alternative if one is stuck with an older version (and can't use boost)?

I see a few options:

1. STL smart pointers (C++11 only)

   std::unique_ptr<double> possiblyFailingCalculation();
   

   • (+) virtually the same usage as optional
   • (−) confusing to have smart pointers to non-polymorphic types or built-in types

2. Pairing it up with a bool

   std::pair<double,bool> possiblyFailingCalculation();
   

3. Old style

   bool possiblyFailingCalculation(double& output);
   

   • (−) incompatible with new C++11 auto value = calculation() style

4. A DIY template: a basic template with the same functionality is easy enough to code, but are there any pitfalls to implement a robust std::optional<T> look-a-like template ?

5. Throw an exception

   • (−) Sometimes "impossible to calculate" is a valid return value.

Answer 1

std::optional, like its boost::optional parent, is a pretty basic class template. It's a bool, some storage, and a bunch of convenience member functions most of which are one line of code and an assert.

The DIY option is definitely preferred. (1) involves allocation and (2), (3) involve having to construct a T even if you want a null value - which doesn't matter at all for double but does matter for more expensive types. With (5), exceptions are not a replacement for optional.

You can always compare your implementation to Boost's. It's a small header-only library, after all.