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optional.md

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std::optional

So, we have

Foo parseFoo(std::string_view input);

What if the parse fails? And you can't parse out a Foo?

  1. throw an exception
  2. return default Foo. ie Foo() (if Foo is default constructible)
  3. bool parseFoo(std::string_view input, Foo & output); // also basically requires Foo()
  4. Foo * parseFoo(std::string_view input); // allocation!? :-(
C++14 C++17 
// returns default Foo on error
Foo parseFoo(std::string_view in);
// throws parse_error
Foo parseFoo(std::string_view in);
// returns false on error
bool parseFoo(std::string_view in, Foo & output);
// returns null on error
unique_ptr<Foo> parseFoo(std::string_view in);
 
std::optional<Foo> parseFoo(std::string_view in);

Usage

C++17 
optional ofoo = parseFoo(str);
if (ofoo)
   use(*ofoo);
 
optional<int> oi = parseInt(str);
std::cout << oi.value_or(0);

Note, optional is not just for errors, and exceptions are still the go-to choice for error handling.
See also boost::optional, Haskell's Maybe, etc.