Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

This is the documentation for a snapshot of the develop branch, built from commit 0f79ae966a.
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Optional data members

Suppose we want to implement a lazy load optimization. This is because we do not want to perform an expensive initialization of our Resource until (if at all) it is really used. We can do it this way:

class Widget
{
  mutable boost::optional<const Resource> resource_;

public:
  Widget() {}

  const Resource& getResource() const // not thread-safe
  {
    if (resource_ == boost::none)
        resource_.emplace("resource", "arguments");

    return *resource_;
  }
};

optional's default constructor creates an uninitialized optional. No call to Resource's default constructor is attempted. Resource doesn't have to be DefaultConstructible. In function getResource we first check if resource_ is initialized. This time we do not use the contextual conversion to bool, but a comparison with boost::none. These two ways are equivalent. Function emplace initializes the optional in-place by perfect-forwarding the arguments to the constructor of Resource. No copy- or move-construction is involved here. Resource doesn't even have to be MoveConstructible.

[Note] Note

Function emplace is only available on compilers that support rvalue references and variadic templates. If your compiler does not support these features and you still need to avoid any move-constructions, use In-Place Factories.


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