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 an old version of Boost. Click here to view this page for the latest version.
PrevUpHomeNext

Optional references

This library allows the template parameter T to be of reference type: T&, and to some extent, T const&.

However, since references are not real objects some restrictions apply and some operations are not available in this case:

Also, even though optional<T&> treats it wrapped pseudo-object much as a real value, a true real reference is stored so aliasing will occur:

[Warning] Warning

On compilers that do not conform to Standard C++ rules of reference binding, operations on optional references might give adverse results: rather than binding a reference to a designated object they may create an unexpected temporary and bind to it. For more details see Dependencies and Portability section.

Rvalue references

Rvalue references and lvalue references to const have the ability in C++ to extend the life time of a temporary they bind to. Optional references do not have this capability, therefore to avoid surprising effects it is not possible to initialize an optional references from a temporary. Optional rvalue references are disabled altogether. Also, the initialization and assignment of an optional reference to const from rvalue reference is disabled.

const int& i = 1;            // legal
optional<const int&> oi = 1; // illegal

PrevUpHomeNext