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

Emplace operations in older compilers

Certain constructors and functions in the interface of optional perform a 'perfect forwarding' of arguments:

template<class... Args> optional(in_place_init_t, Args&&... args);
template<class... Args> optional(in_place_init_if_t, bool condition, Args&&... args);
template<class... Args> void emplace(Args&&... args);

On compilers that do not support variadic templates, each of these functions is substituted with two overloads, one forwarding a single argument, the other forwarding zero arguments. This forms the following set:

template<class Arg> optional(in_place_init_t, Arg&& arg);
optional(in_place_init_t);

template<class Arg> optional(in_place_init_if_t, bool condition, Arg&& arg);
optional(in_place_init_if_t, bool condition);

template<class Arg> void emplace(Arg&& arg);
void emplace();

On compilers that do not support rvalue references, each of these functions is substituted with three overloadss: taking const and non-const lvalue reference, and third forwarding zero arguments. This forms the following set:

template<class Arg> optional(in_place_init_t, const Arg& arg);
template<class Arg> optional(in_place_init_t, Arg& arg);
optional(in_place_init_t);

template<class Arg> optional(in_place_init_if_t, bool condition, const Arg& arg);
template<class Arg> optional(in_place_init_if_t, bool condition, Arg& arg);
optional(in_place_init_if_t, bool condition);

template<class Arg> void emplace(const Arg& arg);
template<class Arg> void emplace(Arg& arg);
void emplace();

This workaround addressess about 40% of all use cases. If this is insufficient, you need to resort to using In-Place Factories.


PrevUpHomeNext