Boost C++ Libraries

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.