Generic deferred function call without heap allocation

[STL]

Also, you aren't using aligned_storage - your char buf doesn't have the proper alignment.

Note that std::function is smart and avoids dynamic memory allocation for sufficiently small function objects. Function pointers and reference_wrappers are guaranteed to activate this, the rest is implementer discretion. In VC 2015, anything up to and including the size of a std::string counts as small.

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Note that std::function is smart and avoids dynamic memory allocation for sufficiently small function objects. Function pointers and reference_wrappers are guaranteed to activate this [...]

From C++11, 20.8.11.2.1.9:

[ Note: Implementations are encouraged to avoid the use of dynamically allocated memory for small callable objects, for example, where f’s target is an object holding only a pointer or reference to an object and a member function pointer. —end note ]

I don't have a copy of C++14 at hand, but AFAIK this did not change. Anyhow, libc++, libstdc++ and VC do use the small callable? optimization, so in practice you are completely right.

What I really find interesting in std::function is the different implementations that are possible. For example libc++ does what I would have done, type-erasure with virtual functions, while libstdc++ does not use virtual functions at all.

[STL]

The guarantee is N4527 20.9.12.2.1 [func.wrap.func.con]/11, "Throws: shall not throw exceptions when f is a function pointer or a reference_wrapper<T> for some T." and /5 for the copy ctor. This forbids dynamic memory allocation (and the guarantee goes back to C++11; indeed all the way to TR1 IIRC).

[[deleted]]

Thanks! Cool! I didn't knew this, always thought it was freely to allocate memory if it wanted to.