<cuda/std/functional>
Omissions
The following facilities in section functional.syn of ISO/IEC IS 14882 (the C++ Standard) are not available in the NVIDIA C++ Standard Library today:
std::function- Polymorphic function object wrapper.std::bind- Generic function object binder / lambda facility.std::reference_wrapper- Reference wrapper type.std::hash- Hash function object.
std::function
std::function is a polymorphic function object wrapper. Implementing it requires both polymorphism (either hand built dispatch tables or the use of C++ virtual functions) and memory allocation. This means that it is non-trivial to implement a heterogeneous version of this facility today. As such, we have deferred it.
std::bind
std::bind is a general-purpose function object binder / lambda facility. It relies on constexpr global variables for placeholders, which presents heterogeneous implementation challenges today due to how global variables work in NVCC. E.g. We cannot easily ensure the placeholders are the same object with the same address in host and device code. Therefore, we’ve decided to hold off on providing this feature for now.
std::hash
std::hash is a function object which hashes entities. While this is an important feature, it is also important that we pick a hash implementation that makes sense for GPUs. That implementation might be different from the default that the upstream libc++ uses. Further research and investigation is required before we can provide this feature.
std::reference_wrapper
std::reference_wrapper is a CopyConstructible and CopyAssignable wrapper around a reference to an object or function of type T. There is nothing that makes this facility difficult to implement heterogeneously today. It is a value type that does not allocate memory, hold pointers, have virtual functions, or make calls to platform specific APIs.
No design or functional changes were required to port the upstream libc++ implementations of this facility. We just had to add execution space specifiers to port it.
However, this feature failed tests involving function pointers with some of the compilers we support. So, we’ve omitted this feature for now.