# C++ standard libraries extensions, version 3

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< cpp‎ | experimental

Version 3 of the C++ Extensions for Library Fundamentals, defines the following new components for the C++ standard library, in addition to those defined in version 1 and version 2.

Components merged into C++17/20 and the following components are removed in LFTS v3:

- std::experimental::make_array,
- std::experimental::erased_type,
- `std::experimental::packaged_task`, and
- `std::experimental::promise`.

### Scope guard

Defined in header `<experimental/scope>`
scope_exit | wraps a function object and invokes it on exiting the scope (class template)
scope_fail | wraps a function object and invokes it on exiting the scope through an exception (class template)
scope_success | wraps a function object and invokes it on exiting the scope normally (class template)
unique_resource | universal RAII resource handle wrapper (class template)

### Feature test macros

Defined in header `<experimental/memory_resource>`
__cpp_lib_experimental_memory_resources | a value of at least 201803 indicates that `resource_adaptor` is supported (macro constant)
Defined in header `<experimental/functional>`
__cpp_lib_experimental_function_polymorphic_allocator | a value of at least 202211 indicates that std::pmr::polymorphic_allocator<> is used for `std::experimental::function` (macro constant)
Defined in header `<experimental/scope>`
__cpp_lib_experimental_scope | a value of at least 201902 indicates that the scope guards are supported (macro constant)

### Notes

The value of `__cpp_lib_experimental_memory_resources` has been increased in LFTS v3, because the design of `resource_adaptor` is rebased onto std::pmr::memory_resource.

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