There are two kinds of implementations defined by the C++ standard:hosted andfreestanding implementations. Forhosted implementations, the set of standard library headers required by the C++ standard is much larger than forfreestanding ones. In afreestanding implementation, execution may happen without an operating system.

The kind of the implementation is implementation-defined. The predefined macro__STDC_HOSTED__ is expanded to1 for hosted implementations and​0​ for freestanding implementations.(since C++11)

[edit]Requirements on themain function

[edit]Requirements onstandard library headers

Afreestanding implementation has an implementation-defined set of headers. This set includes at least the headers in the following table.

For partially freestanding headers, freestanding implementations only needs to provide part of the entities in the corresponding synopsis:

• If an entity is commented// freestanding, it is guaranteed to be provided.

• If an entity is declared in a header whose synopsis begins withall freestanding or// mostly freestanding, it is guaranteed to be provided if the entity itself is not commented.

[edit]Headers required for a freestanding implementation

1. ↑Support for always lock-free integral atomic types and presence of type aliasesstd::atomic_signed_lock_free andstd::atomic_unsigned_lock_free are implementation-defined in a freestanding implementation.(since C++20)

[edit]Notes

Some compiler vendors may not fully support freestanding implementation. For example, GCC libstdc++ has had implementation and build issues before version 13, while LLVM libcxx and MSVC STL do not support freestanding.

In C++23, many features are made freestanding with partial headers. However, it is still up for discussion in WG21 whether some headers will be made freestanding in the future standards. Regardless, containers likevector,list,deque, andmap will never be freestanding due to their dependencies on exceptions and heap.

GCC 13 provides more headers, such as<optional>,<span>,<array>, and<bitset>, for freestanding, although these headers may not be portable or provide the same capabilities as a hosted implementation. It is better to avoid using them in a freestanding environment, even if the toolchain provides them.

[edit]References

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit]See also

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