Freestanding and hosted implementations
There are two kinds of implementations defined by the C++ standard: hosted and freestanding implementations. For hosted implementations, the set of standard library headers required by the C++ standard is much larger than for freestanding ones. In a freestanding implementation, execution may happen without an operating system.
The kind of the implementation is implementation-defined. The predefined macro __STDC_HOSTED__ is expanded to 1 for hosted implementations and 0 for freestanding implementations.(since C++11)
Requirements on multi-threaded executions and data races
|
(since C++11) |
[edit] Requirements on the main function
| freestanding | hosted |
|---|---|
| In a freestanding implementation, it is implementation-defined whether a program is required to define a main function. Start-up and termination is implementation-defined; start-up contains the execution of constructors for objects of namespace scope with static storage duration; termination contains the execution of destructors for objects with static storage duration. | In a hosted implementation, a program must contain a global function called main. Executing a program starts a main thread of execution in which the main function is invoked, and in which variables of static storage duration might be initialized and destroyed.
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[edit] Requirements on standard library headers
A freestanding 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.
|
(since C++26) |
- If an entity is declared in a header whose synopsis begins with all freestanding or // mostly freestanding, it is guaranteed to be provided if the entity itself is not commented.
[edit] Headers required for a freestanding implementation
| Library | Component | Headers | Freestanding |
|---|---|---|---|
| Language support | Common definitions | <cstddef> | All |
| C standard library | <cstdlib> | Partial | |
| Implementation properties | <cfloat> <climits> (since C++11) <limits> <version> (since C++20) |
All | |
| Integer types | <cstdint> (since C++11) | All | |
| Dynamic memory management | <new> | All | |
| Type identification | <typeinfo> | All | |
| Source location | <source_location> (since C++20) | All | |
| Exception handling | <exception> | All | |
| Initializer lists | <initializer_list> (since C++11) | All | |
| Comparisons | <compare> (since C++20) | All | |
| Coroutines support | <coroutine> (since C++20) | All | |
| Other runtime support | <cstdarg> | All | |
| Debugging support | <debugging> (since C++26) | All | |
| Concepts | <concepts> (since C++20) | All | |
| Diagnostics | Error numbers | <cerrno> (since C++26) | Partial |
| System error support | <system_error> (since C++26) | Partial | |
| Memory management | Memory | <memory> (since C++23) | Partial |
| Metaprogramming | Type traits | <type_traits> (since C++11) | All |
| Compile-time rational arithmetic | <ratio> (since C++23) | All | |
| General utilities | Utility components | <utility> (since C++23) | All |
| Tuples | <tuple> (since C++23) | All | |
| Function objects | <functional> (since C++20) | Partial | |
| Primitive numeric conversions | <charconv> (since C++26) | Partial | |
| Bit manipulation | <bit> (since C++20) | All | |
| Strings | String classes | <string> (since C++26) | Partial |
| Null-terminated sequence utilities |
<cstring> (since C++26) | Partial | |
| Text processing | Null-terminated sequence utilities |
<cwchar> (since C++26) | Partial |
| Iterators | <iterator> (since C++23) | Partial | |
| Ranges | <ranges> (since C++23) | Partial | |
| Numerics | Mathematical functions for floating-point types |
<cmath> (since C++26) | Partial |
| Random number generation | <random> (since C++26) | Partial | |
| Concurrency support | Atomics | <atomic> (since C++11) | All[1] |
| Execution control | <execution> (since C++26) | Partial | |
| Deprecated headers | <ciso646> (until C++20) <cstdalign> (since C++11)(until C++20) <cstdbool> (since C++11)(until C++20) |
All | |
- ↑ Support for always lock-free integral atomic types and presence of type aliases std::atomic_signed_lock_free and std::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 like vector, list, deque, and map will never be freestanding due to their dependencies on exceptions and heap.
GCC 13 provides more headers, such as <optional>, <span>, <array>, and