cpp/freestanding

There are two kinds of implementations defined by the C++ standard: hosted and freestanding implementations. For hosted implementations the set of available libraries required by the C++ standard is much larger than for freestanding ones. In a freestanding implementation execution may happen without an operating system.

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

Headers required for a freestanding implementation

	<ciso646>
Types	<cstddef>
Implementation properties	<cfloat> <limits> <climits>
Integer types	<cstdint>
Start and termination	<cstdlib>
Dynamic memory management	<new>
Type identification	<typeinfo>
Exception handling	<exception>
Initializer lists	<initializer_list>
Other runtime support	<cstdarg>
Type traits	<type_traits>
Atomics	<atomic>
Deprecated headers	<cstdalign> <cstdbool>

The following are the differences between freestanding and hosted implementations:

freestanding	hosted
Under a freestanding implementation, it is implementation-defined whether a program can have more than one thread of execution.	Under a hosted implementation, a C++ program can have more than one thread running concurrently.
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.

References

• Working Draft, Standard for Programming Language C++ (2017)