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.

Requirements on multi-threaded executions and data races 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.

(since C++11)

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.

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:

Headers required for a freestanding implementation

Types	<cstddef>
Implementation properties	<limits> <cfloat> <climits> (since C++11) <version> (since C++20)
Integer types	<cstdint> (since C++11)
Start and termination	<cstdlib> (partial)[1]
Dynamic memory management	<new>
Type identification	<typeinfo>
Exception handling	<exception>
Initializer lists	<initializer_list> (since C++11)
Other runtime support	<cstdarg>
Fundamental library concepts	<concepts> (since C++20)
Type traits	<type_traits> (since C++11)
Atomics	<atomic> (since C++11)
Bit manipulation	<bit> (since C++20)
	<ciso646> (since C++11)(until C++20)
Deprecated headers	<cstdalign> <cstdbool> (since C++11)(until C++20)

1. The supplied version of the header <cstdlib> shall declare at least the functions std::abort, std::atexit, std::exit, std::at_quick_exit and std::quick_exit (since C++11).

References

• C++17 standard (ISO/IEC 14882:2017):
• 4.1 Implementation compliance (p: 5)
• 4.7 Multi-threaded executions and data races (p: 15)
• 6.6.1 Main function (p: 66)
• 20.5.1.3 Freestanding implementations (p: 458)
• C++14 standard (ISO/IEC 14882:2014):
• 1.4 Implementation compliance (p: 5)
• 1.10 Multi-threaded executions and data races (p: 11)
• 3.6.1 Main function (p: 62)
• 17.6.1.3 Freestanding implementations (p: 441)
• C++11 standard (ISO/IEC 14882:2011):
• 1.4 Implementation compliance (p: 5)
• 1.10 Multi-threaded executions and data races (p: 11)
• 3.6.1 Main function (p: 58)
• 17.6.1.3 Freestanding implementations (p: 408)
• C++03 standard (ISO/IEC 14882:2003):
• 1.4 Implementation compliance (p: 3)
• 3.6.1 Main function (p: 43)
• 17.4.1.3 Freestanding implementations (p: 326)

See also