Free-standing
- C++ course
- Expert
- Lesson 33/47
- 5 min
Learn free-standing C++ and std lib constraints.
Free-standing
The C++ standard defines two implementation kinds: hosted (an operating system is present, tools like std::cout, std::vector are available) and freestanding (no operating system, only minimal headers).
In a freestanding implementation main is not required; the entry point is hardware-specific (Reset_Handler, for example). Available headers are limited: <cstdint>, <cstddef>, <new>, <type_traits>. These are called freestanding headers; they do not require an operating system.
Freestanding headers
The freestanding headers are: <cstdint> (fixed-width integers), <cstddef> (std::size_t, nullptr_t), <limits>, <type_traits>, <new>. They make no operating-system calls.
By contrast <iostream>, <vector>, <string> are hosted headers; they require dynamic memory and an operating system. Embedded systems avoid them; freestanding types like std::uint32_t are used instead. This lets the program run on any hardware.
Practice Write add with freestanding types
Write std::uint32_t add(std::uint32_t a, std::uint32_t b) with body return a + b;. In main write std::cout << add(3, 4);.
Starter code
#include <iostream>
#include <cstdint>
// Define the add function here
int main() {
// Call add and print it
return 0;
}Sample solution
#include <iostream>
#include <cstdint>
std::uint32_t add(std::uint32_t a, std::uint32_t b) {
return a + b;
}
int main() {
std::cout << add(3, 4);
return 0;
}Expected output
7
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match each header to its type
| Term | Meaning |
|---|---|
freestanding |
No operating system |
hosted |
Operating system present |
<cstdint> |
Freestanding header |
<iostream> |
Hosted header |
2 Complete the uint32_t type
std::_____t counter = 0;std::uint32_t counter = 0;Explanation: std::uint32_t is a 32-bit unsigned integer.
3 Which header is freestanding?
// #include <...>- <cstdint> Correct answer
- <iostream>
- <vector>
- <string>
Explanation: <cstdint> is a freestanding header; it requires no operating system.