Nova for C and C++ programmers

Nova compiles to C, calls C directly, and is designed to live inside C codebases, kernels included. What it adds is a compiler that tracks who owns each piece of memory and who is looking at it, and rejects the programs that would use it wrongly. What it removes is undefined behaviour.

At a glance

C / C++	Nova	Notes
int32_t x = 1;	let x: i32 = 1; (or var to change it)
T *, const T *	borrow mut T, borrow T in safe code; *mut T, *T in unsafe	references are never null and never dangle
malloc / free, new / delete	Box.create(v); freed by its owner	allocation failure is a Result, not NULL or an exception
std::unique_ptr<T>	Box<T>
std::vector<T>	Vector<T>	growing it while an element is referenced is a compile error
std::string, std::string_view	String, StringView	UTF-8 only
std::optional<T>	T?
destructors, RAII	drop { … } blocks; destruction at scope end	moves leave nothing behind to destroy twice
std::move(x)	assignment moves by default; move x to end a use early	using a moved value is a compile error
exceptions, errno, return codes	Result<T, E> and ?
assert, abort	panics: bounds, overflow, unwrap	always on
templates	generics with interface bounds	checked once, before use; no SFINAE
#include, the preprocessor	packages and import	no macros, no conditional compilation
struct layout	the same as C's, in declaration order	@repr(C) to cross the boundary
volatile	Volatile<T>, p.readVolatile()
-fsanitize=address,undefined	needed only for unsafe code; novac run --interp checks it

The same program

#include <cstdio>
#include <vector>

int main() {
    std::vector<int> v = {1, 2, 3};
    int &first = v[0];
    v.push_back(4);          // may reallocate: `first` now dangles
    std::printf("%d\n", first);
}

In C++ this compiles and has undefined behaviour. In Nova it does not compile:

fn main() {
    var v = Vector<i32>.create();
    v.push(1)!;
    let first = borrow v[0];
    v.push(4)!;
    Console.writeLine(first);
}

main.nova:5:5: error[N0241]: cannot borrow 'v' as mutable while 'v[0]' is borrowed
  |
5 |     v.push(4)!;
  |     ^
  = note: 'v[0]' is borrowed here (4:5)
  = note: the borrow is used later here (6:5)

The fixes are the ones you would reach for anyway: copy the value (let first = v[0];), or take the reference after the last push.

No undefined behaviour

Every operation that is undefined in C is defined in Nova, by a check or by a rule:

Undefined in C	In Nova
signed overflow	stops the program (wrappingAdd and friends where wrapping is meant)
out-of-bounds access	stops the program
use after free, double free	compile errors
reading uninitialised memory	a compile error
null dereference	there are no null references; optionals must be checked
shift by the width or more	stops the program
unsequenced modifications	evaluation order is left to right
strict aliasing violations	the generated C never relies on them

The generated C is written so that it has no undefined behaviour itself, and is warning-free under strict flags, so the C compiler's optimiser has nothing to exploit.

Working with an existing C codebase

• Call C functions from an extern "C" block, inside unsafe.
• Export Nova functions with @export; novac writes nova_exports.h for your C code to include.
• Use your codebase's own structs from its headers (@header, @ctype) instead of re-declaring them: the C compiler checks every field Nova uses.
• Build a freestanding object with --profile freestanding --emit obj and link it with your build.

See Unsafe code and C and Kernel code.

Things you will miss

The preprocessor, inline assembly, unions (enums with payloads replace tagged unions), bit fields, pointer arithmetic in safe code, varargs, goto, and in C++, classes with inheritance and virtual functions, operator overloading, overloads, exceptions, and constexpr evaluation.