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As usual, types are divided into basic types and user defined types (enum, union, struct, faults, aliases). All types are defined on a global level.


All user defined types in C3 starts with upper case. So MyStruct or Mystruct would be fine, mystruct_t or mystruct would not. This naming requirement ensures that the language is easy to parse for tools. It is possible to use attributes to change the external name of a type:

struct Stat @extern("stat")
// ...
fn CInt stat(const char* pathname, Stat* buf);

This would for example affect generated C headers.

Differences from C

Unlike C, C3 does not use type qualifiers. const exists, but is a storage class modifier, not a type qualifier. Instead of volatile, volatile loads and stores are used. In order to signal restrictions on parameter usage, parameter preconditions are used. typedef has a slightly different syntax and renamed def.

C3 also requires all function pointers to be used with an alias, so:

def Callback = fn void();
Callback a = null; // Ok!
fn Callback getCallback() { ... } // Ok!
// fn fn void() getCallback() { ... } - ERROR!
// fn void() a = null; - ERROR!```

Basic types

Basic types are divided into floating point types, and integer types. Integer types being either signed or unsigned.

Integer types
Namebit sizesigned

* bool will be stored as a byte.
** size, pointer and pointer sized types depend on platform.

Integer arithmetics

All signed integer arithmetics uses 2’s complement.

Integer constants

Integer constants are 1293832 or -918212. Without a suffix, suffix type is assumed to the signed integer of arithmetic promotion width. Adding the u suffix gives a unsigned integer of the same width. Use ixx and uxx – where xx is the bit width for typed integers, e.g. 1234u16

Integers may be written in decimal, but also

  • in binary with the prefix 0b e.g. 0b0101000111011, 0b011
  • in octal with the prefix 0o e.g. 0o0770, 0o12345670
  • in hexadecimal with the prefix 0x e.g. 0xdeadbeef 0x7f7f7f

In the case of binary, octal and hexadecimal, the type is assumed to be unsigned.

Furthermore, underscore _ may be used to add space between digits to improve readability e.g. 0xFFFF_1234_4511_0000, 123_000_101_100

TwoCC, FourCC and EightCC

FourCC codes are often used to identify binary format types. C3 adds direct support for 4 character codes, but also 2 and 8 characters:

  • 2 character strings, e.g. 'C3', would convert to an ushort or short.
  • 4 character strings, e.g. 'TEST', converts to an uint or int.
  • 8 character strings, e.g. 'FOOBAR11' converts to an ulong or long.

Conversion is always done so that the character string has the correct ordering in memory. This means that the same characters may have different integer values on different architectures due to endianess.

Base64 and hex data literals

Base64 encoded values work like TwoCC/FourCC/EightCC, in that is it laid out in byte order in memory. It uses the format b64'<base64>'. Hex encoded values work as base64 but with the format x'<hex>'. In data literals any whitespace is ignored, so '00 00 11'x encodes to the same value as x'000011'.

In our case we could encode b64'Rk9PQkFSMTE=' as 'FOOBAR11'.

Base64 and hex data literals initializes to arrays of the char type:

char[*] hello_world_base64 = b64"SGVsbG8gV29ybGQh";
char[*] hello_world_hex = x"4865 6c6c 6f20 776f 726c 6421";
String literals, and raw strings

Regular string literals is text enclosed in " ... " just like in C. C3 also offers two other types of literals: multi-line strings and raw strings.

Raw strings uses text between ` `. Inside of a raw string, no escapes are available. To write a ` double the character:

char* foo = `C:\foo\bar.dll`;
char* bar = `"Say ``hello``"`;
// Same as
char* foo = "C:\\foo\\bar.dll";
char* bar = "\"Say `hello`\"";
Floating point types
Namebit size

*support depends on platform

Floating point constants

Floating point constants will at least use 64 bit precision. Just like for integer constants, it is allowed to use underscore, but it may not occur immediately before or after a dot or an exponential.

Floating point values may be written in decimal or hexadecimal. For decimal, the exponential symbol is e (or E, both are acceptable), for hexadecimal p (or P) is used: -2.22e-21 -0x21.93p-10

It is possible to type a floating point by adding a suffix:

f32 or ffloat

C compatibility

For C compatibility the following types are also defined in std::core::cinterop

Namec type
CShortshort int
CUShortunsigned short int
CUIntunsigned int
CLonglong int
CULongunsigned long int
CLongLonglong long
CULongLongunsigned long long
CLongDoublelong double

Note that signed C char and unsigned char will correspond to ichar and char. CChar is only available to match the default signedness of char on the platform.

Other built-in types

Pointer types

Pointers mirror C: Foo* is a pointer to a Foo, while Foo** is a pointer to a pointer of Foo.

The typeid type

The typeid can hold a runtime identifier for a type. Using <typename>.typeid a type may be converted to its unique runtime id, e.g. typeid a = Foo.typeid;. This value is pointer-sized.

The any type

C3 contains a built-in variant type, which is essentially struct containing a typeid plus a void* pointer to a value. It is possible to cast the any pointer to any pointer type, which will return null if the types don’t match, or the pointer value otherwise.

int x;
any y = &x;
double *z = (double*)y; // Returns null
int* w = (int*)y; // Returns the pointer to x

Switching over the any type is another method to unwrap the pointer inside:

fn void test(any z)
// Unwrapping switch
switch (z)
case int:
// z is unwrapped to int* here
case double:
// z is unwrapped to double* here
// Assignment switch
switch (y = z)
case int:
// y is int* here
// Direct unwrapping to a value is also possible:
switch (w = *z)
case int:
// w is int here

any.type returns the underlying pointee typeid of the contained value. any.ptr returns the raw void* pointer.

Array types

Arrays are indicated by [size] after the type, e.g. int[4]. Slices use the type[]. For initialization the wildcard type[*] can be used to infer the size from the initializer. See the chapter on arrays.

Vector types

Vectors use [<size>] after the type, e.g. float[<3>], with the restriction that vectors may only form out of integers, floats and booleans. Similar to arrays, wildcard can be used to infer the size of a vector: int[<*>] a = { 1, 2 }.

Types created using def


Like in C, C3 has a “typedef” construct, def <typename> = <type>

def Int32 = int;
def Vector2 = float[<2>];
Int32 a = 1;
int b = a;

Function pointer types

Function pointers are always used through a def:

def Callback = fn void(int value);
Callback callback = &test;
fn void test(int a) { ... }

To form a function pointer, write a normal function declaration but skipping the function name. fn int foo(double x) -> fn int(double x).

Function pointers can have default arguments, e.g. def Callback = fn void(int value = 0) but default arguments and parameter names are not taken into account when determining function pointer assignability:

def Callback = fn void(int value = 1);
fn void test(int a = 0) { ... }
Callback callback = &main; // Ok
fn void main()
callback(); // Works, same as test(0);
test(); // Works, same as test(1);
callback(.value = 3); // Works, same as test(3)
test(.a = 4); // Works, same as test(4)
// callback(.a = 3); ERROR!

Distinct types

Distinct types is a kind of type alias which creates a new type that has the same properties as the original type but is - as the name suggests - distinct from it. It cannot implicitly convert into the other type using the syntax distict <name> = <type>

distinct MyId = int;
fn void* get_by_id(MyId id) { ... }
fn void test(MyId id)
void* val = get_by_id(id); // Ok
void* val2 = get_by_id(1); // Literals convert implicitly
int a = 1;
// void* val3 = get_by_id(a); // ERROR expected a MyId
void* val4 = get_by_id((MyId)a); // Works
// a = id; // ERROR can't assign 'MyId' to 'int'

Inline distinct

Using inline in the distinct declaration allows a distinct type to implicitly convert to its underlying type:

distinct Abc = int;
distinct Bcd = inline int;
fn void test()
Abc a = 1;
Bcd b = 1;
// int i = a; Error: Abc cannot be implicitly converted to 'int'
int i = b; // This is valid
// However, 'inline' does not allow implicit conversion from
// the inline type to the distinct type:
// a = i; Error: Can't implicitly convert 'int' to 'Abc'
// b = i; Error: Can't implicitly convert 'int' to 'Bcd'

Generic types

import generic_list; // Contains the generic MyList
struct Foo { int x; }
// Using def - usually recommended:
def IntMyList = MyList(<int>);
IntMyList abc;
// Inline type definition
MyList<Foo> bcd = MyList(<Foo>);

Read more about generic types on the page about generics.


Enum (enumerated) types use the following syntax:

enum State : int

Enum constants are namespaces by default, just like C++‘s class enums. So accessing the enums above would for example use State.PENDING rather than PENDING.

Enum type inference

When an enum is used in where the type can be inferred, like in case-clauses or in variable assignment, it is allowed to drop the enum name:

State foo = PENDING; // State.PENDING is inferred.
switch (foo)
case RUNNING: // State.RUNNING is inferred
fn void test(State s) { ... }
test(RUNNING); // State.RUNNING is inferred

In the case that it collides with a global in the same scope, it needs the qualifier:

module test;
fn void testState(State s) { ... }
const State RUNNING = State.TERMINATED; // Don't do this!
test(RUNNING); // Ambiguous
test(test::RUNNING); // Uses global.
test(State.RUNNING); // Uses enum constant.

Enum associated values

It is possible to associate each enum value with a static value.

enum State : int (String state_desc, bool active)
PENDING("pending start", false),
RUNNING("running", true),
TERMINATED("ended", false)
State s = get_state();
io::printfn("Currently the process is %s", s.state_desc);
if ( do_something();


fault defines a set of optional result values, that are similar to enums, but are used for optional returns.

fault IOResult
fault MapResult

Like the typeid, the constants are pointer sized and each value is globally unique, even when compared to other faults. For example the underlying value of MapResult.NOT_FOUND is guaranteed to be different from IOResult.IO_ERROR. This is true even if they are separately compiled.

A fault may be stored as a normal value, but is also unique in that it may be passed as the optional result value using the ! suffix operator.

Optional Result Types

An optional result type is created by taking a type and appending !. An optional result type is a tagged union containing either the expected result or an optional result value (which is a fault).

int! i;
i = 5; // Assigning a real value to i.
i = IOResult.IO_ERROR?; // Assigning an optional result to i.

Only variables and return variables may be optionals. Function and macro parameters may not be optionals.

fn Foo*! getFoo() { ... } // Ok!
fn void processFoo(Foo*! f) { ... } // Error
int! x = 0; // Ok!

Read more about the optional types on the page about optionals and error handling.

Struct types

Structs are always named:

struct Person
char age;
String name;

A struct’s members may be accessed using dot notation, even for pointers to structs.

Person p;
p.age = 21; = "John Doe";
libc::printf("%s is %d years old.", p.age,;
Person* pPtr = &p;
pPtr.age = 20; // Ok!
libc::printf("%s is %d years old.", pPtr.age,;

(One might wonder whether it’s possible to take a Person** and use dot access. – It’s not allowed, only one level of dereference is done.)

To change alignment and packing, optional attributes such as @packed may be used.

Struct subtyping

C3 allows creating struct subtypes using inline:

struct ImportantPerson
inline Person person;
String title;
fn void printPerson(Person p)
libc::printf("%s is %d years old.", p.age,;
ImportantPerson important_person;
important_person.age = 25; = "Jane Doe";
important_person.title = "Rockstar";
printPerson(important_person); // Only the first part of the struct is copied.

Union types

Union types are defined just like structs and are fully compatible with C.

union Integral
byte as_byte;
short as_short;
int as_int;
long as_long;

As usual unions are used to hold one of many possible values:

Integral i;
i.as_byte = 40; // Setting the active member to as_byte
i.as_int = 500; // Changing the active member to as_int
// Undefined behaviour: as_byte is not the active member,
// so this will probably print garbage.
libc::printf("%d\n", i.as_byte);

Note that unions only take up as much space as their largest member, so Integral.sizeof is equivalent to long.sizeof.

Nested sub-structs / unions

Just like in C99 and later, nested anonymous sub-structs / unions are allowed. Note that the placement of struct / union names is different to match the difference in declaration.

struct Person
char age;
String name;
int employee_nr;
uint other_nr;
union subname
bool b;
Callback cb;


Bitstructs allows storing fields in a specific bit layout. A bitstruct may only contain integer types and booleans, in most other respects it works like a struct.

The main differences is that the bitstruct has a backing type and each field has a specific bit range. In addition, it’s not possible to take the address of a bitstruct field.

bitstruct Foo : char
int a : 0..2;
int b : 4..6;
bool c : 7;
Foo f;
f.a = 2;
char x = (char)f;
io::printfn("%d", (char)f); // prints 2
f.b = 1;
io::printfn("%d", (char)f); // prints 18
f.c = true;
io::printfn("%d", (char)f); // prints 146

The bitstruct will follow the endianness of the underlying type:

bitstruct Test : uint
ushort a : 0..15;
ushort b : 16..31;
Test t;
t.a = 0xABCD;
t.b = 0x789A;
char* c = (char*)&t;
io::printfn("%X", (uint)t); // Prints 789AABCD
for (int i = 0; i < 4; i++) io::printf("%X", c[i]); // Prints CDAB9A78

It is however possible to pick a different endianness, in which case the entire representation will internally assume big endian layout:

bitstruct Test : uint @bigendian
ushort a : 0..15;
ushort b : 16..31;

In this case the same example yields CDAB9A78 and 789AABCD respectively.

Bitstruct backing types may be integers or char arrays. The difference in layout is somewhat subtle:

bitstruct Test1 : char[4]
ushort a : 0..15;
ushort b : 16..31;
bitstruct Test2 : char[4] @bigendian
ushort a : 0..15;
ushort b : 16..31;
Test1 t1;
Test2 t2;
t1.a = t2.a = 0xABCD;
t1.b = t2.b = 0x789A;
char* c = (char*)&t1;
for (int i = 0; i < 4; i++) io::printf("%X", c[i]); // Prints CDAB9A78 on x86
c = (char*)&t2;
for (int i = 0; i < 4; i++) io::printf("%X", c[i]); // Prints ABCD789A

Bitstructs can be made to have ovelapping bit fields. This is useful when modelling a layout which has multiple different layouts depending on flag bits:

bitstruct Foo : char @overlap
int a : 2..5;
int b : 1..3; // Only valid due to the @overlap attribute