Cpp2 Language Overview

Disclaimer:

• These docs are unofficial and may be inaccurate or incomplete.
  • Please file bugs at https://github.com/ntrel/cpp2/issues.
• At the time of writing, Cpp2 is an unstable experimental language, see:
  • https://github.com/hsutter/cppfront#cppfront
  • https://github.com/hsutter/cppfront#wheres-the-documentation.

Note: Some examples are snipped/adapted from: https://github.com/hsutter/cppfront/tree/main/regression-tests

Note: Examples here use C++23 std::println instead of std::cout. If you don't have it, you can use this definition:

std: namespace = {
    println: (args...) = (std::cout << ... << args) << "\n";
}

Contents

• Declarations
• Variables
• Modules
• Types
• Memory Safety
• Expressions
• Statements
• Functions
• User-Defined Types
• Templates
• Aliases

Declarations

These are of the form:

• declaration:
  • identifier : type? = initializer

type can be omitted for type inference (though not at global scope).

    x: int = 42;
    y := x;

A global declaration can be used before the line declaring it.

Mixing Cpp1 Declarations

Cpp1 declarations can be mixed in the same file.

// Cpp2
x: int = 42;

// Cpp1
int main() {
    return x; // use a Cpp2 definition
}

A Cpp2 declaration cannot use Cpp1 declaration format internally:

// declare a function
f: () = {
    int x; // error
}

Note: cppfront has a -p switch to only allow pure Cpp2.

Variables

Uninitialized Variables

Use of an uninitialized variable is statically detected.

When the variable declaration specifies the type, initialization can be deferred to a later statement. Both branches of an if statement must initialize a variable, or neither.

    x: int;
    y := x; // error, x is uninitialized
    if f() {
        x = 1; // initialization, not assignment
    } else {
        x = 0; // initialization required here too, otherwise an error
    }
    x = 2; // assignment

Runtime Constants

    x: const int;
    x = 5; // initialization
    x = 6; // error

    y: int = 7;
    z: const _ = y; // z is a `const int`

Note that x does not need to be initialized immediately, it can deferred. This is particularly useful when using if branches to initialize the constant.

https://github.com/ntrel/cppfront/wiki/Design-note:-const-objects-by-default

Implicit Move on Last Use

A variable is implicitly moved on its last use when the use site syntax may accept an rvalue. This includes passing an argument to a function, but not an assignment to the last use of a variable.

inc: (inout v: int) = v++;

test2: () = {
    v := 42;
    inc(v);     // OK, lvalue
    inc(v);     // error, cannot pass rvalue
}

This can be suppressed by adding a statement _ = v; after the final inc call.

Modules

Cpp2 files have the file extensions .cpp2 and .h2.

Imports

C++23 will support:

import std;

This will be implicitly done in Cpp2. For now common std headers are imported.

Types

See also: User-Defined Types.

Arrays

Use:

• std::array for fixed-size arrays.
• std::vector for dynamic arrays.
• std::span to reference consecutive elements from either.

Pointers

A pointer to T has type *T. Pointer arithmetic is illegal.

Postfix Pointer Operators

Address of and dereference operators are postfix:

    x: int = 42;
    p: *int = x&;
    y := p*;

This makes p-> obsolete - use p*. instead.

To distinguish these from binary & and *, use preceeding whitespace.

new<T>

new<T> gives unique_ptr by default:

    p: std::unique_ptr<int> = new<int>;
    q: std::shared_ptr<int> = shared.new<int>;

Note: gc.new<T> will allocate from a garbage collected arena.

There is no delete operator. Raw pointers cannot own memory.

Null Dereferences

Initialization or assignment from null is an error:

    q: *int = nullptr; // error

Instead of using null for *T, use std::optional<*T>.

By default, cppfront also detects a runtime null dereference. For example when dereferencing a pointer created in Cpp1 code.

int *ptr;

f: () -> int = ptr*;

Calling f above produces:

Null safety violation: dynamic null dereference attempt detected

Memory Safety

Cpp2 will not enforce a memory-safety subset 100%. It will diagnose or prevent type, bounds, initialization, and common lifetime memory-safety violations. This is done by:

• Runtime bounds checks
• Requiring each variable is initialized before use in every possible branch
• Not implemented yet: Compile-time tracking of a set of 'points-to' information for each pointer. When a pointed-to variable goes out of scope, the set is updated to replace the variable with an invalid item. Dereferencing a pointer with a set containing an invalid item is a compile-time error. See https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1179r1.pdf.

See:

• https://github.com/hsutter/cppfront#2015-lifetime-safety
• https://www.reddit.com/r/cpp/comments/16ummo8/cppfront_autumn_update/k2r3fto/

Bounds Checks

By default, cppfront does runtime bound checks when indexing:

    v: std::vector = (1, 2);
    i := v[-1]; // aborts program

    s: std::string = ("hi");
    i = s[2]; // aborts program

Expressions

Postfix Operators

Besides the pointer operators, Cpp2 also only uses postfix instead of prefix form for:

• ++
• --
• ~

Unlike Cpp1, the immediate result of postfix increment/decrement is the new value.

    i := 0;
    assert(i++ == 1);

https://github.com/hsutter/cppfront/wiki/Design-note:-Postfix-operators

String Interpolation

A bracketed expression with a trailing $ inside a string will evaluate the expression, convert it to string and insert it into the string.

    a := 2;
    b: std::optional<int> = 2;
    s: std::string = "a^2 + b = (a * a + b.value())$\n";
    assert(s == "a^2 + b = 6\n");

Note: $ means 'capture' and is also used in closures and postconditions: https://github.com/hsutter/cppfront/wiki/Design-note%3A-Capture

Anonymous Variables

• anonymousVariable:
  • : type? = expression

f: (i: int)   = { std::println("int"); }
f: (i: short) = { std::println("short"); }

main: () = {
    f(5);          // int
    f(:short = 5); // short
}

The last statement is equivalent to tmp: short = 5; f(tmp);.

Identifier Expressions

• identifierExpression:
  • identifier
  • identifier < expressions >
  • expression :: identifierExpression

Required Parentheses

Whenever any kind of identifier expression is used where it could parse as a type, it must be enclosed in parentheses:

• id1 - type
• (id1) - expression

An identifier expression does not need parentheses where a type would not be valid. Other expressions never need parentheses as they could not be parsed as a valid type, e.g. literals, unary expressions etc.

as

• asExpression:
  • expression as type

x as T attempts:

• type conversion (if the type of x implicitly converts to T)
• customized conversion (using operator as<T>), useful for std::optional, std::variant etc.
• construction of T(x)
• dynamic casting (equivalent to Cpp1 dynamic_cast<T>(x) when x is a base class of T)

An exception is thrown if the expression is well-formed but the conversion is invalid.

    c := 'A';
    i: int = c as int;
    assert(i == 65);

    v := std::any(5);
    i = v as int;

    s := "hi" as std::string;
    assert(s.length() == 2);

is

• isExpression:
  • type is (type | template)
  • expression is (type | expression | template)

Type Tests

Not implemented yet.

Test a type T matches another type - T is Target attempts:

• true when T is the same type as Target.
• true if T is a type that inherits from Target.

Test a type against a template - T is Template attempts:

• true if T is an instance of Template.
• Template<T> if the result is convertible to bool.

Expression Tests

Note: Testing an identifier expression needs to use parentheses.

Test type of an expression - (x) is T attempts:

• true when the type of x is T
• x.operator is<T>()
  • (x) is void means x is empty

    assert(5 is int);
    i := 5;
    assert((i) is int);
    assert(!((i) is long));

    v := std::any();
    assert((v) is void); // `v.operator is<void>()`
    v = 5;
    assert((v) is int); // `v.operator is<int>()`

Test expression has a particular value - (x) is v attempts:

• x.operator is(v)
• x == v
• x as V == v where V is the type of v
• v(x) if the result is bool

    i := 5;
    assert((i) is 5);
    v := std::any(i);
    assert((v) is 5);

The last lowering allows to test a value by calling a predicate function:

pred: (x: int) -> bool = x < 20;

test_int: (i: int) = {
    if (i) is (pred) {
        std::println("(i)$ is less than 20");
    }
}

main: () = {
    test_int(5);
    test_int(15);
    test_int(25);
}

Note that pred is not a type identifier so it must be parenthesized.

Test an expression against a template - (x) is Template attempts:

• true if the type of x is an instance of Template.
• Template<(x)> if the result is convertible to bool.

inspect

• inspectExpression:
  • inspect constexpr? expression -> type { alternative+ }
• alternative:
  • alt-name? pattern = statement
  • alt-name? pattern { alternative+ }
• alt-name:
  • identifier :
• pattern:
  • is (type | expression | template)
  • as type
  • if expression
  • pattern || pattern
  • pattern && pattern

Only is alternatives without alt-name are implemented ATM.

v : std::any = 12;

main: () = {
    s: std::string;
    s = inspect v -> std::string {
        is 5 = "five";
        is int = "some other integer";
        is _ = "not an integer";
    };
    std::println(s);
}

An inspect expression must have an is _ case.

Unimplemented: an inspect statement has the same grammar except there must be no -> type after the expression.

https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2392r2.pdf

Move Expressions

A variable can be explictly moved. The move constructor of z will destroy x:

    x: std::string = "hi";
    z := (move x);
    assert(z == "hi");
    assert(x == "");

See also Implicit Move on Last Use.

Statements

A condition expression does not require parentheses in Cpp2, though when a statement immediately follows a condition, a blockStatement is required.

if

• ifStatement:
  • if constexpr? expression blockStatement elseClause?
• elseClause:
  • else blockStatement
  • else ifStatement

    if c1 {
        ...
    } else if c2 {
        ...
    } else {
        ...
    }

Assertions

    x := 1
    assert(x == 1);

Parameterized Statement

• parameterizedStatement:
  • parameterList statement

A parameterized statement declares one or more variables that are defined only for the scope of statement.

(tmp := some_complex_expression) func(tmp, tmp);
// tmp no longer in scope

Valid parameterStorage keywords are in, copy, inout.

while

• whileStatement:
  • while expression nextClause? blockStatement
• nextClause:
  • next expression

If next is present, its expression will be evaluated at the end of each loop iteration.

    // prints: 0 1 2
    (copy i := 0) while i < 3 next i++ {
        std::println(i);
    }

Note: The above is a parameterizedStatement.

do

• doWhileStatement:
  • do blockStatement nextClause? while expression ;

    // prints: 0 1 2
    i := 0;
    do {
        std::println(i);
    } next i++ while i < 3;

for

• forStatement:
  • for expression nextClause? do ( parameter ) statement

The first expression must be a range. parameter is initialized from each element of the range. The parameter type is inferred. parameter can have inout parameterStorage.

    vec: std::vector<int> = (1, 2, 3);

    for vec do (inout e)
        e++;

    assert(vec[0] == 2);
    for vec do (e)
        std::println(e);

https://github.com/hsutter/cppfront/blob/main/regression-tests/mixed-intro-for-with-counter-include-last.cpp2

Labelled break and continue

The target of these statements can be a labelled loop.

    outer: while true {
        j := 0;
        while j < 3 next j++ {
            if done() {
                break outer;
            }
        }
    }

Functions

Function Types

• functionType:
  • parameterList returnSpec
• parameterList:
  • ( parameter? )
  • ( parameter (, parameter)+ )
• parameter:
  • parameterStorage? type.
  • parameterStorage? identifier ...? : type.
• returnSpec:
  • -> (forward | move)? type
  • -> parameterList

E.g. (int, float) -> bool.

Function Declarations

• functionDeclaration:
  • identifier? : parameterList returnSpec? ;
  • identifier? : parameterList returnSpec? contracts? = functionInitializer
  • identifier? : parameterList expression ;

Function declarations extend the declaration form. Each parameter must have an identifier.

If returnSpec is missing with the first two forms, the function returns void. The return type can be inferred from the initializer by using -> _.

See also Template Functions.

Function Bodies

• functionInitializer:
  • (expression ; | statement)

A function is initialized from a statement or an expression.

d: (i: int) = std::println(i);
e: (i: int) = { std::println(i); } // same

If the function has a returnSpec, the expression form implies a return statement.

f: (i: int) -> int = return i;
g: (i: int) -> int = i; // same

Lastly, -> _ = together can be omitted:

h: (i: int) i; // same as f and g

This form is useful for lambda functions.

Named Return Values

When a function returns a parameterList, each parameter must be named. A function with multiple named return parameters returns a struct with a member for each parameter.

f: () -> (i: int, s: std::string) = {
    i = 10;
    s = "hi";
}

main: () = {
    t := f();
    assert(t.i == 5);
    assert(t.s == "hi");
}

• Unless a return parameter has a default value, it must be initialized in the function body.
• When only one return parameter is declared, the caller does not use member syntax to access the result.

f: () -> (ret: int = 42) = {}

main: () = {
    assert(f() == 42);
}

main

• mainFunction:
  • main : ( args? ) (-> int)? = functionInitializer

If args is declared, it is a std::vector<std::string_view> containing each command-line argument to the program.

Uniform Call Syntax

If a method doesn't exist when using method call syntax, and there is a function whose first parameter can take the type of the 'object' expression, then that function is called instead.

main: () -> int = {
    // call C functions
    myfile := fopen("xyzzy", "w");
    myfile.fprintf("Hello %d!", 2); // fprintf(myfile, "Hello %d!", 2)
    myfile.fclose(); // fclose(myfile)
}

Parameter Passing

• in - default, read-only. Will pass by reference when more efficient, otherwise pass by value.
• inout - pass by mutable reference.
• out - must be written to. Can accept an uninitialized argument, otherwise destroys the argument. The first assignment constructs the parameter. Used for constructors.
• move - argument can be moved from. Used for destructors.
• copy - argument can be copied from.
• forward - accepts lvalue or rvalue, pass by reference.

e: (i: int) = i++; // error, `i` is read-only
f: (inout i: int) = i++; // mutate argument

g: (out i: int) = {
    v := i; // error, `i` used before initialization
    // error, `i` was not initialized
}

Functions can return by reference:

first: (forward v: std::vector<int>) -> forward int = v[0];

main: () -> int = {
    v : std::vector = (1,2,3);
    first(v) = 4;
}

https://github.com/hsutter/cppfront/blob/main/regression-tests/mixed-parameter-passing.cpp2

Contracts

vec: std::vector<int> = ();

insert_at: (where: int, val: int)
    pre(0 <= where && where <= vec.ssize())
    post(vec.ssize() == vec.ssize()$ + 1) = {
    vec.insert(vec.begin() + where, val);
}

The postcondition compares the vector size at the end of the function call with an expression that captures the vector size at the start of the function call.

https://github.com/hsutter/cppfront/blob/main/regression-tests/mixed-postexpression-with-capture.cpp2

A single named return is useful to refer to a result in a postcondition:

f: () -> (ret: int)
    post(ret > 0) = {
    ret = 42;
}

Function Literals

A function literal is declared like a named function, but omitting the leading identifier. Variables can be captured:

    s: std::string = "Got: ";
    f := :(x) = { std::println(s$, x); };
    f(5);
    f("str");

• s$ means capture s by value.
• s&$* can be used to dereference the captured address of s.

Template Functions

A template function declaration can have template parameters:

• functionTemplate:
  • identifier? : templateParameterList? parameterList returnSpec? requiresClause?

E.g. size: <T> (v: T) -> _ = v.length();

When a function parameter type is _, this implies a template with a corresponding type parameter.

A template function parameter can also be just identifier.

f: (x: _) = {}
g: (x) = {} // same

Variadic Template Functions

print: (a0) = std::print(a0);

print: (a0, args...) = {
    print(a0);
    print(", ");
    print(args...);
}

main: () = print(1, 2, 3);

User-Defined Types

type declares a user-defined type with data members and member functions. When the first parameter is this, it is an instance method.

myclass : type = {
    data: int = 42;
    more: std::string = std::to_string(42);

    // method
    print: (this) = {
        std::println("data: (data)$, more: (more)$");
    }

    // non-const method
    inc: (inout this) = data++;
}

main: () = {
    x: myclass = ();
    x.print();
    x.inc();
    x.print();
}

Data members are private by default, whereas methods are public. Member declarations can be prefixed with private or public.

operator=

Official docs: https://github.com/hsutter/cppfront/wiki/Cpp2:-operator=,-this-&-that.

operator= with an out this first parameter is called for construction. When only one subsequent parameter is declared, assignment will also call this function.

    operator=: (out this, i: int) = {
        this.data = i;
    }
...
    x: myclass = 99;
    x = 1;

With only one parameter move this, it is called to destroy the object:

    operator=: (move this) = {
        std::println("destroying (data)$ and (more)$");
    }

Objects are destroyed on last use, not end of scope.

Inheritance

base: type = {
    operator=: (out this, i: int) = {}
}

derived: type = {
    this: base = (5); // declare parent class & construct with `base(5)`
}

Type Templates

• typeTemplate:
  • identifier? : templateParameterList? type requiresClause?

Templates

• templateParameterList:
  • < templateParameters >
• templateParameter
  • identifier ...? (: type)?
  • identifier : type

The first parameter form accepts a type.

The second parameter form accepts a value. To use a constant identifier as a template parameter, enclose it in parentheses:

f: <i: int> () -> _ = i;
n: int == 5;
...
    std::println(f<(n)>());

n is a constant alias.

Constraints

• requiresClause:
  • requires constExpression

defaultValue: <T> () -> T requires std::regular<T> = { v: T = (); return v; }
...
    assert(defaultValue<int>() == 0);

Note: Using an inline concept for a type parameter is not supported yet.

Concepts

• concept:
  • identifier : templateParameterList concept requiresClause? = constExpression ;

arithmetic: <T> concept = std::integral<T> || std::floating_point<T>;
...
    assert(arithmetic<i32>);
    assert(arithmetic<float>);

Aliases

Aliases are defined using == rather than =.

• alias:
  • identifier : templateParameterList? type? == constExpression
  • identifier : templateParameterList? functionType == functionInitializer
  • identifier : templateParameterList? type == type
  • identifier : namespace == identifierExpression

The forms above are equivalent to the following Cpp1 declarations:

• constexpr variable
• constexpr function
• using type alias
• namespace alias

// constant template
size: <T> size_t == sizeof(T);
// compile-time function
init: <T> () -> T == ();

main: () = {
    static_assert(size<char> == 1);

    // constant aliases
    v := 5;
    //n :== v; // error, cannot read `v` at compile-time
    n :== 6; // OK
    myfunc :== main;

    static_assert(init<int>() == 0);
    view: type == std::string_view;
    N4: namespace == std::literals;
}