Metafunctions

Overview

A metafunction is a compile-time function that can participate in interpreting the meaning of a declaration, and can:

apply defaults (e.g., interface makes functions virtual by default)
enforce constraints (e.g., value enforces that the type has no virtual functions)
generate additional functions and other code (e.g., value generates copy/move/comparison operations for a type if it didn't write them explicitly)

The most important thing about metafunctions is that they are not hardwired language features — they are compile-time library code that uses the reflection and code generation API, that lets the author of an ordinary type easily opt into a named set of defaults, requirements, and generated contents. This approach is essential to making the language simpler, because it lets us avoid hardwiring special "extra" types into the language and compiler.

Applying metafunctions using `@`

Metafunctions provide an easy way for a type author to opt into a group of defaults, constraints, and generated functions: Just write @name afer the : of a declaration, where name is the name of the metafunction. This lets the type author declare (and the human reader see) the intent up front: "This isn't just any type, this is a @value type" which automatically gives the type default/copy/move construction and assignment, <=> with std::strong_ordering comparisons, and guarantees that it has a public destructor and no protected or virtual functions:

Using the value metafunction when writing a type

point2d: @value type = {
    x: i32 = 0;
    y: i32 = 0;
    // @value automatically generates default/copy/move
    // construction/assignment and <=> strong_ordering comparison,
    // and emits an error if you try to write a non-public
    // destructor or any protected or virtual function
}

Generating source code at compile time

A metafunction applied to a definition using @ gets to participate in interpreting the meaning of the definition by inspecting and manipulating the definition's parse tree. For example:

shape.cpp2: Using @interface @print

shape: @interface @print type = {
    draw   : (this);
    move_by: (this, dx: double, dy: double);
}

The above code:

applies @interface, which makes functions pure virtual by default and defines a virtual destructor with a do-nothing body if there isn't already a virtual destructor (among other things), and
then applies @print, which pretty-prints the resulting parse tree as source code to the console so that we can see the results of what the first metafunction did.

The result of compiling this is the following cppfront output, which is the @interface-modified Cpp2 source code as printed by @print:

'cppfront shape.cpp2' output to the console, from @print

shape:/* @interface @print */ type =
{
    public draw:(virtual in this);

    public move_by:(
        virtual in this,
        in dx: double,
        in dy: double
    );

    operator=:(virtual move this) =
    {
    }
}

Finally, cppfront also emits the following in shape.cpp:

'cppfront shape.cpp' output to 'shape.cpp'

class shape {
    public: virtual auto draw() const -> void = 0;
    public: virtual auto move_by(cpp2::in<double> dx, cpp2::in<double> dy) const -> void = 0;
    public: virtual ~shape() noexcept;

    public: shape() = default;
    public: shape(shape const&) = delete; /* No 'that' constructor, suppress copy */
    public: auto operator=(shape const&) -> void = delete;

};

shape::~shape() noexcept{}

Built-in metafunctions

The following metafunctions are provided in the box with cppfront.

For regular value-like types (copyable, comparable)

`ordered`, `weakly_ordered`, `partially_ordered`

An ordered (or weakly_ordered or partially_ordered) type has an operator<=> three-way comparison operator that returns std::strong_ordering (or std::weak_ordering or std::partial_ordering, respectively). This means objects of this type can be used in all binary comparisons: <, <=, ==, !=, >=, and >.

If the user explicitly writes operator<=>, its return type must be the same as the one implied by the metafunction they chose.

If the user doesn't explicitly write operator<=>, a default memberwise operator<=>: (this, that) -> /* appropriate _ordering */; will be generated for the type.

These metafunctions will emit a compile-time error if:

a user-written operator<=> returns a different type than the one implied by the metafunction they chose

Note: This feature derived from Cpp2 was already adopted into Standard C++ via paper P0515, so most of the heavy lifting is done by the Cpp1 C++20/23 compiler, including the memberwise default semantics. In contrast, cppfront has to do the work itself for default memberwise semantics for operator= assignment as those aren't yet part of Standard C++.

`copyable`

A copyable type has (copy and move) x (construction and assignment).

If the user explicitly writes any of the copy/move operator= functions, they must also write the most general one that takes (out this, that).

If the user doesn't write any of the copy/move operator= functions, a default general memberwise operator=: (out this, that) = { } will be generated for the type.

copyable will emit a compile-time error if:

there is a user-written operator= but no user-written operator=: (out this, that)

`basic_value`, `value`, `weakly_ordered_value`, `partially_ordered_value`

A basic_value type is a regular type: copyable, default constructible, and not polymorphic (no protected or virtual functions).

A value (or weakly_ordered_value or partially_ordered_value) is a basic_value that is also ordered (or weakly_ordered or partially_ordered, respectively).

graph TD;
    value---->basic_value;
    weakly_ordered_value---->basic_value;
    partially_ordered_value---->basic_value;
    basic_value-->copyable;
    value-->ordered;
    partially_ordered_value-->partially_ordered;
    weakly_ordered_value-->weakly_ordered;

These metafunctions will emit a compile-time error if:

any function is protected or virtual
the type has a destructor that is not public

`struct`

A struct is a type with only public bases, objects, and functions, with no virtual functions, and with no user-defined constructors (i.e., no invariants) or assignment or destructors.

struct is implemented in terms of cpp1_rule_of_zero.

struct will emit a compile-time error if:

any member is non-public
any function is virtual
there is a user-written operator=

For polymorphic types (interfaces, base classes)

`interface`

An interface type is an abstract base class having only pure virtual functions.

Cpp2 has no interface feature hardwired into the language, as C# and Java do. Instead you apply the @interface metafunction when writing an ordinary type. For a detailed example, see the shape example above.

interface will emit a compile-time error if:

the type contains a data object
the type has a copy or move function (the diagnostic message will suggest a virtual clone function instead)
any function has a body
any function is nonpublic

`polymorphic_base`

A polymorphic_base type is a pure polymorphic base type that is not copyable, and whose destructor is either public and virtual or protected and nonvirtual.

Unlike an interface, it can have nonpublic and nonvirtual functions.

polymorphic_base will emit a compile-time error if:

the type has a copy or move function (the diagnostic message will suggest a virtual clone function instead)
the type has a destructor that is not public and virtual, and also not protected and nonvirtual

For enumeration types

`enum`

Cpp2 has no enum feature hardwired into the language. Instead you apply the @enum metafunction when writing an ordinary type.

enum will emit a compile-time error if:

any member has the reserved name operator= or operator<=>, as these will be generated by the metafunction
an enumerator is not public or does not have a deduced type

For example:

Using the @enum metafunction when writing a type

// skat_game is declaratively a safe enumeration type: it has
// default/copy/move construction/assignment and <=> with
// std::strong_ordering, a minimal-size signed underlying type
// by default if the user didn't specify a type, no implicit
// conversion to/from the underlying type, in fact no public
// construction except copy construction so that it can never
// have a value different from its listed enumerators, inline
// constexpr enumerators with values that automatically start
// at 1 and increment by 1 if the user didn't write their own
// value, and conveniences like to_string()... the word "enum"
// carries all that meaning as a convenient and readable
// opt-in, without hardwiring "enum" specially into the language
//
skat_game: @enum<i16> type = {
    diamonds := 9;
    hearts;  // 10
    spades;  // 11
    clubs;   // 12
    grand    := 20;
    null     := 23;
}

Consider hearts: It's a member object declaration, but it doesn't have a type (or a default value) which is normally illegal, but here it's okay because the @enum<i16> metafunction fills them in: It iterates over all the data members and gives each one the underlying type (here explicitly specified as i16, otherwise it would be computed as the smallest signed type that's big enough), and an initializer (by default one higher than the previous enumerator).

Unlike C enum, this @enum is scoped and strongly typed (does not implicitly convert to the underlying type).

Unlike C++11 enum class, it's "just a type" which means it can naturally also have member functions and other things that a type can have:

An @enum type with a member function

janus: @enum type = {
    past;
    future;

    flip: (inout this) == {
        if this == past { this = future; }
        else { this = past; }
    }
}

`flag_enum`

flag_enum is a variation on enum that has power-of-two default enumerator values, a default signed underlying type that is large enough to hold the values, and supports bitwise operators to combine and test values.

flag_enum will emit a compile-time error if:

any member has the reserved name operator=, operator<=>, has, set, clear, to_string, get_raw_value, or none, as these will be generated by the metafunction
an enumerator is not public or does not have a deduced type
the values are outside the range that can be represented by the largest default underlying type

For example:

Using the @flag_enum metafunction when writing a type

// file_attributes is declaratively a safe flag enum type:
// same as enum, but with a minimal-size unsigned underlying
// type by default, and values that automatically start at 1
// and rise by powers of two if the user didn't write their
// own value, and bitwise operations plus .has(flags),
// .set(flags), and .clear(flags)... the word "flag_enum"
// carries all that meaning as a convenient and readable
// opt-in without hardwiring "[Flags]" specially into the
// language
//
file_attributes: @flag_enum<u8> type = {
    cached;     // 1
    current;    // 2
    obsolete;   // 4
    cached_and_current := cached | current;
}

For dynamic types

`union`

@union declaratively opts into writing a safe discriminated union/variant dynamic type.

union will emit a compile-time error if:

any alternative is not public or has an initializer
any member starts with the reserved name prefix is_ or set_, as these will be generated by the metafunction

For example:

Using the @union metafunction when writing a type

// name_or_number is declaratively a safe union/variant type:
// it has a discriminant that enforces only one alternative
// can be active at a time, members always have a name, and
// each member has .is_member(), .set_member(), and .member()
// accessors using the member name... the word "union"
// carries all that meaning as a convenient and readable
// opt-in without hardwiring "union" specially into the
// language
//
name_or_number: @union type = {
    name: std::string;
    num : i32;
}

main: () = {
    x: name_or_number = ();

    x.set_name("xyzzy");            // now x is a string
    assert( x.is_name() );
    std::cout << x.name();          // prints the string

    // trying to use x.num() here would cause a Type safety
    // contract violation, because x is currently a string

    x.set_num( 120 );               // now x is a number
    std::cout << x.num() + 3;       // prints 123
}

Unlike C union, this @union is safe to use because it always ensures only the active type is accessed.

Unlike C++11 std::variant, this @union is easier to use because its alternatives are anonymous, and safer to use because each union type is a distinct type. ¹

Each @union type has its own type-safe name, has clear and unambiguous named members, and safely encapsulates a discriminator to rule them all. Sure, it uses unsafe casts in the implementation, but they are fully encapsulated, where they can be tested once and be safe in all uses.

Because a @union type is still a type, it can naturally have other things normal types can have, such as template parameter lists and member functions:

A templated custom safe union type

name_or_other: @union <T:type> type
= {
    name  : std::string;
    other : T;

    // a custom member function
    to_string: (this) -> std::string = {
        if is_name()       { return name(); }
        else if is_other() { return other() as std::string; }
        else               { return "invalid value"; }
    }
}

main: () = {
    x: name_or_other<int> = ();
    x.set_other(42);
    std::cout << x.other() * 3.14 << "\n";
    std::cout << x.to_string(); // prints "42" here, but is legal
                                // whichever alternative is active
}

Helpers and utilities

`cpp1_rule_of_zero`

A cpp1_rule_of_zero type is one that has no user-written copy/move/destructor functions, and for which Cpp2 should generate nothing so that the Cpp1 defaults for generated special member functions are accepted.

C.20: If you can avoid defining default operations, do. Reason: It's the simplest and gives the cleanest semantics. This is known as "the rule of zero". — Stroustrup, Sutter, et al. (C++ Core Guidelines)

`print`

print prints a pretty-printed visualization of the type to the console.

This is most useful for debugging metafunctions, and otherwise seeing the results of applying previous metafunctions.

For a detailed example, see the shape example above.

With variant, there's no way to distinguish in the type system between a variant<int,string> that stores either an employee id or employee name, and a variant<int,string> that stores either a lucky number or a pet unicorn's dominant color. ↩

Metafunctions

Overview

Applying metafunctions using @

Generating source code at compile time

Built-in metafunctions

For regular value-like types (copyable, comparable)

ordered, weakly_ordered, partially_ordered

copyable

basic_value, value, weakly_ordered_value, partially_ordered_value

struct

For polymorphic types (interfaces, base classes)

interface

polymorphic_base

For enumeration types

enum

flag_enum

For dynamic types

union

Helpers and utilities

cpp1_rule_of_zero

print

Applying metafunctions using `@`

`ordered`, `weakly_ordered`, `partially_ordered`

`copyable`

`basic_value`, `value`, `weakly_ordered_value`, `partially_ordered_value`

`struct`

`interface`

`polymorphic_base`

`enum`

`flag_enum`

`union`

`cpp1_rule_of_zero`

`print`