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Showing content from https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2593.htm below:

So-Magic - typeof(…) in C

Document: n2593
Previous Revisions: None
Audience: WG14
Proposal Category: New Features
Target Audience: General Developers, Compiler/Tooling Developers
Latest Revision: https://thephd.github.io/vendor/future_cxx/papers/source/n2593.html

Abstract:

Getting the type of an expression in Standard C code.

typeof is a extension featured in many implementations of the C standard to get the type of an expression. It works similarly to sizeof, which runs the expression in an “unevaluated context” to understand the final type, and thusly produce a size. typeof stops before producing a byte size and instead just yields a type name, usable in all the places a type currently is in the C grammar.

There are many uses for typeof that have come up over the intervening decades since its first introduction in a few compilers, most notably GCC. It can, for example, help produce a type-safe generic printing function that even has room for user extension (see: https://slbkbs.org/tmp/fmt/fmt.h). It can also help write code that can use the expansion of a macro expression as the return type for a function, or used within a macro itself to correctly cast to the desired result of a specific computation’s type (for width and precision purposes). The use cases are vast and endless, and many people have been locking themselves into implementation-specific vendorship that have locked them out of other compilers (for example, Microsoft’s Visual C Compiler).

Every implementation in existence since C89 has an implementation of typeof. Some compilers (GCC, Clang, EDG, tcc, and many, many more) expose this with the implementation extension typeof. But, the Standard already requires typeof to exist. Notably, with underlined emphasis added,

The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type. The size is determined from the type of the operand. — N2573, §6.5.3.4 The sizeof and _Alignof operators, Semantics

Any implementation that can process sizeof("foo") is already doing sizeof(typeof("foo")) internally. This feature is the most “existing practice”-iest feature to be proposed to the C Standard, possibly in the entire history of the C standard.

Furthermore, putting a type or a VLA-type computation results in an idempotent type computation that simply yields that type in most implementations that support the feature.

The following wording is relative to N2573.

Adjust the Syntax grammar of §6.7.2 Type specifiers

type-specifier:
    void
    …
    typedef-name
    typeof-specifier

Add a new §6.7.2.5 The Typeof specifier

§6.7.2.5     The Typeof specifier Syntax

typeof-specifier:
    _Typeof unary-expression
    _Typeof ( type-name )

Constraints

The typeof-specifier shall not be applied to an expression that has function type or an incomplete type, to the parenthesized name of such a type, or to an expression that designates a bit-field member.

Semantics

The typeof-specifier applies the _Typeof operator to a unary-expression (6.5.3) or a type-specifier. If the _Typeof operator is applied to a unary-expression, it yields the type-name representing the type of its operand^11�0). Otherwise, it produces the type-name with any nested typeof-specifier evaluated ^11�1). If the type of the operand is a variable length array type, the operand are evaluated; otherwise, the operand is not evaluated.

Type qualifiers (6.7.3) of the type from the result of a _Typeof operation are preserved.

^11�0) _{When applied to a parameter declared to have array or function type, the _Typeof operator yields the adjusted (pointer) type (see 6.9.1).}

^11�1) _{If the operand is a _Typeof operator, the operand will be evaluated before evaluating the current _Typeof operation. This happens recursively until a typeof-specifier is no longer the operand.}

Add the following examples to the new Typeof section

⁵ EXAMPLE 1 Type of an expression

_Typeof(1) main () {
    return 0;
}
// equivalent to:
// int main() {
//   ...
// }

⁶ EXAMPLE 2 Equivalence of sizeof and typeof.

int main (int argc, char* argv[]) {
    // this program has no constraint violations
    _Static_assert(sizeof(_Typeof('p')) == sizeof(char));
    _Static_assert(sizeof(_Typeof('p')) == sizeof('p'));
    _Static_assert(sizeof(_Typeof("meow")) == sizeof(char[5]));
    _Static_assert(sizeof(_Typeof("meow")) == sizeof("meow"));
    _Static_assert(sizeof(_Typeof(argc)) == sizeof(int));
    _Static_assert(sizeof(_Typeof(argc)) == sizeof(argc));
    _Static_assert(sizeof(_Typeof(argv)) == sizeof(char**));
    _Static_assert(sizeof(_Typeof(argv)) == sizeof(argv));
    return 0;
}

⁷ EXAMPLE 3 Nested _Typeof(...).

int main (int argc, char*[]) {
    float val = 6.0f;
    // equivalent to a cast and return
    return (_Typeof(_Typeof(_Typeof(argc))))val;
    // return (int)val;
}

⁸ EXAMPLE 4 Variable Length Arrays and _Typeof.

#include <stddef.h>

size_t vla_size (int n) {
    typedef char vla_type[n + 3];
    vla_type b; // variable length array
    return sizeof(
        _Typeof(b)
    ); // execution-time sizeof, translation-time _Typeof
}

int main () {
    return (int)vla_size(10); // vla_size returns 13
}

Add a new §7.� Typeof <stdtypeof.h>

The header <stdtypeof.h> defines two macros.

The macro

typeof

expands to _Typeof.

The macro

__typeof_is_defined

is suitable for use in #if preprocessing directives. It expands to the integer constant 1.

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