可变修改类型仅是 VLA 吗？

Question

一个简单的问题：可变修改类型是VLA（可变长度数组）只有吗？

C11, 6.10.8.3 条件特征宏，1（强调已加）：

__STDC_NO_VLA__ The integer constant 1, intended to indicate that the implementation does not support variable length arrays or variably modified types.

这是否意味着除了 VLA 之外还有一个可变修改类型？有例子吗？

“可变修饰类型”和“可变长度数组”有什么关系？

额外：“变长数组”的定义依赖于“已知常量大小”的定义：

If the size is an integer constant expression and the element type has a known constant size, the array type is not a variable length array type; otherwise, the array type is a variable length array type.

但是，“已知常数大小”的定义取决于“可变长度数组”的定义：

A type has known constant size if the type is not incomplete and is not a variable length array type.

有点迷糊。

相关 DR：http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_312.htm。

Answer 1

Does it mean that there is a variably-modified type, other than VLA? Any examples?

根据 C 标准（6.7.6 声明符）

3 A full declarator is a declarator that is not part of another declarator. If, in the nested sequence of declarators in a full declarator, there is a declarator specifying a variable length array type, the type specified by the full declarator is said to be variably modified. Furthermore, any type derived by declarator type derivation from a variably modified type is itself variably modified.

这是一个演示程序

#include <stdio.h>

int main( void )
{
    for ( size_t n = 1; n < 10; n++ )
    {
        typedef int ( *Ptr )[n];
        Ptr p;

        printf( "sizeof( *p ) = %zu\n", sizeof( *p ) );
    }
}

程序输出为

sizeof( *p ) = 4
sizeof( *p ) = 8
sizeof( *p ) = 12
sizeof( *p ) = 16
sizeof( *p ) = 20
sizeof( *p ) = 24
sizeof( *p ) = 28
sizeof( *p ) = 32
sizeof( *p ) = 36

在此程序中，指针类型 Ptr 定义为 int( * )[n] 是可变修改类型。

这句话

A type has known constant size if the type is not incomplete and is not a variable length array type.

表示此类类型的 sizeof 运算符在 compile-time 处求值，与在 run-time 处对可变长度数组类型的求值相反，并且此类类型的大小未更改在程序执行期间。

引用与下面的引用有不同的含义

If the size is an integer constant expression and the element type has a known constant size, the array type is not a variable length array type; otherwise, the array type is a variable length array type.

说明了如何区分可变长度数组的声明和 non-variable 长度数组的声明。

Answer 2

C 2018 6.7.6 3 说：

If, in the nested sequence of declarators in a full declarator, there is a declarator specifying a variable length array type, the type specified by the full declarator is said to be variably modified. Furthermore, any type derived by declarator type derivation from a variably modified type is itself variably modified.

因此int (*)[n]，对于某些non-constantn来说，是一个可变修改类型，即使它是一个指针。此外，int [3][n] 是可变修改类型。

可变修改类型仅是 VLA 吗？

Is variably-modified type a VLA only?

c

language-lawyer

variable-length-array