For some unclear reason, when it comes to the VLA, there is not much to suggest that pre-emptive sizes can be created in local memory, that is, in the glass. This is a misunderstanding, because, in fact, the possibility of localizing such masses is a completely bypassive and secondary VLA, there is no significant role in VLA functionality. Typically, this opportunity and the surrounding underwater stones are to be wilfully criticized by the VLA in order to avoid the discussion.The point is that support for VLA is primarily a powerful expansion. Typical system language. This introduction to language C is a fundamentally new conceptual group of types, such as variably modified types (for the purposes of this statement, I will translate as "variable types." All the most important internal implementation details of the VLA are related to its type rather than to the object itself as such. It is a language introduction. Variable types is a presidious iceberg VLA, and the possibility of creating such objects in local memory is not more than a small (and not necessarily) tip of the iceberg. For example, whenever a type is declared in the programme/* Внутри блока */
int n = 10;
...
typedef int A[n];
of this variable type - value n - fixed at the time when the control passes on the typedef announcement. Change of value n After pseudonym A no longer affects the type characteristics A♪ In terms of implementation, this means that with a variable type A a hidden internal variable describing the size of the mass will be associated. This hidden variable is initiated at the time of the type-approval board.This gives this typedef-declaration an unusually interesting characteristic - it does not simply produce a complete code, it creates critically needed A complete code. For this reason, in language C, an invisible characteristic (known to us from C+++) appears to be present: language C prohibits the transfer of external visibility of a variable species in this field of visibility/* Внутри блока */
int n = 10;
goto skip; /* ОШИБКА: недопустимая передача управления */
typedef int A[n];
skip:;
Stress once again that there is no definition of any VLA Massive in the code listed above, but only a typedef pseudonym for a variable type. However, the transmission of control through such typedef announcement is not permitted.In determining the actual VLA Massives, the same hidden variables stored the size of the mass are being created, apart from the actual remembrance of the body. The body itself is carried out through an ordinary index, and memory is given by a mechanism like alloca/* Внутри блока */
int n = 10, m = 20;
typedef int A[n][m];
A a;
a[5][6] = 42;
/* ... транслируется в ... */
int n = 10, m = 20;
size_t _internal_A1 = n, _internal_A2 = m;
int a = alloca(_internal_A1 * _internal_A2 * sizeof(int));
a[5 * _internal_A2 + 6] = 42;
(In addition, the code for the release of the memory will be generating at the end of the cluster in which the mass is determined a) However, in this case, it should be understood that these hidden variables are associated not so much with the body itself, but with it. variable type♪ If the code declares several VLA Massives and/or variable types with identical performance dates, they may in principle use the same hidden variables to store their sizes.One important consequence is that additional information on the size of the mass associated with the VLA Massive is not built into the objective image of the body itself, but is stored in a very separate, independent manner. This results in an objective VLA submission with any number of measurements being fully compatible with the objective presentation of the classic " fixed " mass with the same number of measurements and the same size. For example/ Внутри блока /
unsigned n = 5;
int a[n][n + 1][n + 2]; / VLA */
int (p)[5][6][7]; / Указатель на "классический" массив */
p = &a;
/* Присваивание корректно, т.к. размеры массивов совпадают */
(p)[1][2][3] = 42;
/ Поведение определено: a[1][2][3] получает значение 42 */
Or an example of the often-expired need to transfer the mass into a functionvoid foo(unsigned n, unsigned m, unsigned k, int a[n][m][k]) {}
void bar(int a[5][5][5]) {}
int main(void)
{
unsigned n = 5;
int vla_a[n][n][n];
bar(a);
int classic_a[5][6][7];
foo(5, 6, 7, classic_a);
}
Both the challenge of the functions in the above code is entirely correct and their behaviour is fully defined by the language, despite the fact that we are transferring VLA to a classic area and vice versa. Of course, the compiler in such a situation will not control the correctness of the challenges, i.e., the coincidence of the actual dimensions of the parameters and arguments (although, if desired, the possibility of generating the check code in the appropriate mode is both for him and for the user). (Remark: As usual, the "massiv" parameters, whether they are VLA or not, are always not clearly transformed into "indicator," which means that in the above example, the parameter a He's actually got a type. int ()[m][k] and meaning n It doesn't affect this type. I specifically added a lot of measurements to the mass so as not to lose its variability.) Compatibility is also ensured that the transfer of VLA to the compiler does not need to accompany the VLA itself with any hidden additional information on its size. Syntaxis of the language makes the author a code of volley-neutrals to transmit the information independently to the public. In the above example, the author of the code was forced to list the functions in the first instance. foo parameters n♪ m and k'Cause without them, he wouldn't be able to declare a parameter. a (see also comment above on n) It's these clearly-transmitted parameters and "take" information on the actual amount of the mass. a♪ VLA Massive announcements do not permit initialling, which also prevents the use of VLA in composite literatureint n = 10;
int a[n] = { 0 }; / ОШИБКА: нельзя указывать инициализатор */
The reason for this limitation, as I recall, is that there is no good answer to the question of what to do if some initials turn out to be redundant.Using the values of the VLA, we can write, for example, the next code.#include <stdio.h>
#include <stdlib.h>
void init(unsigned n, unsigned m, int a[n][m])
{
for (unsigned i = 0; i < n; ++i)
for (unsigned j = 0; j < m; ++j)
a[i][j] = rand() % 100;
}
void display(unsigned n, unsigned m, int a[n][m])
{
for (unsigned i = 0; i < n; ++i)
for (unsigned j = 0; j < m; ++j)
printf("%2d%s", a[i][j], j + 1 < m ? " " : "\n");
printf("\n");
}
int main(void)
{
int a1[5][5] = { 42 };
display(5, 5, a1);
init(5, 5, a1);
display(5, 5, a1);
unsigned n = rand() % 10 + 5, m = rand() % 10 + 5;
int (*a2)[n][m] = malloc(sizeof *a2);
init(n, m, *a2);
display(n, m, a2);
free(a2);
}
Note: This programme is actively and substantially using the valuable properties provided by variable types. What this code does cannot be easily implemented without the use of variability. However, this programme does not create any VLA in local memory (!), which is a popular criticism of the VLA in this code. The existence of a VLA language makes it possible to develop mysterious designs whose practical value is questionable and whose behaviour is not always clear. For example, options for posting functions are allowed/ На уровне файла */
int n = 100;
void foo(int a[n++]) {}
void bar(int m, int a[++n][++m]) {}
int hello(void) { return printf("Hello World\n"); }
void baz(int a[hello()]) {}
The expressions used by the VLA advertisements within the functions will be accurately calculated, together with their side effects, in each function challenge. Please note that, despite the fact that the parameters of the type of massiv will be transformed into the parameters of the type of "indicator," this does not obviate the need to compute the expression used to produce the size of the mass in the original announcement. In this example, each challenge to the function baz to be accompanied by the withdrawal of the line "Hello World\n"♪The reference to the teg [C++] on the issue is wrong. In populist contexts, it is often possible to hear allegations that some VLA compilers (GCC) are also supported in C++. In fact, the fact that some C+++ compilers allow the designation of non-constant expressions as the size of the languages does not reflect the support of the C99 VLA in the C+++ code area. C and C+++ substantially different languages and C99 VLA support in C++ is difficult or impossible. Simple experiments show that pseud-VLA behaviour in GCC C++ is fundamentally different from standard C99 VLA behaviour. Like this code.#include <stdio.h>
int main(void)
{
int n = 10;
typedef int A[n];
n = 20;
A a;
printf("%zu\n", sizeof a / sizeof *a);
}
♪ 10 in C mode (as should be) but will 20 when compiling GNU C+++. Obviously, the concept of a typedef that produces a complete code is not consistent with the fundamental ideas of the C+++ language.
