Что такое длинная версия = {0} в C?

Question

я задал этот вопрос:

= { 0 } является «ярлык» для того, что более кода?

Итак, после многих поисков и рутинга через мою книгу C я знаю все, что он делает с агрегатной инициализацией, но что, если бы я хотел написать более длинную версию этого кода. Вот реальный код из программы только в случае, если

static char vid[25][100] = { 0 } ; 

Answer 1

Длинная версия представляет собой цикл, который инициализирует все члены массива:

for (int i = 0; i < ARRAY_SIZE; ++i) 
{ 
    myArray[i] = 0; 
} 

Компилятор, вероятно, генерировать вызов функции memset, который может или не может содержать более оптимизированную версию этот наивный цикл.

Но, как указывает Дитрих, в примере, который вы указали в вопросе, вы объявляете массив static. Это делает вещи немного сложнее. Неинициализированные статические данные помещаются в специальный раздел исполняемого файла, который называется секцией .bss, и для обоих стандартов C и C++ требуется инициализация по умолчанию для нуля. Если вы явно инициализируете данные, компоновщик помещает их в раздел .data исполняемого файла. Явная инициализация до 0 - это немного особый случай. Смарт-линкеры будут пропускать явный инициализатор и поместить данные в раздел .bss. Туманные линкеры могут по-прежнему размещать данные в разделе .data. Так что, вероятно, лучше не для явно нулевой инициализации объектов со статической связью.

Answer 2

Стандарт не определяет, как zero'ing делается так, что это зависит от конкретной реализации. На практике для локальных переменных компиляторы обычно генерируют memset/RtlZeroMemory/и т. Д. для этого. Для статических/глобальных переменных код не требуется, потому что переменная будет помещена в нулевую инициализированную память.

Answer 3

Инициализирует все элементы массива до 0 за один раз. Вы можете представить себе, как напряженность делает это индивидуально для всех из них.

Answer 4

«Approximayely» «очень долго» версия о том, что вы делаете:

static char vid[25][100]; 
static firstTime = 1; 

if(firstTime) { 
    int ii, jj; 

    for(ii = 0; i < 25; ii++) { 
    for (jj = 0; jj < 100; jj++) { 
     vid[ii][jj] = 0; 
    } 
    } 
    firstTime = 0; 
} 

Это учитывает тот факт, что это должно быть сделано только в самый первый раз, так как переменная объявлена ​​как static. В действительности, как указано в комментариях, ключевое слово static подразумевает инициализацию до нуля и отображение в специальную область, называемую сегментом .bss. Это устанавливается равным нулю загрузчиком программы или каким-либо другим механизмом, даже до того, как будет вызываться main(). Таким образом, код, который запускается, не похож на приведенный выше (обнуление блоков памяти может быть сделано довольно эффективно на большинстве процессоров и не связано с чередованием нескольких измерений!).

Но:

Initialization это то, что происходит до того, как функция main называется, в то время как мой «примерно эквивалентно» код на самом деле выполняется во время выполнения; поэтому, как указал Йенс, это не то же самое. WhozCraig фактически показывает явное initiozation, для которого {0} является сокращением; это лучший ответ.

Answer 5

Длинная версия (и вы будете в затруднении, чтобы получить больше):

static const char vid[25][100] = 
{ 
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}; 

Тем не менее, Децл статична и, следовательно, находится в памяти нулевой инициализируется с самого начала, так что это все несколько избыточно, но по крайней мере вы знаете, как это выглядит.