[英]Constant size array initalization with sizeof
Why does the following work just fine with gcc c99 为什么以下工作在gcc c99上能正常工作
int a[] = {1,2,3};
int b[sizeof a / sizeof *a] = {0};
But this gives compilation errors 但这会导致编译错误
int n = sizeof a / sizeof *a;
int b[n] = {0};
Error 错误
file.c:14:2: error: variable-sized object may not be initialized
file.c:14:2: warning: excess elements in array initializer [enabled by default]
file.c:14:2: warning: (near initialization for 'b') [enabled by default]
n
is a variable unlike sizeof a / sizeof *a
because latter is calculate at compile time. n
是一个不同于sizeof a / sizeof *a
的变量,因为后者是在编译时计算的。
int b[n]
declares a variable length array. int b[n]
声明一个可变长度的数组。 You can't initialize it by using initializer list. 您无法使用初始化列表初始化它。 You can use a loop or
memeset
function to initialize all of its elements to 0
. 您可以使用循环或
memeset
函数将其所有元素初始化为0
。
memset(b, 0, sizeof(b));
The first example works because sizeof a / sizeof *a
is a constant expression, and it's OK to be used as array dimension. 第一个示例起作用是因为
sizeof a / sizeof *a
是一个常量表达式,可以用作数组维。
In the second example, n
is NOT a constant expression, so the compiler treats b
as a definition of variable length array, the error is saying VLAs may not be initialized. 在第二个示例中,
n
不是常量表达式,因此编译器将b
视为可变长度数组的定义,错误是说VLA可能未初始化。
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