为什么恒定初始化需要静态char *而不是静态char ** [英] Why is constant initialization need for static char* but not static char**
问题描述
有人可以解释一下为什么这样的代码...
Can someone please explain why this code...
// main.c
#include <stddef.h>
static const int g_a = 1;
static const char* g_b = "hello";
static const char* g_c[] = { "a", "b", NULL };
typedef struct Foo
{
int a;
const char* b;
const char** c;
} Foo;
static Foo f[] =
{
{ g_a,
g_b,
g_c }
};
int main( int argc, char* argv[] )
{
return 0;
}
...产生此错误：
> gcc --version && gcc -g main.c
gcc (GCC) 8.2.1 20181215 (Red Hat 8.2.1-6)
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
main.c:19:5: error: initializer element is not constant
g_b,
^~~
main.c:19:5: note: (near initialization for 'f[0].b')
我了解编译器正在要求常数来初始化 f [0] .b ，因此以下初始化是解决编译错误的方法：
I understand that the compiler is asking for a constant to initialize f[0].b, so the following initialization is a solution to the compile error:
static Foo f[] = { { g_a, "hello", g_c } };
但是为什么编译器在初始化以下内容时不发出类似的 constant required错误 f [0] .c ？（或者就此而言， f [0] .a ？）为什么这只是 f [0] .b 的问题？
But why does the compiler not issue a similar "constant required" error for the initialization of f[0].c? (Or, for that matter, f[0].a?) Why is this only an issue for f[0].b?
推荐答案
f [0] .a 类型为非恒定 int ，并由类型为 const int 的 g_a 值初始化，该值在运行时无法更改，并且在编译时已知。因此，这里没有错误。
f[0].a has type "non-constant int" and is initialized by the value of g_a that is of type "const int" which cannot change at run time and is known at compile time. So no error here.
f [0] .b 具有类型指向<$ c的非恒定指针$ c> const char ，并应使用 g_b 的值进行初始化，该值也是 g_b 可以在运行时更改，并且IIRC的初始化顺序也不确定。因此它的值在编译时未知，因此会出错。
f[0].b has type "non-constant pointer to const char" and should be initialized by the value of g_b that is also a "non-constant pointer to const char". Even if it has an initializer g_b can change at run time, and IIRC the initialization sequence is not determined. So its value is not known at compile time, hence the error.
f [0] .c 具有类型指向 const char 的非恒定指针，并由 g_c 初始化一个数组类型为 << c $ c> const char 的非恒定指针的元素的数组。数组的符号可以用作在编译时已知的常量指针。因此，这里没有错误。
f[0].c has type "non-constant pointer to non-constant pointer(s) to const char" and is initialized by g_c that is an array of elements of type "non-constant pointer to const char". The symbol of an array can be used as a constant pointer which is known at compile time. So no error here.
我想这就是您所缺少的：如果您希望将指针设为 const ，将修饰符放在指针的 * 后面，而不是指向的值，如下所示： [const ] char * const指针。
This is what you are missing, I think: If you want a pointer to be const, place the modifier at the pointer, behind the *, not at the pointed value, like this: [const] char * const pointer.
这篇关于为什么恒定初始化需要静态char *而不是静态char **的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
