Can't initialize array with struct containing function pointer -error: initializer element is not constant

Question

我正在尝试初始化结构数组。该结构包含一个函数指针作为其数据成员之一。但是我的编译器给我一个错误，抱怨 The initializer element is not constant。如何使用声明的函数指针初始化数组？

typedef void (*write_func_ptr_t)( void**, size_t*, char*, const size_t);
typedef bool (*read_func_ptr_t)( char*, const void*, const size_t);

write_func_ptr_t generate_basic_msg_ptr;
read_func_ptr_t handle_basic_msg_ptr;
write_func_ptr_t generate_reg_msg_ptr;
read_func_ptr_t handle_reg_msg_ptr;

struct supported_msg_info
{
    const char* const type;
    const write_func_ptr_t write_func;
    const read_func_ptr_t read_func;
};

static struct supported_msg_info SUPP_MESSAGES[] = {
    { "basic", generate_basic_msg_ptr, handle_basic_msg_ptr },
    { "registration", generate_reg_msg_ptr, handle_reg_msg_ptr }
};

Answer 1

generate_basic_msg_ptr 和 friends 只是甚至没有初始化的变量（好吧，严格来说它们可能已经被初始化为零，但这可能不是你的意图）。所以它们没有编译时定义的值，显然不能用于数组初始化。您可能想要声明一个函数，如 void func(<whatever>);，并在初始化中使用 func。

Answer 2

你可以这样做，添加函数声明并使它们匹配函数指针类型，就像这样

void generate_basic_msg_ptr(void **, size_t *, char *, const size_t);
bool handle_basic_msg_ptr(char *, const void *, const size_t);
void generate_reg_msg_ptr(void **, size_t *, char *, const size_t);
bool handle_reg_msg_ptr(char *, const void *, const size_t);

但是根据你的评论

The definitions of the functions are going to be pulled in at runtime from a shared library using dlsym – agranum 59 secs ago

你真正需要的是这个

struct supported_msg_info
{
    const char *const type;
    /* remove the const qualifier */
    write_func_ptr_t write_func;
    read_func_ptr_t read_func;
};

static struct supported_msg_info SUPP_MESSAGES[] = {
    {"basic", NULL, NULL},
    {"registration", NULL, NULL}
};

然后当您调用 dlsym 时，您可以这样做

SUPP_MESSAGES[0].write_func = dlsym(dlhandle, "symbol_name");

函数 poitner SUPP_MESSAGES[0].write_func 然后将使用您的 struct 定义和函数指针 typedef 指定的签名调用，请记住 void * 可转换为无需转换的任何类型的指针，所以这就是您所需要的。

Can't initialize array with struct containing function pointer -error: initializer element is not constant

Can't initialize array with struct containing function pointer -error: initializer element is not constant

c

arrays

function-pointers

constants

array-initialization