将char指针初始化为字符串，将其他类型指针初始化为数组

Question

I've got a question about initializing char pointers vs other data type pointers. Specifically, we are allowed to initialize char pointers as follows:

char *char_ptr = "Hello World";

As far as I know, the only thing special about a string is that it is a '0' terminated character array. However, we are not allowed to do the following:

int *int_ptr = {1,2,3,4};

but we must do this:

int int_arr[] = {1,2,3,4};
int_ptr = int_arr;

in order to let int_ptr point to the first element of int_array.

In the char case, we have not explicitly defined the string "Hello World" as a char array before letting char_ptr point to the string, but have initialized the char_ptr directly using the string "Hello World".

My question is, why is this the case, what is special about strings that allows us to do this but can't do so with other types?

Thanks in advance,

Sriram

Answer 1

You can do the same in C99 or later for other types using compund literals:

#include <stdio.h>

int main(void) {
    int *ptr = (int[]){ 1, 2, 3, 4 };
    for(int i = 0; i < 4; ++i) {
        printf("%d: %d\n",i, ptr[i]);
    }
    return 0;
}

The compound literal (int[]){ 1, 2, 3, 4 } creates an unnamed int[4] , just like "Hello" creates an unnamed char[6] .

So as of C99, char is only special in providing a more convenient syntax.

Answer 2

The answer is:

Strings(literals) are treated as special.

Believe it! Accept it, and Live with it.

Answer 3

It is because string are used so much that there's an easier way to define them made available. It's way easier than making explicitly an array of character every time.

Answer 4

"Hello World" is a string literal. It has type "array of 12 const char " and static storage duration. That is, it gives you an array in memory that lasts for the duration of your program. It has 12 char because of the null character, which you correctly point out. Now lets consider the two cases.

With const char *char_ptr = "Hello World"; you are relying on the fact that an array can be converted to a pointer to its first element. So your char_ptr points to the first element of the static data that the literal represents.

With const char char_arr[] = "Hello World"; , what happens is a special rule for initializing arrays of characters. What you get is char_arr which is of type "array of 12 const char " again, but the characters from the static string data are used to initialize each element in the array (§8.5.2). That means, your array char_arr is a copy of the static string data.

When we do int *int_ptr = {1,2,3,4}; we get an error. Why? Because {1, 2, 3, 4} isn't like the string literal above. {1, 2, 3, 4} is an initializer, not a literal at all. In fact, it's an initializer list and can only be used to initialize things. It is not a literal. It does not create some static array with the values 1 , 2 , 3 , and 4 in it and then give you a pointer. It is simply used to directly initialize an array. So what would your int_ptr point to? The array does not exist anywhere for it to point to.

However, int int_arr[] = {1,2,3,4}; is the correct use of this initializer list. You get an array, int_arr , where each of the elements are initialized to the elements in the initializer list.

Answer 5

When you set char *char_ptr = "Hello World", you are setting the address of the string literal to char_ptr. But, in case of other data types, there is nothing analogous to string literals. So, it is not allowed.