Initialization discards qualifiers from pointer target type

问题

I'm trying to print the list of a singly linked list that I referred to in link text. It works, but I do get the compiler warnings:

Initialization discards qualifiers from pointer target type

(on declaration of start = head) and

return discards qualifiers from pointer target type

(on return statement) in this code:

/* Prints singly linked list and returns head pointer */
LIST *PrintList(const LIST *head) 
{
    LIST *start = head;

    for (; start != NULL; start = start->next)
        printf("%15s %d ea\n", head->str, head->count);

    return head;
}

I am using XCode. Any thoughts?

回答1:

It's this part:

LIST *start = head;

The parameter for the function is a pointer to a constant, const LIST *head; this means you cannot change what it is pointing to. However, the pointer above is to non-const; you could dereference it and change it.

It needs to be const as well:

const LIST *start = head;

The same applies to your return type.

All the compiler is saying is: "Hey, you said to the caller 'I won't change anything', but you're opening up opportunities for that."

回答2:

In following function, would get the warning that you encountered with.

void test(const char *str) {
  char *s = str;
}

There are 3 choices:

Remove the const modifier of param:

void test(char *str) {
  char *s = str;
}

Declare the target variable also as const:

void test(const char *str) {
  const char *s = str;
}

Use a type convert:

void test(const char *str) {
  char *s = (char *)str;
}

来源：https://stackoverflow.com/questions/2316387/initialization-discards-qualifiers-from-pointer-target-type

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linked-list