C - Setting a struct to null (incompatible types in assignment)

问题

I have the following struct:

struct elem {
  int number;
  char character;
};

struct item {
  struct elem element;
};

and the following function:

void init(struct item *wrapper) {
  assert(wrapper != NULL);
  wrapper->element = NULL;
}

item->element = NULL yields a incompatible types in assignment. Why is that? Shouldn't setting a struct to NULL be okay?

回答1:

In C NULL is generally defined as the following

#define NULL ((void*)0)

This means that it's a pointer value. In this case your attempting to assign a pointer (NULL) to a non-pointer value item::element and getting the appropriate message. It seems like your intent is to have element be a pointer here so try the following

struct item {
  struct elem* element;
};

回答2:

NULL is a pointer value, wrapper->element is not a pointer, therefore you cannot assign it NULL

回答3:

In addition to the previous answers, sometimes it makes sense to create a NULL struct. http://en.wikipedia.org/wiki/Null_Object_pattern

回答4:

1. element is not a pointer and hence cannot be assigned NULL.
2. main->element is wrong usage. You cannot access a structure's element using it's name. It should be wrapper->element. You should access it using the variable name.

来源：https://stackoverflow.com/questions/11416359/c-setting-a-struct-to-null-incompatible-types-in-assignment