Question

Here is the structure declare code.

struct list_el {
    int val;
    struct list_el * next;
};

typedef struct list_el item;

And when I write a function like this, the compiler gives a error. It says cur undeclared before first use.

bool delete(item* item)
{
    assert(item != NULL);

    item* cur = NULL;
    cur = head;
    item* prev = NULL;
    while (cur) {
        if (cur == item) {
            if (prev == NULL) {
                head = item->next;
            } else {
                prev->next = item->next;
            }
            free(item);
            return true;
        }

        prev = cur;
        cur = cur->next;
    }
    return false;
}

After I look up the reference, it says the typedef works out just a bit like #define. It simply makes a substitution at compile time. Is that the reason the code can’t be compiled?

Answer 1

In this code:

bool delete(item* item)
{
    item *cur = NULL;

the item in the third line is taken to be the name of the variable item (the parameter to the function), and not the type. Consequently, the third line looks as if it starts out as an expression that multiplies item by the undefined variable cur, which leads to problems; the rest of the expression is also bogus.

If this isn’t what you want, don’t use the same name for a type and a variable. You’ll confuse other people even if you don’t confuse yourself and the compiler.

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Whichever reference source said that typedef and #define are ‘the same’ should be dropped from your list of references now! If it can’t differentiate two such fundamentally different constructs, it is dangerous because you won’t know when it is misleading you (but this is one case where it is misleading you).