Question

I encountered a for loop where the condition is the parameter (p) itself. When will the loop stop? I don’t see this case in my C++ books.

for (PDFS *p = e->prev ; p ; p = p->prev) {
    push_back (p->edge);
    edge[p->edge->id] = vertex[p->edge->from] = vertex[p->edge->to] = 1;
}

Answer 1

The loop will stop when p is NULL. In the loop, you don’t need to explicitly check for the condition p !=NULL, or in C++11, p != nullptr.

The similar code is also written you’ve null-terminated c-string:

 char str[] = "its null-terminated string";
 for(size_t i = 0 ; str[i] ;  ++i)
        std::cout << str[i] << std::endl;

In this case, you don’t need to check for the condition i < strlen(str) or something like that.