What are good strategies in C for dealing with pointer errors of this type?

Question

In C, I often come across errors with code like this

prio_queue->head[index] = newEntry;  

Yields a " "Cannot assign Entry** to Entry* " How do you prevent/debug errors of this type, in general?

Answer 1

If head is an array of elements, and index is < size of that array, and newEntry is of the same type as the array, and prio_queue is a pointer to a data structure containing head, then there nothing wrong with this code.

A lot of ifs, but without knowing your data structures, I (and no one else) can tell if it's right or wrong.

Update: Based on the update to the question, a strategy would be to keep careful track of how many levels of indirection a variable has.

Answer 2

There are two types of problems you may run into:

1. The compilation level where pointer data types are not matching

2. The run time issues where array bounds overflows and hence either violates the memory access or corrupts the internal data structure which will result in memory problem later in the execution.

Your problem looks like compilation problem than run time so it looks more like data type mismatch problem mostly but we need more information to dig up this. Please elaborate.

Answer 3

I've been away from a C compiler for quite a few years, but isn't it telling you that prio_queue->head[index] is a pointer to Entry and newEntry is a pointer to a pointer to Entry? Try:

prio_queue->head[index] = *newEntry;

Answer 4

How do you prevent/debug errors of this type, in general?

This particular issue comes down to knowing the types of the respective expressions, and there are no shortcuts to that; either you know it or you don't. The question to answer is why the types are different in the first place; that indicates that someone was a bit confused when they wrote the code.