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13. Unusual Data Types
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Add explicit redundancies An alternative to using a tag field is to use certain fields twice. If the data in the redundant fields doesn t match, you know memory has been corrupted. This can result in a lot of overhead if you manipulate pointers directly. If you isolate pointer operations in routines, however, it adds duplicate code in only a few places. Use extra pointer variables for clarity By all means, don t skimp on pointer variables. The point is made elsewhere that a variable shouldn t be used for more than one purpose. This is especially true for pointer variables. It s hard enough to figure out what someone is doing with a linked list without having to figure out why one genericLink variable is used over and over again or what pointer->next->last->next is pointing at. Consider this code fragment:
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C++ Example of Traditional Node Insertion Code
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void InsertLink( Node *currentNode, Node *insertNode ) { // insert "insertNode" after "currentNode" insertNode->next = currentNode->next; insertNode->previous = currentNode; if ( currentNode->next != NULL ) { currentNode->next->previous = insertNode; } currentNode->next = insertNode; }
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3 This line is needlessly difficult. 4
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This is traditional code for inserting a node in a linked list, and it s needlessly hard to understand. Inserting a new node involves three objects: the current node, the node currently following the current node, and the node to be inserted between them. The code fragment explicitly acknowledges only two objects insertNode, and currentNode. It forces you to figure out and remember that currentNode->next is also involved. If you tried to diagram what is happening without the node originally following currentNode, you would get something like this:
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A better diagram would identify all three objects. It would look like this:
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Here s code that explicitly references all three of the objects involved:
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C++ Example of More Readable Node-Insertion Code
void InsertLink( Node *startNode, Node *newMiddleNode ) { // insert "newMiddleNode" between "startNode" and "followingNode" Node *followingNode = startNode->next; newMiddleNode->next = followingNode; newMiddleNode->previous = startNode; if ( followingNode != NULL ) { followingNode->previous = newMiddleNode; } startNode->next = newMiddleNode; }
This code fragment has an extra line of code, but without the first fragment s currentNode->next->previous, it s easier to follow.
Simplify complicated pointer expressions Complicated pointer expressions are hard to read. If your code contains expressions like p->q->r->s.data, think about the person who has to read the expression. Here s a particularly egregious example:
CODING HORROR
C++ Example of a Pointer Expression That s Hard to Understand
for ( rateIndex = 0; rateIndex < numRates; rateIndex++ ) { netRate[ rateIndex ] = baseRate[ rateIndex ] * rates->discounts->factors->net; }
Complicated expressions like the pointer expression in this example make for code that has to be figured out rather than read. If your code contains a complicated expression, assign it to a well-named variable to clarify the intent of the operation. Here s an improved version of the example:
C++ Example of Simplifying a Complicated Pointer Expression
quantityDiscount = rates->discounts->factors->net; for ( rateIndex = 0; rateIndex < numRates; rateIndex++ ) { netRate[ rateIndex ] = baseRate[ rateIndex ] * quantityDiscount; }
With this simplification, not only do you get a gain in readability, but you might also get a boost in performance from simplifying the pointer operation inside the loop. As usual, you d have to measure the performance benefit before you bet any folding money on it.
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Draw a picture Code descriptions of pointers can get confusing. It usually helps to draw a picture. For example, a picture of the linked-list insertion problem might look like the one shown in Figure 13-2.
Diag rams such as this can become part of the external documentation of your program. For details on good documentation practices, see 32, SelfDocumenting Code.
CROSS-REFERENCE
Initial Linkage
startNode->next startNode followingNode->previous followingNode
Desired Linkage
startNode->next startNode newMiddleNode->previous newMiddleNode followingNode->previous followingNode newMiddleNode->next
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Figure 13-2 An example of a picture that helps think through the steps involved in relinking pointers.
Free pointers in linked lists in the right order A common problem in working with dynamically allocated linked lists is freeing the first pointer in the list first and then not being able to get to the next pointer in the list. To avoid this problem, make sure that you have a pointer to the next element in a list before you free the current one. Allocate a reserve parachute of memory If your program uses dynamic memory, you need to avoid the problem of suddenly running out of memory, leaving your user and your user s data lost in RAM space. One way to give your program a margin of error is to pre-allocate a memory parachute. Determine how much memory your program needs to save work, clean up, and exit gracefully. Allocate that amount of memory at the beginning of the program as a reserve parachute, and leave it alone. When you run out of memory, free the reserve parachute, clean up, and shut down. Free pointers at the same scoping level as they were allocated Keep allocation and deallocation of pointers symmetric. If you use a pointer within a single scope, call new to allocate and delete to deallocate the pointer within the same scope. If you allocate a pointer inside a routine, deallocate it inside a sister routine. If you allocate a pointer inside an object s constructor,
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