It might seem limiting that FLET can't be used to define recursive functions, but Common Lisp provides both FLET and LABELS because sometimes it's useful to be able to write local functions that can call another function of the same name, either a globally defined function or a local function from an enclosing scope. Because the local functions can refer to variables from the enclosing scope, they can often be written to take fewer parameters than the equivalent helper functions. This is particularly handy when you need to pass a function that takes a single argument as a functional parameter. For example, in the following function, which you'll see again in Chapter 25, the FLETed function, count-version, takes a single argument, as required by walk-directory, but can also use the variable versions, introduced by the enclosing LET:
Also, sort is generic, so that it can be used for any reasonable combination of container type, element type, and comparison criterion. The primary reason that sort tends to outperform qsort is that the comparison inlines better.
What is a function object? An object that in some way behaves like a function, of course. Typically, that would mean an object of a class that defines the application operator - operator.
A function object is a more general concept than a function because a function object can have state that persist across several calls like a static local variable and can be initialized and examined from outside the object unlike a static local variable.
Function objects are extensively used to provide flexibility in the standard library. By writing code that doesn't have any.
Clearly, if your code has new operations, delete operations, and pointer arithmetic all over the place, you are going to mess up somewhere and get leaks, stray pointers, etc. This is true independently of how conscientious you are with your allocations: It follows that successful techniques rely on hiding allocation and deallocation inside more manageable types.
Good examples are the standard containers.
They manage memory for their elements better than you could without disproportionate effort. Consider writing this without the help of string and vector: What would be your chance of getting it right the first time? And how would you know you didn't have a leak?
Note the absence of explicit memory management, macros, casts, overflow checks, explicit size limits, and pointers. By using a function object and a standard algorithm, I could have eliminated the pointer-like use of the iterator, but that seemed overkill for such a tiny program.
These techniques are not perfect and it is not always easy to use them systematically. However, they apply surprisingly widely and by reducing the number of explicit allocations and deallocations you make the remaining examples much easier to keep track of.
As early asI pointed out that by reducing the number of objects that I had to keep track of explicitly from many tens of thousands to a few dozens, I had reduced the intellectual effort needed to get the program right from a Herculean task to something manageable, or even easy.
If your application area doesn't have libraries that make programming that minimizes explicit memory management easy, then the fastest way of getting your program complete and correct might be to first build such a library.
Templates and the standard libraries make this use of containers, resource handles, etc. The use of exceptions makes it close to essential. Here is an example where I need to return an object allocated on the free store from a function.
This is an opportunity to forget to delete that object.
After all, we cannot tell just looking at pointer whether it needs to be deallocated and if so who is responsible for that. Think about resources in general, rather than simply about memory. If systematic application of these techniques is not possible in your environment you have to use code from elsewhere, part of your program was written by Neanderthals, etc.
Why can't I resume after catching an exception? Basically, someone resuming from an exception handler can never be sure that the code after the point of throw was written to deal with the execution just continuing as if nothing had happened.
An exception handler cannot know how much context to "get right" before resuming. To get such code right, the writer of the throw and the writer of the catch need intimate knowledge of each others code and context.Based on our collection of resume samples, Machine Operators usually hold a high school diploma and complete on-the-job training.
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