[Effective-cpp] Item 1: Uses and Abuses of vector

[Gregory Haley]

> ...then I guess the call to vint.end() may just be 
> significant. But I don't really like the usual solutions:
> 
> Solution 1, const iterator outside the loop:
> const vector<int>::const_iterator end = vint.begin(); for 

Did you mean end = vint.end()  here?

> (vector<int>::const_iterator it = vint.begin(); it != end; ++it)
> 
> Solution 2, non-const iterator inside the loop:
> for (vector<int>::const_iterator it = vint.begin(), end = 
> vint.end(); it != end; ++it)
> 
> I don't like solution 1 because the iterator 'end' has too 
> large a scope. I only want it to be accessible up to the end 
> of the loop, not to the end of the enclosing block. I don't 
> like solution 2 because the iterator is not const. The 
> compiler would let me accidentally change its value inside 
> the loop. Other languages (I think Fortran is one of them) 
> solve this problem by having the language specification say 
> that the compiler calculates the step and end values just 
> once at the start of the loop and uses a saved copy of the 
> values in each iteration.
> Programmers need to be aware that these loop-control 
> parameters never change even if the variables used do change. 
> But that's not a problem as it's just part of the language. 
> Not sure what C-family programmers would think of this idea.


I must confess, I took Herb at his word on this when I brought it up for
summary, but the discussion caused me to examine the question more.

It made me curious about what the performance might be, so I wrote a couple
of little programs to build up a really long vector of a million ints, then
loop through it and print out the values (dumping the output to /dev/null)
and timed it.  There was no appreciable difference between the two variants
-- indeed what I had thought would be the more efficient loop was slightly
slower.  The platform was linux, the compiler g++ 3.2.3.  So my guess would
be that this compiler does not recalculate the value of the end iterator
each time.

On your observation about the scope, setting a vint.end() to have a larger
scope than just within the loop, I can foresee dangerous curves ahead, if
someone altered the vector after setting a global end, and then the range
would no longer be correct.  The programmer would have to be very careful to
reset it each time.

On another point raised, I would use this sort of loop for a vector that I
intend to modify, especially if the modification involved removing items.  I
learned this the difficult way, once trying to iterate through a vector to
remove elements, and at times, the removal would blow the "end iterator"
away.  

ciao!
greg.

Gregory Haley

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