在MFC CArray上的std :: reverse

Question

I have an array of points like this:

CArray<CPoint,CPoint> points;

And I need to reverse the order of the points. I've tried this method:

std::reverse( &points[0], &points[0] + points.GetSize() );

And it works. Then I've tried this other method:

std::reverse( &points[0], &points[points.GetUpperBound()] );

But it doesn't work: the last item is not ordered correctly. Why?

Answer 1

That is because STL algorithms take ranges in the form [b, e) (that is, e exclusive), whereas the function you used returns the position of the last actual last element .

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It should be further noted that your second form is even more problematic in the case where the array is empty. The function, according to the documentation, returns -1 in this case. BOOM!

Answer 2

First, while STL's algorithms (including std::reverse() ) are designed to work fine with STL containers or STL-compatible containers (ie those providing STL-compatible iterators), I'm not sure about combining them with MFC containers .
Certainly, MFC containers were not designed with STL algorithm compatibility in mind.

I'd suggest to move your code from using MFC containers like CArray to more modern containers like std::vector .

That said, in the second case here:

 std::reverse( &points[0], &points[points.GetUpperBound()] ); 

the second "iterator" argument that you pass to std::reverse() is not pointing to one-past the last valid item (like in the first case of &points[0] + points.GetSize() ), but it's actually pointing to the last valid item .

In fact, CArray::GetUpperBound() returns the last valid index (from MSDN documentation):

Because array indexes are zero-based, this function returns a value 1 less than GetSize .

You may be tempted to use something like &points[points.GetSize()] or &points[points.GetUpperBound() + 1] , but those would fail, since CArray overloads operator[] , implementing bound checking at least in debug builds.
And with these aforementioned alternatives, you end up using an index out of the valid range.

But, let me repeat: consider moving your code from CArray to std::vector . You can still use MFC for the front-end GUI of your application; but for the "core" of your application, for the "business logic", using modern C++ and STL containers is a better option.

Answer 3

文档说GetUpperBound()返回最后一个元素的索引，因此&points[points.GetUpperBound()]表示最后一个元素的迭代器，而STL算法需要半开范围，如[begin, end) ，即end必须直接指向最后一个元素 。