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  • 指定VC中std::sort的比较函数时发生"invalid operator<"错误原因

    在用stl的sort模板函数的时候遇到一个运行时错误,觉得很诡异,搜索了一下,原来VC05以后的版本会运行时检查比较函数是否为strict weak ordering,由此顺便了解了下strict weak ordering。而VC的运行时检查机制也耐人寻味。此为文章链接:

    http://hi.baidu.com/haochaoqing/item/00b40cf2b8c4efc0a835a255

    http://support.microsoft.com/kb/949171

    把第二篇的内容贴出来如下:

    Action

    Sort any STL collection using stable_sort() or sort() with duplicate items using the following custom predicate

    bool CustPredicate (int elem1, int elem2 )
    {
        if(elem1 > elem2)
            return true; 

        if (elem1 < elem2)
            return false;
        return true;
    }

    Result

    It works fine in Visbual C++ 2002 and Visual C++ 2003.
    It throws and assertion failed error, "Expression: invalid operator < " in Visual C++  2005 and Visual C++ /2008.
     

    Cause

    The STL algorithms for stable_sort() and sort() require the binary predicate to be strict weak ordering.

    For example: 

    · Strict: pred(X, X) is always false.

    · Weak: If !pred(X, Y) && !pred(Y, X), X==Y.

    · Ordering: If pred(X, Y) && pred(Y, Z), then pred(X, Z).

    Resolution

    Below either of the options will resolve the issue

    First Option:-

    bool CustPredicate (int elem1, int elem2 )
    {
        if(elem1 > elem2)
            return true; 

        if (elem1 < elem2)
            return false;

        return false; //Should return false if both the vaules are same
    }

    Second Option:-
    bool CustPredicate (int elem1, int elem2 )
    {
        return elem1 > elem2;
    }
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  • 原文地址:https://www.cnblogs.com/waytofall/p/2844750.html
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