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;
}