c++ - 我的代码中的 binary_search 谓词有什么问题

Question

我正在阅读有关 binary_search 的内容，然后我尝试使用谓词来实现它。这是我的代码(我还包括了我正在使用的排序谓词)。我知道小于是默认值。这是粗略的测试代码

class person
{
public:
    int age;
};

//sort predicate
class less_than_key
{
public:
    inline bool operator()(const person& pa , const person& pb)
    {
        return (pa.age < pb.age);
    }
};

//Binary Search predicate
class bsearch_predicate
{
    public:
    bool operator()(const person& pa)
    {
        return pa.age == n;
    }
    bsearch_predicate(int i):n(i) {}
    int n;
};

实现

    person p;
p.age = 24;

std::vector<person> vec;
vec.push_back(p);

std::sort(vec.begin(),vec.end(),less_than_key());

std::binary_search(vec.begin(),vec.end(),bsearch_predicate(24));

现在这里的 binary_search 会产生错误，但是如果我这样尝试使用 std::find_if

std::find_if(vec.begin(),vec.end(),bsearch_predicate(24));

以上作品。如果有人能告诉我为什么我的代码中出现 std::binary_search 链接器错误，我将不胜感激。链接器错误是:

Error   12  error C2676: binary '<' : 'const bsearch_predicate' does not define this operator or a conversion to a type acceptable to the predefined operator   d:\program files (x86)\microsoft visual studio 10.0\vc\include\algorithm    2978    1
Error   4   error C2784: 'bool std::operator <(const _Elem *,const std::basic_string<_Elem,_Traits,_Alloc> &)' : could not deduce template argument for 'const _Elem *' from 'const bsearch_predicate'  d:\program files (x86)\microsoft visual studio 10.0\vc\include\algorithm    2978    1
Error   10  error C2784: 'bool std::operator <(const std::_Revranit<_RanIt,_Base> &,const std::_Revranit<_RanIt2,_Base2> &)' : could not deduce template argument for 'const std::_Revranit<_RanIt,_Base> &' from 'const bsearch_predicate' d:\program files (x86)\microsoft visual studio 10.0\vc\include\algorithm    2978    1
Error   7   error C2784: 'bool std::operator <(const std::_Tree<_Traits> &,const std::_Tree<_Traits> &)' : could not deduce template argument for 'const std::_Tree<_Traits> &' from 'const bsearch_predicate'  d:\program files (x86)\microsoft visual studio 10.0\vc\include\algorithm    2978    1
Error   3   error C2784: 'bool std::operator <(const std::basic_string<_Elem,_Traits,_Alloc> &,const _Elem *)' : could not deduce template argument for 'const std::basic_string<_Elem,_Traits,_Alloc> &' from 'const bsearch_predicate'    d:\program files (x86)\microsoft visual studio 10.0\vc\include\algorithm    2978    1
Error   5   error C2784: 'bool std::operator <(const std::basic_string<_Elem,_Traits,_Alloc> &,const std::basic_string<_Elem,_Traits,_Alloc> &)' : could not deduce template argument for 'const std::basic_string<_Elem,_Traits,_Alloc> &' from 'const bsearch_predicate'  d:\program files (x86)\microsoft visual studio 10.0\vc\include\algorithm    2978    1
Error   1   error C2784: 'bool std::operator <(const std::list<_Ty,_Ax> &,const std::list<_Ty,_Ax> &)' : could not deduce template argument for 'const std::list<_Ty,_Ax> &' from 'const bsearch_predicate' d:\program files (x86)\microsoft visual studio 10.0\vc\include\algorithm    2978    1
Error   2   error C2784: 'bool std::operator <(const std::move_iterator<_RanIt> &,const std::move_iterator<_RanIt2> &)' : could not deduce template argument for 'const std::move_iterator<_RanIt> &' from 'const bsearch_predicate'    d:\program files (x86)\microsoft visual studio 10.0\vc\include\algorithm    2978    1
Error   11  error C2784: 'bool std::operator <(const std::pair<_Ty1,_Ty2> &,const std::pair<_Ty1,_Ty2> &)' : could not deduce template argument for 'const std::pair<_Ty1,_Ty2> &' from 'const bsearch_predicate'   d:\program files (x86)\microsoft visual studio 10.0\vc\include\algorithm    2978    1
Error   9   error C2784: 'bool std::operator <(const std::reverse_iterator<_RanIt> &,const std::reverse_iterator<_RanIt2> &)' : could not deduce template argument for 'const std::reverse_iterator<_RanIt> &' from 'const bsearch_predicate'   d:\program files (x86)\microsoft visual studio 10.0\vc\include\algorithm    2978    1
Error   8   error C2784: 'bool std::operator <(const std::unique_ptr<_Ty,_Dx> &,const std::unique_ptr<_Ty2,_Dx2> &)' : could not deduce template argument for 'const std::unique_ptr<_Ty,_Dx> &' from 'const bsearch_predicate' d:\program files (x86)\microsoft visual studio 10.0\vc\include\algorithm    2978    1
Error   6   error C2784: 'bool std::operator <(const std::vector<_Ty,_Ax> &,const std::vector<_Ty,_Ax> &)' : could not deduce template argument for 'const std::vector<_Ty,_Ax> &' from 'const bsearch_predicate'   d:\program files (x86)\microsoft visual studio 10.0\vc\include\algorithm    2978    1

Answer 1

binary_search 函数不采用相等运算符，而是采用定义顺序的不等式 迭代器。您必须将谓词建模为小于比较。

原因是binary_search 算法需要 了解参数的相对顺序和它正在查看的元素，以决定继续搜索的方向。当 pred(*it,value) 和 pred(value,*it) 都不为真(如果两个值都不小于另一个，则它们是一样)