c++ - std::binary_search 的自定义比较函数

Question

这段代码有什么问题吗？

bool Spellcheck::smart_comp(string value, string key){
    return true;
}

void func(){
    std::string aprox_key = "hello";
    if(std::binary_search(this->words.begin(), this->words.end(), aprox_key, smart_comp)){
        std::cout << "Found" << std::endl;
    }
}

我正在尝试编写自己的比较函数来比较二进制搜索中的字符串

我收到以下错误:

xyz.cpp:40:85: error: no matching function for call to ‘binary_search(std::vector<std::basic_string<char> >::iterator, std::vector<std::basic_string<char> >::iterator, std::string&, <unresolved overloaded function type>)’
xyz.cpp:40:85: note: candidates are:
/usr/include/c++/4.6/bits/stl_algo.h:2665:5: note: template<class _FIter, class _Tp> bool std::binary_search(_FIter, _FIter, const _Tp&)
/usr/include/c++/4.6/bits/stl_algo.h:2698:5: note: bool std::binary_search(_FIter, _FIter, const _Tp&, _Compare) [with _FIter = __gnu_cxx::__normal_iterator<std::basic_string<char>*, std::vector<std::basic_string<char> > >, _Tp = std::basic_string<char>, _Compare = bool (Spellcheck::*)(std::basic_string<char>, std::basic_string<char>)]
/usr/include/c++/4.6/bits/stl_algo.h:2698:5: note:   no known conversion for argument 4 from ‘<unresolved overloaded function type>’ to ‘bool (Spellcheck::*)(std::basic_string<char>, std::basic_string<char>)’

感谢任何帮助...

Answer 1

Is there any problem with this code?

bool Spellcheck::smart_comp(string const value, string const key){
  return true;
}

除了它总是返回 true 之外？是的，基本问题是成员函数有一个隐式参数 this，因此签名与预期谓词的签名不匹配。您应该执行此函数 static 或什至是一个免费函数(如果需要，请编辑 friend)。此外，您每次都在复制 strings，最好通过 const 引用获取参数以避免不需要的拷贝。

如果谓词的实际结果取决于 Spellcheck 对象的状态，您必须将该状态绑定(bind)到成员函数，以便创建具有适当签名的函数对象:

std::binary_search(
    this->words.begin(), this->words.end()
  , std::bind( &Spellcheck::smart_comp, this )
);