Custom Compare function for std::binary_search - c++

Question

Is there any problem with this code?
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;
}
}
I am trying to write my own compare function for comparing strings in binarysearch
I am getting following error:
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>)’
Any help is appreciated...

Answer 1

Is there any problem with this code?
bool Spellcheck::smart_comp(string const value, string const key){
return true;
}
Other than it always returns true? Yes, the basic problem is that a member function has an implicit parameter this, so the signature does not match that of the expected predicate. You should be doing this function static or even a free function (friended if needed). Also you are copying the strings each time, it would be best if you take the arguments by const reference to avoid unneeded copies.
In case the real result of the predicate depends on the state of the Spellcheck object, you will have to bind that state to the member function in order to create a function object with the appropiate signature:
std::binary_search(
this->words.begin(), this->words.end()
, std::bind( &Spellcheck::smart_comp, this )
);

Answer 2

You're trying to pass a non-static member function, which is not convertible to the required binary function (on account of having three actual parameters).
Try declaring your smart_comp function static. (Of course then you can't refer to instance members; if you need statefulness, you'll have to write a full functor.)

Answer 3

Assuming this->words's type is std::vector<std::string> and func is a member of Spellcheck, you can work it around declaring smart_comp to be static. But I would think twice on your class design.