Binary search on the vector of structs - c++

Question

I have a vector of structs that contains struct with this architecture
struct Main{
int mainID;
string mainDIV;
string mainNAME;
}
is it possible to use binary search on struct? I know its easy to use on value using
binary_search( vector.begin() , vector.end() , 5 )
But is there a way how to pass callback or something to actually find attribute of struct? I fail to find anything related to thi topic.

Answer 1

Yes, it's possible. The value that std::binary_search takes is only meaningful when compared to the elements of the container. In the simple case (if Main supports operator< somewhere), you would provide an element of type Main as the value:
// check if this specific Main exists
bool yes = std::binary_search(v.begin(), v.end(), Main{0, "some", "strings"});
// does exactly the same thing as above
bool yes = std::binary_search(v.begin(), v.end(), Main{0, "some", "strings"}
, std::less<Main>{});
If it doesn't support operator< (or your container is ordered by something else, e.g. mainID), then you will have to provide a comparator yourself that the algorithm will use:
// check if there is a Main with mainID 5
bool yes = std::binary_search(v.begin(), v.end(), 5,
[](const Main& element, const int value) {
return element.mainID < value;
});

Answer 2

You have to provide information to binary_search() to tell it how to compare your objects. The two most common ways, are to either add an operator<() to the struct if that is possible, or provide a helper function that can compare two structs.
The first form would look something like this:
struct Main {
int mainID ;
string mainDIV ;
string mainNAME ;
bool operator<(const Main & other) const
{
return mainID < other.mainID ;
}
}
This will only compare on on mainID, but you can expand it from there.
Also, this only teaches the compiler how to compare two struct Main, while #Barry's answer above will match an int and a struct Main. But lets keep going with this answer.
Now to find the record for 5, we have to make it into a struct Main:
struct Main search_key = { 5 } ;
bool yes = std::binary_search( v.begin(), v.end(), search_key ) ;
Now, this isn't very elegant, and besides if you have a constructor for struct Main ( and haven't put it in your example ), this won't even work. So we add another constructor just for int.
struct Main
{
Main(int id, const string & a_div, const string & a_name ) : id(id), div(a_div), name(a_name) { }
Main(int id) : id(id) { }
int id ;
string div, name ;
bool operator<(const Main &o) const { return id < o.id ; }
} ;
Now we can do a slightly shorter form:
bool has_3 = std::binary_search( v.begin(), v.end(), Main( 3) ) ;
Historical note: Bjarne has been trying for quite some time to get default comparison operators into the standard, but not everyone was excited about it at the standards meetings. I though there was some progress on it at the last meeting and so it may eventually appear when C++17 is a thing.