std::set custom comparator for non null strings - c++

Question

As the title suggests am trying to implement comparison functor for my defined structure. Here is the sample snippet
#include<set>
struct testData
{
char * data;
int size;
};
class compare
{
public:
bool operator()(const testData & lhs,
const testData & rhs) const noexcept
{
return memcmp(lhs.data, rhs.data, lhs.size<rhs.size?lhs.size:rhs.size) < 0;
}
};
int main()
{
std::set<testData,compare>S;
....
return 0;
}
Issue in the comparison function is since am taking lesser size,this
case fails
suppose there is already this data present {"test",4},and i am trying
to find {"test1",5}.it will say as matched.How can i modify comparison
to overcome this?
Update:
changed to this
class compare
{
public:
bool operator()(const testData & lhs,
const testData & rhs) const noexcept
{
if (lhs.size == rhs.size)
return memcmp(lhs.data, rhs.data, lhs.size) < 0;
return lhs.size < rhs.size;
}
};
will this work?

Answer 1

You'd use std::lexicographical_compare.
Lexicographical comparison is a operation with the following properties:
Two ranges are compared element by element.
The first mismatching element defines which range is lexicographically less or greater than the other.
If one range is a prefix of another, the shorter range is lexicographically less than the other.
If two ranges have equivalent elements and are of the same length, then the ranges are lexicographically equal.
An empty range is lexicographically less than any non-empty range.
Two empty ranges are lexicographically equal.
class compare
{
public:
bool operator()(const testData & lhs,
const testData & rhs) const noexcept
{
return std::lexicographical_compare(lhs.data, lhs.data + lhs.size, rhs.data, rhs.data + rhs.size);
}
};

Answer 2

the reason it failes is because you compare only the length of the smaller string.
In your example only the "test" part of "test1" will be compared.
You can try this:
bool operator()(const testData & lhs,
const testData & rhs) const noexcept
{
int ret = memcmp(lhs.data, rhs.data, lhs.size<rhs.size?lhs.size:rhs.size);
return ret == 0 ? lhs.size < rhs.size : ret < 0 ;
}