How can I create a <string> set that supports C string searching without an intermediary std :: string?
Let's say I have std::set<std::string>
and I want to know if it contains the string "name":
#include <string>
#include <set>
using namespace std;
bool has_name(const set<string> &s) {
return s.find("name") != s.end();
}
The above function constructs and destroys a temporary std :: string with the value "name". This inefficiency seems unnecessary since std :: string has the ability to compare against const char * directly. I would like to remove this temporary.
I tried to use a custom comparator with overloads:
struct str_comp_t {
bool operator()(const string &s1, const char *s2) const {
return s1.compare(s2) < 0;
}
bool operator()(const string &s1, const string &s2) const {
return s1.compare(s2) < 0;
}
};
typedef std::set<string, str_comp_t> string_set_t;
bool has_name_2(const string_set_t &s) {
return s.find("name") != s.end();
}
However, only the variant that accepts std :: string is called; const char * is ignored.
How can I do this constant string comparison directly, instead of creating an intermediate string?
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In C ++ 14, use transparent comparators :
std::set<std::string, std::less<>> s;
// ^^^^^^^^^^^
s.find("Hello"); // no temporary
The transparent comparison predicate std::less<>
is templated operator()
, and the C ++ 14 containers that specialize in transparent predicates expose template overloads find
.
Transparent comparators are strictly selectable so they std::set<std::string>
don't automatically get new overloads.
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