C++14 allows to lookup into a map or set without creating a temporary key when the provided needle is convertible to the key

heterogeneous_lookup.cpp

// See https://www.bfilipek.com/2019/05/heterogeneous-lookup-cpp14.html

#include <map>
#include <string>

std::map<std::string, int> m1;

int f1() {
    // Constructs a temporary string for key.
    auto it = m1.find("foo"); 
    if (it != m1.end()) {
        return it->second;
    }
    return 0;
}

std::map<std::string, int, std::less<>> m2;

// std::less<> enables heterogeneous lookup because it has a
// is_transparent member type which allows comparing different operands of
// different types and enables additional find() overloads.

template<class T>
constexpr bool is_heterogeneous_compare(typename T::key_compare::is_transparent*) {
    return true;
}

template<class T>
constexpr bool is_heterogeneous_compare(...) {
    return false;
}

static_assert(!is_heterogeneous_compare<decltype(m1)>(0));
static_assert(is_heterogeneous_compare<decltype(m2)>(0));

int f2() {
    // Heterogeneous lookup: uses const char[] as key => no temporary string
    auto it = m2.find("foo"); 
    if (it != m2.end()) {
        return it->second;
    }
    return 0;
}