User-defined literals [C++]

literals.cpp

#include <iostream>
#include <string>

using namespace std;

auto operator "" _x2(unsigned long long number) {
	return number * 2;
}

string operator "" _upper(char const * str, size_t size) {
	string output = str;
	transform(output.begin(), output.end(), output.begin(), ::toupper);
	return output;
}

string operator "" _lower(char const * str, size_t size) {
	string output = str;
	transform(output.begin(), output.end(), output.begin(), ::tolower);
	return output;
}

// Metric system conversion
auto operator "" _km(uint64_t km) { return km * 1000.0; }
auto operator "" _hm(uint64_t hm) { return hm * 100.0; }
auto operator "" _dam(uint64_t dam) { return dam * 10.0; }
auto operator "" _m(uint64_t meters) { return meters; }
auto operator "" _dm(uint64_t dm) { return dm / 10.0; }
auto operator "" _cm(uint64_t cm) { return cm / 100.0; }
auto operator "" _mm(uint64_t mm) { return mm / 1000.0; }

auto operator "" _km(long double km) { return km * 1000.0; }
auto operator "" _hm(long double hm) { return hm * 100.0; }
auto operator "" _dam(long double dam) { return dam * 10.0; }
auto operator "" _m(long double meters) { return meters; }
auto operator "" _dm(long double dm) { return dm / 10.0; }
auto operator "" _cm(long double cm) { return cm / 100.0; }
auto operator "" _mm(long double mm) { return mm / 1000.0; }

long double km(const long double& metres) { return metres / 1000.0; }
long double hm(const long double& metres) { return metres / 100.0; }
long double dam(const long double& metres) { return metres / 10.0; }
long double m(const long double& metres) { return metres; }
long double dm(const long double& metres) { return metres * 10.0; }
long double cm(const long double& metres) { return metres * 100.0; }
long double mm(const long double& metres) { return metres * 1000.0; }

int main(int argc, char *argv[]) {
	
	cout << 10_x2 << endl;	
	
	cout << "hello"_upper << endl;
	
	cout << "How are you"_upper << endl;
	
	cout << "THIS IS SPARTA!"_lower << endl;
	
	cout << 10_km << " m" << endl;
	cout << 1_km + 1_m + 1_cm << " m" << endl;
	cout << km(1_km + 1_m + 1_cm) << " km" << endl;
	cout << cm(1_km + 10_m) << " cm" << endl;
	cout << km(1_cm) << " km" << endl;
	
	cout << km(100_hm) << " km" << endl;
	
	cout << m(100_km) << " m" << endl;
}