User-defined literals

cppreference User-defined literals

microsoft User-defined literals

// UDL_Distance.cpp

#include <iostream>
#include <string>

struct Distance
{
private:
    explicit Distance(long double val)
            : kilometers(val)
    {
    }

    friend Distance operator"" _km(long double val);
    friend Distance operator"" _mi(long double val);

    long double kilometers { 0 };
    public:
    const static long double km_per_mile;
    long double get_kilometers()
    {
        return kilometers;
    }

    Distance operator+(Distance other)
    {
        return Distance(get_kilometers() + other.get_kilometers());
    }
};

const long double Distance::km_per_mile = 1.609344L;

Distance operator"" _km(long double val)
{
    return Distance(val);
}

Distance operator"" _mi(long double val)
{
    return Distance(val * Distance::km_per_mile);
}

int main()
{
    // Must have a decimal point to bind to the operator we defined!
    Distance d { 402.0_km }; // construct using kilometers
    std::cout << "Kilometers in d: " << d.get_kilometers() << std::endl; // 402

    Distance d2 { 402.0_mi }; // construct using miles
    std::cout << "Kilometers in d2: " << d2.get_kilometers() << std::endl;  //646.956

    // add distances constructed with different units
    Distance d3 = 36.0_mi + 42.0_km;
    std::cout << "d3 value = " << d3.get_kilometers() << std::endl; // 99.9364

    // Distance d4(90.0); // error constructor not accessible

    std::string s;
    std::getline(std::cin, s);
    return 0;
}
// g++ --std=c++11 test.cpp

Use case

nlohmannjson中就使用了literal。

chrono literals

chrono literals

std::literals::chrono_literals::operator""ms