C++11 新特性之operator "" xxx

从C++11开始，我们可以使用以下形式通过常量字符串构造自定义类型， 比如：

class Person
{
public:
    Person(const std::string& name): _name(name){}
    std::string name() const {
        return _name;
    }
private:
    std::string name;
}

inline Person operator "" _psn (const char *name, size_t n)
{
    return Person(std::string(name, n));
}

int main()
{
    auto person = "Xiao Ming"_psn;
    std::cout<< "The person' name is " << person.name() << std::endl;
    return 0;
}

Output:

The person' name is Xiao Ming

在std::literals::chrono_literals的实现用，也用了类似的手段：

constexpr std::chrono::milliseconds operator "" ms(unsigned long long ms);

实现如下：

constexpr std::chrono::milliseconds operator ""ms(unsigned long long ms)
{
    return chrono::milliseconds(ms);
}

例子：

#include <iostream>
#include <chrono>
 
int main()
{
    using namespace std::chrono_literals;
    auto d1 = 250ms;
    std::chrono::milliseconds d2 = 1s;
    std::cout << "250ms = " << d1.count() << " milliseconds\n"
              << "1s = " << d2.count() << " milliseconds\n";
}

Output：

250ms = 250 milliseconds
1s = 1000 milliseconds