如何从const char16_t *转换为std :: string

Question

在N4606 ISO / IEC JTC1 SC22 WG21的标准文件中， http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/n4606.pdf p.34有一个例子。

long double operator "" _w(long double);
std::string operator "" _w(const char16_t*, std::size_t);
unsigned operator "" _w(const char*);
int main() {
  1.2_w; // calls operator "" _w(1.2L)
  u"one"_w; // calls operator "" _w(u"one", 3) 
  12_w; // calls operator "" _w("12")
  "two"_w; // error: no applicable literal operator
}

如何创建std :: string运算符的示例＆＃34;＆＃34; _w（const char16_t *，std :: size_t）;

如何从const char16_t *转换为std :: string。

其他2个功能是这样的。

long double operator "" _w(long double ld){return ld;}
unsigned operator "" _w(const char*cc){return (unsigned)*cc};