c++ - How to use `duration_cast` for a derived class? -


i'm cleaning timer class using std::chrono. else went smooth except cannot seem apply duration_cast derived classes. well, made work in different way, i'd still know missed.

see here error message.

#include <iostream> #include <chrono>  typedef std::chrono::high_resolution_clock clock;  class milliseconds : public std::chrono::milliseconds { public:     typedef std::chrono::milliseconds base;     typedef base::rep type;      using base::base; };  inline milliseconds::type millisecondssinceepoch() {     return std::chrono::duration_cast<milliseconds::base>(clock::now().time_since_epoch()).count();     //duration_cast<milliseconds> error! }  int main() {     using namespace std;     cout << millisecondssinceepoch() << endl;     return 0; }

the standard duration_cast can used convert specialization of duration. [time.duration.cast]/p1:

template <class toduration, class rep, class period> constexpr toduration duration_cast(const duration<rep, period>& d);

remarks: function shall not participate in overload resolution unless toduration instantiation of duration.

if toduration not specialization of duration, signature removed overload resolution (by sfinae or similar technique), , call fails unless function template called duration_cast available.


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