任何類型的C++用戶定義文字

Question

現在，用戶定義文字接受一組有限的類型作爲輸入參數（請參閱here）。有沒有計劃接受任何類型作爲輸入參數，如果不是爲什麼呢？

例如，我可能希望能夠得到不同的格式（秒，毫秒，等等），一個std ::時辰::持續時間，並會做這樣的事情

constexpr double operator"" _s(std::chrono::nanosecond time) 
{ 
    return std::chrono::duration_cast<std::chrono::duration<double, std::chrono::seconds::period>>(time).count(); 
} 

constexpr long operator"" _us(std::chrono::nanoseconds time) 
{ 
    return std::chrono::duration_cast<std::chrono::microseconds>(time).count(); 
} 

// And so on ... 

int main() 
{ 
    auto t0 = std::chrono::high_resolution_clock::now(); 
    // do some stuff 
    auto t1 = std::chrono::high_resolution_clock::now(); 

    std::cout << "Time in seconds : " << (t1 - t0)_s << "s\n"; 
    std::cout << "Time in microseconds : " << (t1 - t0)_us << "µs\n"; 

    return 0; 
} 

Answer 1

也許你可以做使用輔助結構，而不是：

#include <chrono> 
#include <iostream> 

using namespace std::literals::chrono_literals; 

template <class Duration> 
struct dc { 
    using rep = typename Duration::rep; 
    const std::chrono::nanoseconds time; 
    constexpr dc(std::chrono::nanoseconds time):time(time) { } 
    constexpr operator rep() { 
     return std::chrono::duration_cast<Duration>(time).count(); 
    } 
}; 

using s_ = dc<std::chrono::seconds>; 
using us_ = dc<std::chrono::microseconds>; 

// And so on ... 

template <us_::rep N> 
struct S { 
}; 

int main() 
{ 
    auto t0 = std::chrono::high_resolution_clock::now(); 
    // do some stuff 
    auto t1 = std::chrono::high_resolution_clock::now(); 
    std::cout << "Time in seconds : " << s_(t1 - t0) << "s\n"; 
    std::cout << "Time in microseconds : " << us_(t1 - t0) << "µs\n"; 
    S<us_(10us)> us; 
    (void)us; 
    return 0; 
} 

[live demo]