在模板中，有没有办法为每个计时实例只编写一个专业化？ （纳秒、毫秒、秒等）

Question

我有一个模板需要使用以下类型：int、float、double、std::chrono::nanoseconds、std::chrono::milliseconds 和 std::chrono::seconds。

该模板有一个成员函数可与int、float 和double 一起使用，但我需要编写一个专门针对纳秒、另一个针对毫秒和另一个针对秒。

为了得到一个工作代码，我不得不为每个std::chrono“类型”编写一个专业化的代码，代码基本相同。所以我失去了使用模板的优势。

我已经读到 chrono “类型”是持续时间实例化的 typedef。

有没有办法只为纳秒、毫秒和秒编写一个特化？

我的代码如下所示：

// file: myclass.hh

template<typename T> 
class Myclass {
public:
    // Variable (I want to initialize m_max to the maximum value possible for the 
    // T type)
    T m_max ;

    // Constructor
    Myclass ( ) ;
};

// file: myclass.cc

#include <limits>
#include <chrono>

// Constructor for int, float, double, etc...
template<typename T> 
Myclass<T>::Myclass ( ) :
    // Here we are setting m_max to the maximum value
    m_max( std::numeric_limits<T>::max( ) )
{}

// Constructor: Specialization number 1 for std::chrono::nanoseconds
template<> 
Myclass<std::chrono::nanoseconds>::Myclass()  :
    // Here we are setting m_max to the maximum value
    m_max( std::chrono::nanoseconds::max( ) )
{}

// Constructor: Specialization number 2 for std::chrono::milliseconds
template<> 
Myclass<std::chrono::milliseconds>::Myclass ( ) :
    // Here we are setting m_max to the maximum value
    m_max( std::chrono::milliseconds::max() )
{}

// Constructor: Specialization number 3 for std::chrono::seconds
template<> 
Myclass<std::chrono::seconds>::Myclass ( ) :
    // Here we are setting m_max to the maximum value
    m_max( std::chrono::seconds::max( ) )
{}

// Forcing instantiations
template class Myclass<int>;
template class Myclass<float>;
template class Myclass<double>;

template class Myclass<std::chrono::nanoseconds>;
template class Myclass<std::chrono::milliseconds>;
template class Myclass<std::chrono::seconds>;

// file: main.cc

#include <iostream>

int main() {
    Myclass<int>                       intobj;
    Myclass<float>                     floatobj;
    Myclass<double>                    doubleobj;
    Myclass<std::chrono::nanoseconds>  nanosecondseobj;
    Myclass<std::chrono::milliseconds> millisecondseobj;
    Myclass<std::chrono::seconds>      secondseobj;

    using std::cout;
    using std::endl;
    cout << "Maximum value for int = "          << intobj.m_max << endl;
    cout << "Maximum value for float = "        << floatobj.m_max << endl;
    cout << "Maximum value for double = "       << doubleobj.m_max << endl;
    cout << "Maximum value for nanoseconds = "  << nanosecondseobj.m_max.count( ) << endl;
    cout << "Maximum value for milliseconds = " << millisecondseobj.m_max.count( ) << endl;
    cout << "Maximum value for seconds = "      << secondseobj.m_max.count( ) << endl;
    return 0;
}

Answer 1

你可以对std::chrono::duration进行部分特化

template<typename T, typename Ratio> 
class Myclass<std::chrono::duration<T,Ratio> > {
public:
  // Variable (I want to initialize m_max to the maximum value possible for the 
  // T type)
  std::chrono::duration<T,Ratio> m_max ;

  // Constructor
  Myclass ( ) : m_max(std::chrono::duration<T,Ratio>::max()) {}
};

使用演示： https://wandbox.org/permlink/Q8R5pz2UPawnZ1NG

Answer 2

完整的使用示范发布者：chtz

#include <iostream>
#include <limits>
#include <chrono>

template<typename T> 
class Myclass {
public:
  // Variable (I want to initialize m_max to the maximum value possible for the 
  // T type)
  T m_max ;

  // Constructor
  Myclass ( ) : m_max(std::numeric_limits<T>::max()) {}
};

template<typename T, typename Ratio> 
class Myclass<std::chrono::duration<T,Ratio> > {
public:
  // Variable (I want to initialize m_max to the maximum value possible for the 
  // T type)
  std::chrono::duration<T,Ratio> m_max ;

  // Constructor
  Myclass ( ) : m_max(std::chrono::duration<T,Ratio>::max()) {}
};

int main() {
    Myclass<int>                       intobj;
    Myclass<float>                     floatobj;
    Myclass<double>                    doubleobj;
    Myclass<std::chrono::nanoseconds>  nanosecondseobj;
    Myclass<std::chrono::milliseconds> millisecondseobj;
    Myclass<std::chrono::seconds>      secondseobj;

    using namespace std;
    cout << "Maximum value for int = "          << intobj.m_max << endl;
    cout << "Maximum value for float = "        << floatobj.m_max << endl;
    cout << "Maximum value for double = "       << doubleobj.m_max << endl;
    cout << "Maximum value for nanoseconds = "  << nanosecondseobj.m_max.count( ) << endl;
    cout << "Maximum value for milliseconds = " << millisecondseobj.m_max.count( ) << endl;
    cout << "Maximum value for seconds = "      << secondseobj.m_max.count( ) << endl;
}