模板以一种聪明的方式：

 模板< typename T，typename Duration = std :: chrono :: seconds，int duration_value = 30> 
 class MyClass 
 {
 //现在你可以在这里使用持续时间：
 // auto duration = Duration（duration_value）; 
} 
 

然后你可以将你的模板实例化为

  MyClass< std :: string，std :: chrono :: seconds，30>目的; 
 

或者，实际上这些值为默认值，只需

  MyClass< std :: string>目的; 
  $ p> 

 编辑：
 

考虑到PaperBirdMaster 的要求，您可以限制模板的 Duration 类型， std :: chrono :: duration ，这样：
  template< typename T> 
 struct is_chrono_duration 
 {
 static constexpr bool value = false; 
} 

 template< typename Rep，typename Period> 
 struct is_chrono_duration< std :: chrono :: duration< Rep，Period>> 
 {
 static constexpr bool value = true; 
} 

 template< typename T，typename Duration = std :: chrono :: seconds，int duration_value = 30> 
 class MyClass 
 {
 static_assert（is_chrono_duration< Duration> :: value，duration必须为std :: chrono :: duration）; 
 //现在你可以在这里使用持续时间：
 // auto duration = Duration（duration_value）; 
} 

 int main（int argc，char ** argv）
 {
 MyClass< std :: string，std :: chrono :: seconds，1> obj1; // Ok 
 MyClass< std :: string，std :: chrono :: milliseconds，1> obj2; // Ok 
 MyClass< std :: string，int，1> obj3; //错误
 return 0; 
} 
 
 
I have a template class, something like:
template < typename T, size_t Seconds > class MyClass {}
Now, I would like to change Seconds to be a duration, so the class can be parametrized with std::chrono::duration. For example, I'd like to be able to do this:
MyClass < std::string, std::chrono::seconds(30) > object;
Also, in the template, I'd like to specify a default value, something like std::chrono::seconds(30).
You can design your template in a clever way:
template < typename T, typename Duration = std::chrono::seconds, int duration_value = 30 > 
class MyClass 
{
    // Now you can use duration here:
    // auto duration = Duration(duration_value);
};
And then you can instantiate your template as
MyClass < std::string, std::chrono::seconds, 30 > object;
Or, actually having these values as defaults, simply
MyClass < std::string > object;
Edit:

Taking into account PaperBirdMaster's request, you can limit template's Duration type, to be std::chrono::duration only, this way:
template <typename T>
struct is_chrono_duration
{
    static constexpr bool value = false;
};

template <typename Rep, typename Period>
struct is_chrono_duration<std::chrono::duration<Rep, Period>>
{
    static constexpr bool value = true;
};

template < typename T, typename Duration = std::chrono::seconds, int duration_value = 30 >
class MyClass
{
    static_assert(is_chrono_duration<Duration>::value, "duration must be a std::chrono::duration");
    // Now you can use duration here:
    // auto duration = Duration(duration_value);
};

int main(int argc, char ** argv)
{
    MyClass < std::string, std::chrono::seconds, 1> obj1;       // Ok
    MyClass < std::string, std::chrono::milliseconds, 1> obj2;  // Ok
    MyClass < std::string, int, 1> obj3;                        // Error
    return 0;
}