C ++：对于除最后的其它模板参数的默认值？ [英] C++: Default values for template arguments other than the last ones?

问题描述

我有我的模板容器类，看起来像这样：

I have my templated container class that looks like this:

template<
   class KeyType, 
   class ValueType, 
   class KeyCompareFunctor   = AnObnoxiouslyLongSequenceOfCharacters<KeyType>, 
   class ValueCompareFunctor = AnObnoxiouslyLongSequenceOfCharacters<ValueType> 
>
   class MyClass
   {
      [...]
   }

这意味着当我实例化这个类的一个对象，我能做到这一点几种不同的方式：

Which means that when I instantiate an object of this class, I can do it several different ways:

MyClass<MyKeyType, MyValueType> myObject;
MyClass<MyKeyType, MyValueType, MyCustomKeyCompareFunctor> myObject;
MyClass<MyKeyType, MyValueType, MyCustomKeyCompareFunctor, MyCustomValueCompareFunctor> myObject;

这些都是很好的。问题是当我想要实例化使用ValueCompareFunctor参数的非缺省版本MyClass的，但我还是想用KeyCompareFunctor参数的默认值。然后，我必须写这篇文章：

Those are all good. The problem comes when I want to instantiate a MyClass that uses a non-default version of the ValueCompareFunctor argument, but I still want to use the default value of the KeyCompareFunctor argument. Then I have to write this:

MyClass<MyKeyType, MyValueType, AnObnoxiouslyLongSequenceOfCharacters<MyKeyType>, MyCustomValueCompareFunctor> myObject;

这将是方便多了，如果我能以某种方式省略了第三个参数，只是这样写：

It would be much more convenient if I could somehow omit the third argument and just write this:

MyClass<KeyType, ValueType, MyCustomValueCompareFunctor> myObject;

由于MyCustomValueCompareFunctor仅适用于类型MyValueType的对象，而不是类型MyKeyType的对象，好像编译器可能至少在理论上锻炼了我的意思在这里。

Since the MyCustomValueCompareFunctor works only on objects of type MyValueType and not on objects of type MyKeyType, it seems like the compiler could at least theoretically work out what I meant here.

有没有办法在C这样做++？

Is there a way to do this in C++?

推荐答案

在一般情况下，无论是在模板和函数或方法，C ++，您可以使用默认值（从而省略）唯一的结尾的参数 - - 没有出路

In general, both in templates and functions or methods, C++ lets you use default for (and thereby omit) only trailing parameters -- no way out.

我推荐一个模板或宏缩短 AnObnoxiouslyLongSequenceOfCharacters＆LT; MyKeyType＆GT; 到美孚＆LT; MyKeyType＆GT; - 不完美，但有总比没有好。

I recommend a template or macro to shorten AnObnoxiouslyLongSequenceOfCharacters<MyKeyType> to Foo<MyKeyType> -- not perfect, but better than nothing.

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