问题描述
对于模板，我已经看到了两个声明:
For templates I have seen both declarations:
template < typename T >
template < class T >
有什么区别?
在下面的例子中这些关键字到底是什么意思(取自德语维基百科关于模板的文章)?
And what exactly do those keywords mean in the following example (taken from the German Wikipedia article about templates)?
template < template < typename, typename > class Container, typename Type >
class Example
{
Container< Type, std::allocator < Type > > baz;
};
推荐答案
typename 和 class 在指定模板的基本情况下是可以互换的:
typename and class are interchangeable in the basic case of specifying a template:
template<class T>
class Foo
{
};
和
template<typename T>
class Foo
{
};
是等价的.
话虽如此，在某些特定情况下，typename 和 class 之间存在差异.
Having said that, there are specific cases where there is a difference between typename and class.
第一个是依赖类型的情况.typename 用于在引用依赖于另一个模板参数的嵌套类型时声明，例如本示例中的 typedef:
The first one is in the case of dependent types. typename is used to declare when you are referencing a nested type that depends on another template parameter, such as the typedef in this example:
template<typename param_t>
class Foo
{
typedef typename param_t::baz sub_t;
};
您在问题中实际展示的第二个，尽管您可能没有意识到:
The second one you actually show in your question, though you might not realize it:
template < template < typename, typename > class Container, typename Type >
当指定一个模板模板时，class关键字必须像上面一样使用——它不能与typename<互换/code> 在这种情况下(注意:由于 C++17 在这种情况下允许两个关键字).
When specifying a template template, the class keyword MUST be used as above -- it is not interchangeable with typename in this case (note: since C++17 both keywords are allowed in this case).
在显式实例化模板时，您还必须使用 class:
You also must use class when explicitly instantiating a template:
template class Foo<int>;
我确定我遗漏了其他一些情况，但最重要的是:这两个关键字并不等效，而且这些是您需要使用其中一个的一些常见情况.
I'm sure that there are other cases that I've missed, but the bottom line is: these two keywords are not equivalent, and these are some common cases where you need to use one or the other.
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