模板中关键字“typename”和“class”的C ++差异 [英] C++ difference of keywords 'typename' and 'class' in templates

问题描述

对于模板，我已经看到两个声明：

For templates I have seen both declarations:

template < typename T >

和：

template < class T >

有什么区别？

这些关键字在下面的例子中是什么意思（取自关于模板的德语维基百科文章）？

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
{
};

等同。

，有特定的情况下， typename 和 class 之间有区别。

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 用于在您引用依赖于另一个模板参数的嵌套类型时声明，例如 typedef 此示例：

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;
};

你在问题​​中实际显示的第二个，虽然你可能没有意识到：

The second one you actually show in your question, though you might not realize it:

template < template < typename, typename > class Container, typename Type >

指定模板模板时， 关键字必须如上所述使用 - 在此情况下不可与 typename 互换。

When specifying a template template, the class keyword MUST be used as above -- it is not interchangeable with typename in this case.

在显式实例化模板时，还必须使用 class ：

You also must use class when explicitly instantiating a template:

template class Foo<int>;

我确定还有其他情况，我错过了，但底线是：这两个关键字不等同，这些是一些常见的情况，你需要使用一个或另一个。

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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