"typename ="是什么?在模板参数中表示什么? [英] What does "typename =" mean in the template parameters?

问题描述

我在《有效的现代C ++》一书的第189页中看到了这种表达方式:

I have seen this expression in page 189 of the book "Effective Modern C++":

    template<typename T,
             typename = typename std::enable_if<condition>::type>
    explicit Person(T&& n);

我只是想知道"typename ="部分是什么意思.它肯定看起来像模板参数的默认参数.但是，您是否不需要在默认参数中添加诸如"typename some_name = ..."之类的东西?第二个模板参数没有名称，在这种情况下我看不到使用第二个模板参数.

I am just wondering what does the part "typename =" mean. It certainly looks like a default argument for a template parameter. But don't you need something like "typename some_name = ..." in a default argument? There is no name for the second template argument, and I don't see the second template argument being used in this case.

P.S.当我在google(或任何其他搜索引擎)上搜索答案时，等号始终会被丢弃，这几乎使寻找答案几乎不可能...

P.S. When I search on google (or any other search engine) for an answer, the equal sign is always discarded, and this just makes finding an answer almost impossible...

推荐答案

这是一个可选的模板参数，没有名称，且为默认值.
它用于应用enable_if条件；如果不满足条件，将产生编译器错误.

That's an optional template parameter with no name and a default value.
It's used to apply the enable_if condition; it will create a compiler error if the condition is not met.

对于普通的方法参数，您可以使用完全相同的语法.

You can use exactly the same syntax for normal method arguments.

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