[英]C++ Templates: When to include template parameters in templatized struct?
I'm new to C++ and templates and I'm having trouble with this concept. 我是C ++和模板的新手,我对这个概念感到困惑。 I had a question about this implementation of the C++11 is_same feature, written by Dirk Holsopple here: https://stackoverflow.com/a/13071396/5199724
我对Dirk Holsopple在此处编写的C ++ 11 is_same功能的实现有疑问: https ://stackoverflow.com/a/13071396/5199724
This is his code, which compiles: 这是他的代码,编译如下:
1 template<class T, class U>
2 struct is_same {
3 enum { value = 0 };
4 };
5
6 template<class T>
7 struct is_same<T, T> {
8 enum { value = 1 };
9 };
My question is, why is this not valid: 我的问题是,为什么这是无效的:
1 template<class T, class U>
2 struct is_same<T, U> { // template parameters in struct declaration -- not valid?
3 enum { value = 0 };
4 };
5
6 template<class T>
7 struct is_same<T, T> {
8 enum { value = 1 };
9 };
The inclusion of template parameters in both struct declarations would make intuitive sense to me, but the second code block gives me the following errors in Visual Studio: 对我来说,在两个struct声明中都包含模板参数对我来说很直观,但是第二个代码块在Visual Studio中给了我以下错误:
error C2143: syntax error : missing ';' before '<'
error C2059: syntax error : '<'
error C2334: unexpected token(s) preceding '{'; skipping apparent function body
error C3412: cannot specialize template in current scope
So why would template parameters be needed in Line 7 but not in Line 2? 那么,为什么在第7行而不是在第2行中需要模板参数? What is the general rule for when to include template parameters?
何时包括模板参数的一般规则是什么? Thanks for your help!
谢谢你的帮助!
template<class T>
struct is_same<T, T> {
enum { value = 1 };
};
The reason why this is not valid is because this is a template specialization, and you can't specialize something that doesn't exist (is_same). 之所以无效,是因为这是模板专门化,您不能专门化不存在的内容(is_same)。 First you have to create the primary template, then specialize it.
首先,您必须创建主模板,然后对其进行专门化。
The first one is a template-definition, where the template-keyword already says it will be a template. 第一个是模板定义,其中template-keyword已经说过它将是一个模板。
The second is a template specialization. 第二个是模板专业化。 It is a template as well, but the parameters must match the previous declared definition.
它也是一个模板,但是参数必须与先前声明的定义匹配。
Eg in 例如
template<typename S, typename T, typename U>
struct whatever{};
template<typename V, typename W>
struct whatever<V, W, W>{};
you need to specify which parameter of the specialisation (V,W) matches which parameter of the definition (S,T,U). 您需要指定专业化(V,W)的哪个参数与定义(S,T,U)的哪个参数匹配。
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