在构造函数中为 class 名称添加和省略模板参数的区别

Question

我想知道两者之间是否存在本质区别：

template <typename T> 
class foo{
  foo<T>(){};
};

template<typename T>
class foo{
  foo(){};
};

两者似乎都有效，但我不明白两者之间的区别。第一个把我搞糊涂了，我不明白这里的是什么作用

Answer 1

根据injected-class-name的规则，它们是完全一样的。

.6.1/1 Locally declared names [temp.local]:

Like normal (non-template) classes, class templates have an injected-class-name (Clause [class]). The injected-class-name can be used as a template-name or a type-name. When it is used with a template-argument-list, as a template-argument for a template template-parameter, or as the final identifier in the elaborated-type-specifier of a friend class template declaration, it refers to the class template itself. Otherwise, it is equivalent to the template-name followed by the template-parameters of the class template enclosed in <>.

所以foo和foo<T>在这里指的是同一件事。更具体地说，

The first one is confusing me, I don't understand what's the role of the < T > here.

您正在使用注入-class-名称 foo 及其模板参数 T（即 foo<T>），它指的是模板 class本身。

Answer 2

请注意，自 C++20 起，较长的形式不再有效，您必须使用依赖于 injected-class-name 的较短形式.

见[diff.cpp17]#class-2:

a simple-template-id is no longer valid as the declarator-id of a constructor or destructor

使用规范中的以下示例：

template<class T>
struct A {
  A<T>();           // error: simple-template-id not allowed for constructor
  A(int);           // OK, injected-class-name used
  ~A<T>();          // error: simple-template-id not allowed for destructor
};

确实是 GCC trunk 版本 rejects code using the longer form (while Clang 11 and GCC 10.2 still accepts it).

在构造函数中为 class 名称添加和省略模板参数的区别

Difference between adding and omitting template parameter for class name inside constructor

c++

templates

constructor

typename

injected-class-name