c++ - 异构可变参数非类型模板参数计数灵活吗？

Question

下面定义了一个可变的非类型嵌套类模板，DEF。根据为 Ts 提供的类型参数，非类型模板参数可能是异构的。

template <typename ...Ts>
struct ABC {
  template <Ts ...Xs>
  struct DEF {};
};

DEF 对象可以声明如下:

ABC<int,bool>::DEF<17,true> x;

我的问题是，提供给DEF的非类型模板参数的数量是否可以少于提供给ABC的类型模板参数的数量？例如，这些声明是否有效:

ABC<int,bool>::DEF<17> y;
ABC<int,bool>::DEF<  > z;

Answer 1

不，因为Ts...是一个包扩展。

§14.5.3 [temp.variadic]

p4 A pack expansion consists of a pattern and an ellipsis, the instantiation of which produces zero or more instantiations of the pattern in a list (described below). The form of the pattern depends on the context in which the expansion occurs. Pack expansions can occur in the following contexts:

• [...]
• In a template parameter pack that is a pack expansion (14.1):
  • if the template parameter pack is a parameter-declaration; the pattern is the parameter-declaration without the ellipsis;
  • [...]

p6 The instantiation of a pack expansion [...] produces a list E₁, E₂, ..., E_N, where N is the number of elements in the pack expansion parameters. [...]

所以你的两个例子都是错误的，因为 DEF正好需要 <int, bool> .