混淆在ISO C ++标准中的std :: tuple的默认构造函数描述 [英] Confused by default constructor description of std::tuple in the ISO C++ Standard

问题描述

标准表示 std :: tuple 有以下成员函数

The Standard says that std::tuple has the following member functions

constexpr tuple();
explicit tuple(const Types&...);

有人可以解释一下 std :: tuple& > ？

推荐答案

我认为这是标准中的一个小错误。显然，当 Types 参数包为空时，两个构造函数调用是等效的，不能重载（参见C ++ 11第13节）。 （进一步注意，使用 Types 的构造函数不是一个成员模板 - 如果是，那么它将是一个合法的重载。）

I believe this is a minor error in the standard. Clearly, when the Types parameter pack is empty, the two constructor calls are equivalent and cannot be overloaded (see C++11 section 13). (Further note that the constructor using Types is not a member template either --if it was, then it would be a legal overload.).

换句话说，这段代码不会编译：

In other words, this code will not compile:

template <typename... Types>
struct Test
{
  constexpr Test() {}
  explicit Test(Types const&...) { /* etc. */ }
};

int main()
{
  Test<> a;
  Test<int> b;
}

例如，g ++ v4.8快照输出：

e.g., a g++ v4.8 snapshot outputs:

tt.cxx: In instantiation of ‘struct Test<>’:
tt.cxx:10:10:   required from here
tt.cxx:5:12: error: ‘Test<Types>::Test(const Types& ...) [with Types = {}]’ cannot be overloaded
   explicit Test(Types const&...) { /* etc. */ }
            ^
tt.cxx:4:13: error: with ‘constexpr Test<Types>::Test() [with Types = {}]’
   constexpr Test() {}
             ^

部分专业化：

template <typename... Types>
struct Test
{
  constexpr Test() {} // default construct all elements
  explicit Test(Types const&...) { /* etc. */ }
  // and all other member definitions
};

template <>
struct Test<>
{
  constexpr Test() {}
  // and any other member definitions that make sense with no types
};

int main()
{
  Test<> a;
  Test<int> b;
}

这将正确编译。

看起来标准想要 constexpr 默认构造函数是 std :: tuple< var; 可以写成 std :: tuple<> var（）; 或 std :: tuple<> var {}; ，因为显式与其他构造函数一起使用。不幸的是，它的定义 std :: tuple 不适用于大小为零的元组。该标准允许在第20.4.2.7节（关系运算符）中使用对于任何两个零长度元组[...]。糟糕！ ： - ）

It appears the standard wanted a constexpr default constructor was so that std::tuple<> var; could be written instead of writing std::tuple<> var(); or std::tuple<> var{}; because of the use of explicit with the other constructor. Unfortunately, its definition of std::tuple does not work for tuples of size zero. The standard does permit such in section 20.4.2.7 (relational operators) though, "For any two zero-length tuples, [...]". Oops! :-)

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