Question

我一直在使用可变参数模板，并注意到以下内容。

这很好用：

auto t = std::make_tuple(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16);

这将给出错误（ ~~gcc 4.8.2~~ （编辑：Clang 3.4）默认最大深度为256）：

auto t2 = std::make_tuple(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17);

但是，直接创建元组将起作用：

std::tuple<int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int> t3(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17);

我在尝试创建一个返回模板化类的模板化函数时注意到了这一点。

template <typename...Arguments>
struct Testing {
  std::tuple<Arguments...> t;
  Testing(Arguments...args) : t(args...) {}
};

template <typename... Arguments>
Testing<Arguments...> create(Arguments... args) {
  return Testing<Arguments...>(args...);
}

在这种情况下，这将起作用：

auto t4 = create(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16);

这不会：

auto t5 = create(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17);

Answer 1

问题不是make_tuple，而是libstdc ++（gcc4.8.2）中tuple的移动构造函数。

对于类模板，成员函数仅在使用时实例化。 noexcept规范类似地延迟，参见例如CWG issue 1330

当从make_tuple初始化变量时，移动构造函数被实例化，即使它被省略（例如，检查它是否格式错误）。这就是为什么您看到仅使用tuple 定义make_tuple变量和之间存在差异。

移动构造函数具有递归实现的条件noexcept。因此，对于每个模板参数，需要恒定数量的附加实例化。当超过最大实例化深度时，clang ++的错误输出的摘录:(支撑自己，文本墙传入）

/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/type_traits:803:24: note: in instantiation of default argument for '__test, std::_Tuple_impl &&>' required here static true_type __test(int); ^~~~~~~~~~~ /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/type_traits:803:24: note: while substituting deduced template arguments into function template '__test' [with _Tp = std::_Tuple_impl, _Arg = std::_Tuple_impl &&, $2 = ] static true_type __test(int); ^ /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/type_traits:117:14: note: in instantiation of template class 'std::__is_direct_constructible_impl, std::_Tuple_impl &&>' requested here : public conditional::type ^ /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/type_traits:818:14: note: in instantiation of template class 'std::__and_ >, std::__is_direct_constructible_impl, std::_Tuple_impl &&> >' requested here : public __and_, ^ /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/type_traits:896:14: note: in instantiation of template class 'std::__is_direct_constructible_new_safe, std::_Tuple_impl &&>' requested here : public conditional::value, ^ /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/type_traits:904:39: note: in instantiation of template class 'std::__is_direct_constructible_new, std::_Tuple_impl &&>' requested here : public integral_constant, std::_Tuple_impl &&>' requested here : public __is_direct_constructible ^ /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/type_traits:956:39: note: in instantiation of template class 'std::__is_constructible_impl, std::_Tuple_impl &&>' requested here : public integral_constant, std::_Tuple_impl &&>' requested here : public conditional::type ^ /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/type_traits:1042:14: note: in instantiation of template class 'std::__and_, std::_Tuple_impl &&>, std::__is_nt_constructible_impl, std::_Tuple_impl &&> >' requested here : public __and_, ^ /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/type_traits:1073:14: note: in instantiation of template class 'std::is_nothrow_constructible, std::_Tuple_impl &&>' requested here : public is_nothrow_constructible ^ /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/type_traits:1079:14: note: in instantiation of template class 'std::__is_nothrow_move_constructible_impl, false>' requested here : public __is_nothrow_move_constructible_impl ^ /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/type_traits:117:14: note: in instantiation of template class 'std::is_nothrow_move_constructible >' requested here : public conditional::type ^ /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/tuple:268:16: note: in instantiation of template class 'std::__and_, std::is_nothrow_move_constructible > >' requested here noexcept(__and_, ^ /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/type_traits:802:24: note: in instantiation of exception specification for '_Tuple_impl' requested here = decltype(::new _Tp(declval()))>

我们可以在这里看到实施例如is_nothrow_move_constructible的{{1}}就is_nothrow_constructible而言，以__is_nt_constructible等方式实现，适用于15个实例化级别。它打印得像一个调用堆栈，因此您可以从底部开始执行实例化。

这意味着tuple的每个模板参数都需要15个额外的实例化级别才能进行此检查。最重要的是，总是需要9个级别（恒定深度）。

因此，17个参数要求实例化深度为17 * 15 + 9 == 264。

为什么这会超过最大递归模板深度？

1 个答案: