为什么这会超过最大递归模板深度？

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,这将工作:

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tuple在libstdc ++(gcc4.8.2)中移动构造函数.

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.

当初始化变量时make_tuple,移动构造函数被实例化,即使它被省略(例如,检查它是否是不正确的).这就是为什么你看到只是定义tuple变量和使用make_tuple之间的区别.

移动构造函数具有noexcept递归实现的条件.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针对15个实例化级别的实现,例如is_nothrow_move_constructible就is_nothrow_constructible其实现的__is_nt_constructible等等.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.

这意味着每个模板参数tuple需要15个额外的实例化级别来进行此检查.最重要的是,总是需要9个级别(恒定深度).

因此,17个参数需要实例化深度为17*15 + 9 == 264.