在c++中初始化参数化类型数组

我正在研究一种泛型类型，其中我需要一个参数类型的堆栈分配数组。它应该有尽可能少的开销。这是考试的一部分，但现在它激起了我的好奇心。我试着到处找，但没找到任何有用的东西。

我试图创建一个数组并初始化成员，但我不完全确定如何正确地做到这一点。我有以下代码:

T b[_dim]; // typeof(_dim) == size_t

现在T可以是任何具有标准算术运算符的东西，一个默认构造函数。当T是双精度类型时，用

memset(b, 0, _dim*sizeof(T));

对于double类型可以正常工作。现在的问题是T可以是一个理性的，例如:

方法1

struct Rational
{
   int numerator;  
   int denominator;
   Rational(): numerator(0), denominator(1) {}
};

如果在所有内容都设置为0之后将其转换为双精度类型，可能会导致问题。另一种方法是像这样遍历所有元素:

方法2

for(size_t i = 0; i < _dim; i++)
{
    b[i] = T();
}

正确初始化成员的有效方法是什么?

边节点是当我对除有理数以外的其他类型都不这样做时的节点。比如双精度。程序运行正常，但是valgrind报错了以下错误消息(很多，重复):

==24450== Conditional jump or move depends on uninitialised value(s)
==24450==    at 0x5878B60: __printf_fp (printf_fp.c:731)
==24450==    by 0x5876B4B: vfprintf (vfprintf.c:1654)
==24450==    by 0x589B654: vsnprintf (vsnprintf.c:119)
==24450==    by 0x53938FD: std::__convert_from_v(__locale_struct* const&, char*, int, char const*, ...) (c++locale.h:93)
==24450==    by 0x53A118C: std::ostreambuf_iterator<char, std::char_traits<char> > std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::_M_insert_float<double>(std::ostreambuf_iterator<char, std::char_traits<char> >, std::ios_base&, char, char, double) const (locale_facets.tcc:997)
==24450==    by 0x53A141F: std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::do_put(std::ostreambuf_iterator<char, std::char_traits<char> >, std::ios_base&, char, double) const (locale_facets.tcc:1144)
==24450==    by 0x53A5966: std::ostream& std::ostream::_M_insert<double>(double) (locale_facets.h:2398)
==24450==    by 0x433E9D: void boost::io::detail::put_last<char, std::char_traits<char>, double>(std::basic_ostream<char, std::char_traits<char> >&, double&) (feed_args.hpp:115)
==24450==    by 0x43273E: void boost::io::detail::put<char, std::char_traits<char>, std::allocator<char>, double&>(double&, boost::io::detail::format_item<char, std::char_traits<char>, std::allocator<char> > const&, boost::basic_format<char, std::char_traits<char>, std::allocator<char> >::string_type&, boost::basic_format<char, std::char_traits<char>, std::allocator<char> >::internal_streambuf_t&, std::locale*) (feed_args.hpp:176)
==24450==    by 0x43141F: void boost::io::detail::distribute<char, std::char_traits<char>, std::allocator<char>, double&>(boost::basic_format<char, std::char_traits<char>, std::allocator<char> >&, double&) (feed_args.hpp:253)
==24450==    by 0x42F22F: boost::basic_format<char, std::char_traits<char>, std::allocator<char> >& boost::io::detail::feed<char, std::char_traits<char>, std::allocator<char>, double&>(boost::basic_format<char, std::char_traits<char>, std::allocator<char> >&, double&) (feed_args.hpp:263)
==24450==    by 0x42DB1E: boost::basic_format<char, std::char_traits<char>, std::allocator<char> >& boost::basic_format<char, std::char_traits<char>, std::allocator<char> >::operator%<double>(double&) (format_class.hpp:68)
==24450== 
==24450== Use of uninitialised value of size 8
==24450==    at 0x5878B6A: __printf_fp (printf_fp.c:731)
==24450==    by 0x5876B4B: vfprintf (vfprintf.c:1654)
==24450==    by 0x589B654: vsnprintf (vsnprintf.c:119)
==24450==    by 0x53938FD: std::__convert_from_v(__locale_struct* const&, char*, int, char const*, ...) (c++locale.h:93)
==24450==    by 0x53A118C: std::ostreambuf_iterator<char, std::char_traits<char> > std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::_M_insert_float<double>(std::ostreambuf_iterator<char, std::char_traits<char> >, std::ios_base&, char, char, double) const (locale_facets.tcc:997)
==24450==    by 0x53A141F: std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::do_put(std::ostreambuf_iterator<char, std::char_traits<char> >, std::ios_base&, char, double) const (locale_facets.tcc:1144)
==24450==    by 0x53A5966: std::ostream& std::ostream::_M_insert<double>(double) (locale_facets.h:2398)
==24450==    by 0x433E9D: void boost::io::detail::put_last<char, std::char_traits<char>, double>(std::basic_ostream<char, std::char_traits<char> >&, double&) (feed_args.hpp:115)
==24450==    by 0x43273E: void boost::io::detail::put<char, std::char_traits<char>, std::allocator<char>, double&>(double&, boost::io::detail::format_item<char, std::char_traits<char>, std::allocator<char> > const&, boost::basic_format<char, std::char_traits<char>, std::allocator<char> >::string_type&, boost::basic_format<char, std::char_traits<char>, std::allocator<char> >::internal_streambuf_t&, std::locale*) (feed_args.hpp:176)
==24450==    by 0x43141F: void boost::io::detail::distribute<char, std::char_traits<char>, std::allocator<char>, double&>(boost::basic_format<char, std::char_traits<char>, std::allocator<char> >&, double&) (feed_args.hpp:253)
==24450==    by 0x42F22F: boost::basic_format<char, std::char_traits<char>, std::allocator<char> >& boost::io::detail::feed<char, std::char_traits<char>, std::allocator<char>, double&>(boost::basic_format<char, std::char_traits<char>, std::allocator<char> >&, double&) (feed_args.hpp:263)
==24450==    by 0x42DB1E: boost::basic_format<char, std::char_traits<char>, std::allocator<char> >& boost::basic_format<char, std::char_traits<char>, std::allocator<char> >::operator%<double>(double&) (format_class.hpp:68)

Update变量_dim确实是变量，并被解析为。g++ 11说:

error: variable-sized object ‘b’ may not be initialized

解决方案及总结如上所述，这将调用所有元素的元素:

b T[_dim];

由于_dim是变量，这不起作用:

b T[_dim] = {};

但是我们可以用do this::

T b[_dim];
if(std::is_fundamental<T>::value)
{
    memset(b, 0, _dim*sizeof(T));
}