boost元组：增加元素的最大数量 [英] boost tuple: increasing maximum number of elements

问题描述

boost元组文档说：

当前版本支持元组
和0-10个元素。如果需要，
的上限可以增加到，
说几十个元素。

The current version supports tuples with 0-10 elements. If necessary, the upper limit can be increased up to, say, a few dozen elements.

我想要元组有 BOOST_MPL_LIMIT_VECTOR_SIZE 元素（默认为20）。这是因为我在 mpl :: vectors 和 boost :: tuples 之间映射，并希望所有的容器相同数量的元素。

I would like the tuple to have BOOST_MPL_LIMIT_VECTOR_SIZE elements (20 by default). This is because I am mapping between mpl::vectors and boost::tuples and would like all the containers to have the same number of elements.

推荐答案

元组类以下列方式声明：

The tuple class is declared in the following way:

// - tuple forward declaration -----------------------------------------------
template <
  class T0 = null_type, class T1 = null_type, class T2 = null_type,
  class T3 = null_type, class T4 = null_type, class T5 = null_type,
  class T6 = null_type, class T7 = null_type, class T8 = null_type,
  class T9 = null_type>
class tuple;

因此，其模板参数计数设置为上限10.但是，在未来（C ++ 0x）中声明它像这样：

Thus, its template parameter count is set to a upper limit of 10. However, it would be possible in the future (C++0x) to declare it like that:

template<class... Params> class tuple;

因此，我认为目前不可能增加实际上限。这将是可能的以下方式：

So, I don't think it's currently possible to increase the upper limit in practice. It would been possible in the following way:

#define VARIADIC_PARAMS_DEFVAL0(type_name, value)
#define VARIADIC_PARAMS_DEFVAL1(type_name, value)   type_name##0 = value
#define VARIADIC_PARAMS_DEFVAL2(type_name, value)   VARIADIC_PARAMS_DEFVAL1(type_name, value), type_name##1 = value
#define VARIADIC_PARAMS_DEFVAL3(type_name, value)   VARIADIC_PARAMS_DEFVAL2(type_name, value), type_name##2 = value
#define VARIADIC_PARAMS_DEFVAL4(type_name, value)   VARIADIC_PARAMS_DEFVAL3(type_name, value), type_name##3 = value
#define VARIADIC_PARAMS_DEFVAL5(type_name, value)   VARIADIC_PARAMS_DEFVAL4(type_name, value), type_name##4 = value
#define VARIADIC_PARAMS_DEFVAL6(type_name, value)   VARIADIC_PARAMS_DEFVAL5(type_name, value), type_name##5 = value
#define VARIADIC_PARAMS_DEFVAL7(type_name, value)   VARIADIC_PARAMS_DEFVAL6(type_name, value), type_name##6 = value
#define VARIADIC_PARAMS_DEFVAL8(type_name, value)   VARIADIC_PARAMS_DEFVAL7(type_name, value), type_name##7 = value
#define VARIADIC_PARAMS_DEFVAL9(type_name, value)   VARIADIC_PARAMS_DEFVAL8(type_name, value), type_name##8 = value
#define VARIADIC_PARAMS_DEFVAL10(type_name, value)  VARIADIC_PARAMS_DEFVAL9(type_name, value), type_name##9 = value
// ...
#define VARIADIC_PARAMS_DEFVAL100(type_name, value) VARIADIC_PARAMS_DEFVAL99(type_name, value), type_name##99 = value
#define VARIADIC_PARAMS_DEFVAL(type_name, value, n) VARIADIC_PARAMS_DEFVAL##n(type_name, value)

#define VARIADIC_MACRO_INVOKE0(macro)    macro(0)
#define VARIADIC_MACRO_INVOKE1(macro)    VARIADIC_MACRO_INVOKE0(macro); macro(1)
#define VARIADIC_MACRO_INVOKE2(macro)    VARIADIC_MACRO_INVOKE1(macro); macro(2)
#define VARIADIC_MACRO_INVOKE3(macro)    VARIADIC_MACRO_INVOKE2(macro); macro(3)
#define VARIADIC_MACRO_INVOKE4(macro)    VARIADIC_MACRO_INVOKE3(macro); macro(4)
#define VARIADIC_MACRO_INVOKE5(macro)    VARIADIC_MACRO_INVOKE4(macro); macro(5)
#define VARIADIC_MACRO_INVOKE6(macro)    VARIADIC_MACRO_INVOKE5(macro); macro(6)
#define VARIADIC_MACRO_INVOKE7(macro)    VARIADIC_MACRO_INVOKE6(macro); macro(7)
#define VARIADIC_MACRO_INVOKE8(macro)    VARIADIC_MACRO_INVOKE7(macro); macro(8)
#define VARIADIC_MACRO_INVOKE9(macro)    VARIADIC_MACRO_INVOKE8(macro); macro(9)
#define VARIADIC_MACRO_INVOKE10(macro)   VARIADIC_MACRO_INVOKE9(macro); macro(10)
// ...
#define VARIADIC_MACRO_INVOKE100(macro)  VARIADIC_MACRO_INVOKE99(macro); macro(100)
#define VARIADIC_MACRO_INVOKE(macro, n)  VARIADIC_MACRO_INVOKE##n(macro)

#define TUPPLE_UPPER_LIMIT      50

#define FORWARD_DECLARE_TUPPLE(N)                       \
template<VARIADIC_PARAMS_DEFVAL(class, null_type, N)    \
class tuple;

FS_VARIADIC_MACRO_INVOKE(FORWARD_DECLARE_TUPPLE, TUPPLE_UPPER_LIMIT);

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