我找到了 C++14 make_index_sequence '算法'的实现:
template< int ... > struct index_sequence{ using type = index_sequence; };
template< typename T> using invoke = typename T :: type ;
template< typename T, typename U > struct concate;
template< int ...i, int ... j>
struct concate< index_sequence<i...>, index_sequence<j...> >
: index_sequence< i... , (j + sizeof ... (i ) )... > {};
// \ /
// ----------
// I think here is slowly.
template< int n>
struct make_index_sequence_help : concate<
invoke< make_index_sequence_help<n/2>>,
invoke< make_index_sequence_help<n-n/2>>
> {};
template<> struct make_index_sequence_help <0> : index_sequence<>{};
template<> struct make_index_sequence_help <1> : index_sequence<0>{};
template< int n> using make_index_sequence = invoke< make_index_sequence_help<n> >;
int main()
{
using iseq = make_index_sequence< 1024 > ; // successfull
using jseq = make_index_sequence< 1024 * 16 > ; // a lot of compile time!!!
using kseq = make_index_sequence< 1024 * 64 > ; // can't compile: memory exhauted!!!
};
但是,当我将 sizeof...(i) 从 'concate' 替换为具体数字时,make_index_sequence<1024 *64> - 编译得非常快。
template< int s, typename T, typename U > struct concate;
template< int s, int ...i, int ...j >
struct concate< s, index_sequence<i...>, index_sequence<j...> >
: index_sequence< i..., ( j + s ) ... > {};
// and
template< int n >
struct make_index_sequence_help : concate<
n / 2 ,
invoke< make_index_sequence_help< n / 2 > >,
invoke< make_index_sequence_help< n - n/2 > >
>{};
问:为什么 sizeof ... (i) 这么慢?
我用 gcc 4.8.1 更新测试:
对于第一种情况:(仅 1024 和 1024*16 )。
g++ -Wall -c "ctx_fptr.cpp" -g -O2 -std=c++11 -ftime-report
Execution times (seconds)
garbage collection : 0.06 ( 1%) usr 0.00 ( 0%) sys 0.06 ( 0%) wall 0 kB ( 0%) ggc
preprocessing : 0.03 ( 0%) usr 0.04 ( 2%) sys 0.09 ( 1%) wall 293 kB ( 0%) ggc
parser : 10.41 (97%) usr 1.61 (95%) sys 12.01 (96%) wall 2829842 kB (99%) ggc
name lookup : 0.12 ( 1%) usr 0.04 ( 2%) sys 0.23 ( 2%) wall 7236 kB ( 0%) ggc
dead store elim1 : 0.01 ( 0%) usr 0.00 ( 0%) sys 0.00 ( 0%) wall 0 kB ( 0%) ggc
symout : 0.15 ( 1%) usr 0.00 ( 0%) sys 0.15 ( 1%) wall 12891 kB ( 0%) ggc
unaccounted todo : 0.00 ( 0%) usr 0.01 ( 1%) sys 0.00 ( 0%) wall 0 kB ( 0%) ggc
TOTAL : 10.78 1.70 12.55 2850835 kB
对于第二种情况:(所有 1024、1024*16 和 1024 * 64)
g++ -Wall -c "ctx_fptr.cpp" -g -O2 -std=c++11 -ftime-report
Execution times (seconds)
preprocessing : 0.02 ( 2%) usr 0.01 ( 5%) sys 0.05 ( 4%) wall 293 kB ( 0%) ggc
parser : 0.54 (45%) usr 0.10 (53%) sys 0.71 (50%) wall 95339 kB (58%) ggc
name lookup : 0.47 (39%) usr 0.04 (21%) sys 0.47 (33%) wall 20197 kB (12%) ggc
tree PRE : 0.01 ( 1%) usr 0.00 ( 0%) sys 0.00 ( 0%) wall 1 kB ( 0%) ggc
varconst : 0.00 ( 0%) usr 0.01 ( 5%) sys 0.00 ( 0%) wall 17 kB ( 0%) ggc
symout : 0.17 (14%) usr 0.03 (16%) sys 0.18 (13%) wall 47092 kB (29%) ggc
TOTAL : 1.21 0.19 1.41 163493 kB