c ++可变参数模板包按任意大小的组扩展 [英] c++ variadic template pack expansion by groups of arbitrary size
问题描述
(以某种方式与此先前的问题)
(Somehow related to this previous question)
我想按 N
个参数组评估模板函数的参数。像这样的东西:
I want to evaluate the parameters of a template function by groups of N
parameters. Something like this:
template <size_t N, typename ... Ts>
void evaluate(Ts const & ... fn)
{
for( int i=0; i<2; i++ )
runH<N>::func(i, fn...);
}
int main()
{
run<3>( // N = 2
[](int i){ cout << "lambda func 1: " << std::to_string( i ) << endl; },
[](int i){ cout << "lambda func 2: " << std::to_string( i ) << endl; },
[](int i){ cout << "lambda func 3: " << std::to_string( i ) << endl; },
[](int i){ cout << "lambda func 4: " << std::to_string( i ) << endl; },
[](int i){ cout << "lambda func 5: " << std::to_string( i ) << endl; }
);
}
应输出:
lambda func 1: 0
lambda func 2: 0
lambda func 1: 1
lambda func 2: 1
lambda func 3: 0
lambda func 4: 0
lambda func 3: 1
lambda func 4: 1
lambda func 5: 0
lambda func 5: 1
剩余物必须正确处理。到目前为止,我设法使用以下方法评估了第一组 N
参数:
Remainder must be handled properly. So far I managed to evaluate just the first group of N
parameters with this:
template <std::size_t N>
struct runH
{
template <typename T0, typename ... Ts>
static void func (const int i, T0 const & f0, Ts const & ... fn)
{
f0(i);
runH<N-1U>::func(i, fn...);
}
};
template <>
struct runH<0>
{
template <typename ... Ts>
static void func (const int i, Ts const & ... fn) { }
};
template <size_t N, typename ... Ts>
void evaluate(Ts const & ... fn)
{
for( int i=0; i<2; i++ )
runH<N>::func(i, fn...);
}
template <std::size_t N, typename ... Ts>
void run (Ts const & ... fn)
{
using unused = int[];
(void)unused { 0, (evaluate<N>(fn...), 0) };
}
运行是否有任何方式
函数来不断扩展参数?我试图在末尾添加另一个省略号,但它不能编译。
Is there any way for the run
function to keep expanding the parameters? I tried to add another ellipsis at the end but it doesn't compile.
推荐答案
我提出了<$ c的定义$ c> runSkip 结构
template <std::size_t N, std::size_t I>
struct runSkip
{
template <typename T0, typename ... Ts>
static void func (T0 const &, Ts const & ... fn)
{ runSkip<N, I-1U>::func(fn...); }
static void func ()
{ }
};
template <std::size_t N>
struct runSkip<N, 0U>
{
template <typename ... Ts>
static void func (Ts const & ... fn)
{ run<N>(fn...); }
};
所以 run()
成为
template <std::size_t N, typename ... Ts>
void run (Ts const & ... fn)
{
using unused = int[];
std::size_t j;
for ( auto i = 0 ; i < 2 ; ++i )
{
j = 0U;
(void)unused { 0, ((++j <= N ? ((void)fn(i), 0) : 0), 0)... };
}
runSkip<N, N>::func(fn...);
}
下面是一个完整的示例
#include <iostream>
template <std::size_t, typename ... Ts>
void run (Ts const & ...);
template <std::size_t N, std::size_t I>
struct runSkip
{
template <typename T0, typename ... Ts>
static void func (T0 const &, Ts const & ... fn)
{ runSkip<N, I-1U>::func(fn...); }
static void func ()
{ }
};
template <std::size_t N>
struct runSkip<N, 0U>
{
template <typename ... Ts>
static void func (Ts const & ... fn)
{ run<N>(fn...); }
};
template <std::size_t N, typename ... Ts>
void run (Ts const & ... fn)
{
using unused = int[];
std::size_t j;
for ( auto i = 0 ; i < 2 ; ++i )
{
j = 0U;
(void)unused { 0, ((++j <= N ? ((void)fn(i), 0) : 0), 0)... };
}
runSkip<N, N>::func(fn...);
}
int main()
{
run<2>( // N = 2
[](int i){ std::cout << "lambda func 1: " << i << std::endl; },
[](int i){ std::cout << "lambda func 2: " << i << std::endl; },
[](int i){ std::cout << "lambda func 3: " << i << std::endl; },
[](int i){ std::cout << "lambda func 4: " << i << std::endl; },
[](int i){ std::cout << "lambda func 5: " << i << std::endl; },
[](int i){ std::cout << "lambda func 6: " << i << std::endl; }
);
}
Ps:不需要 std :: to_string
发送一个整数到输出流运算符。
P.s.: there is no need of std::to_string
to send an integer to an output stream operator.
这篇关于c ++可变参数模板包按任意大小的组扩展的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!