c++ - 非类型模板参数包扩展

Question

我只是打印出一个非类型模板参数包。但是如果我想在元素之间有空格，我找不到有效的扩展。

例子：

template < int ... N > 
struct X
{
    static void Check() { 
        // this will become std::cout << ( 1 << 2 ) << std::endl; -> prints "4"
        std::cout << "X" << (N << ...) <<  std::endl; 

        // this will become: ((std::cout << 1 ) << 2 ) << std::endl; -> prints "12"
        (std::cout << ... << N) << std::endl;

        // this becomes: (( std::cout << " " << 1 ) << 2 ) -> prints " 12"
        (std::cout << " " << ... << N) << std::endl;

        // the following is not allowed
        (std::cout << ... << " " << N) << std::endl;

        // also this one is not compiling
        (std::cout << ... << N << " ") << std::endl;
    }   
};

int main()
{
    X<1,2>::Check();
}

是否有机会在不使用辅助函数的情况下扩展参数包以打印中间有空格的元素？

Answer 1

使用辅助变量。

std::size_t count{};
((std::cout << (count++? " " : "") << N), ...);

Answer 2

另一种选择是额外定义第一个模板参数以方便格式化。

#include <iostream>

template<int... N> 
struct X {
  static void Check() { 
    if constexpr (sizeof...(N)) 
      [](auto first, auto... rest) {
        std::cout << first;
        ((std::cout << " " << rest), ...);
        std::cout << "\n";
      }(N...);
  }
};

int main() {
  X<1,2>::Check();
}