使用 std::ostream 打印可变参数包的最简单方法是什么?

问题描述：

使用 std::ostream 打印以逗号分隔的参数包的最简单方法是什么?

What is the easiest way to print a parameter pack, separated by commas, using std::ostream?

示例:

template<typename... Args>
void doPrint(std::ostream& out, Args... args){
   out << args...; // WRONG! What to write here?
}

// Usage:
int main(){
   doPrint(std::cout,34,"bla",15); // Should print: 34,bla,15
}

注意:可以假设 运算符的相应重载可用于所有类型的参数包.

Note: It may be assumed that a corresponding overload of the << operator is available for all types of the parameter pack.

答

无需递归调用和您想要的逗号.

Without recursive calls and commas where you wanted.

在c++11/c++14通过参数包扩展:

template <typename Arg, typename... Args>
void doPrint(std::ostream& out, Arg&& arg, Args&&... args)
{
    out << std::forward<Arg>(arg);
    using expander = int[];
    (void)expander{0, (void(out << ',' << std::forward<Args>(args)), 0)...};
}

演示

在c++17 使用折叠表达式:

template <typename Arg, typename... Args>
void doPrint(std::ostream& out, Arg&& arg, Args&&... args)
{
    out << std::forward<Arg>(arg);
    ((out << ',' << std::forward<Args>(args)), ...);
}

演示 2

使用 std::ostream 打印可变参数包的最简单方法是什么?

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