在参数包中的每个元素上应用功能 [英] Apply function on each element in parameter pack

问题描述

我具有以下专门化的模板功能:

I have the following template function with specialization:

// Pass the argument through ...
template<typename T, typename U=T>
U convert(T&& t) {
  return std::forward<T>(t);
}

// ... but convert std::strings
const char* convert(std::string s) {
  return s.c_str();
}

如果我那时有一个可变参数模板函数，如:

If I then have a variadic template function like:

template<typename ... Args>
void doSomething(Args ... args) {
  // Convert parameter pack using convert function above
  // and call any other variadic templated function with
  // the converted args.
}

是否可以使用注释中的convert函数转换参数包?

Is there any way to convert the parameter pack using the convert function as in the comment?

我最初的目标是能够在类似printf的函数中将std :: string传递给'％s'，而不必首先手动在字符串上调用.c_str().但是我也总体上对是否可以通过简单的方式完成操作感兴趣，到目前为止，我的尝试都失败了.

My original goal was being to be able to pass std::string to '%s' in a printf like function without having to manually calling .c_str() on the strings first. But I am also interested in general if this can be done in a simple way, my tries so far failed.

推荐答案

template<typename ... Args>
void doSomething(Args ... args) {
  something(convert(args)...);
}

其中something(convert(args)...)是参数包扩展，其扩展为:

// pseudocode
something(convert(arg0), convert(arg1), convert(arg2), ...)

---

顺便说一句，您可能想通过转发引用来获取args，以避免不必要的复制并正确传播左值引用:

---

BTW, you might want to take args by forwarding references in order to avoid unnecessary copies and to properly propagate lvalue references:

template<typename... Args>
void doSomething(Args&& ... args) {
  something(convert(std::forward<Args>(args))...);
}

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