首先，我不知道为什么你需要这么多的代码 make_string 。我只需将其定义为

  template< class ... Args> 
 inline std :: string make_string（Args& ... args）
 {
 ostringstream ostr; 
 _do {ostr<< std :: forward< Args>（args）...}; 
 return std :: move（ostr.str（））; 
} 
 

其中

  struct _do {template< typename ... T> _do（T&& ...）{}}; 
 

是一个助手结构，允许以正确的顺序计算表达式href =http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51253 =nofollow>不正确地评估从右到左，直到4.9）

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现在，您的问题。正如我在我的评论中说的，我觉得你的问题与 make_string 无关。在静态类成员的未定义引用中，在我的问题通过通用引用传递静态constexpr变量？，在所有相关问题中， ve看到，建议的答案是一个定义变量在某处的类：

  constexpr int foo :: max_offset; 
 

我不知道这是否对你有问题。这是我的一个问题，因为在模板化的代码中，它意味着太多的重复（见下面的讨论我的问题）。无论如何，如果它是一个问题，我看到一些其他简单的解决方案，以确保call-by-value：

• use make_string（...，max_offset）而不是 make_string = $ http：// ***。




• 作为快捷方式， + max_offset com / a / 272996/2644390>这里）




• define static constexpr int max_offset（）{return some_value; } ，然后使用 max_offset（），而不是 max_offset li>
  

• 让代码（函数或模板）的一部分推导出 max_offset 作为非类型 int 模板参数，然后直接使用



• 最后，定义 make_string（Args ... args） code>（这是最简单的，但不适用于此，因为您不想复制所有这些字符串）

我不是在讨论使用 make_string 抛出异常;这是一个不同的问题。

edit This is not a duplicate of Undefined reference to static class member. That question explored the cause of the problem (which I explain below). Here, I'm looking for a different solution from those proposed in the answers to that questions (which implied changing the declaration/definition of the constexpr variable to be used -- essentially by adding a definition in a compilation unit).

I have created a little variadic template function make_string() to generate a std::string from any number of io-able arguments as follows.

using std::ostringstream; // just for this example

inline ostringstream&write(ostringstream&ostr, const char*x)
{ if(x) ostr<<x;  return ostr; }

template<class T>
inline ostringstream&write(ostringstream&ostr, T const&x)
{ ostr<<x;  return ostr; }

inline ostringstream&write(ostringstream&ostr) noexcept
{ return ostr; }

template<class T, class... R>
inline ostringstream&write(ostringstream&ostr, T const&x, R&&... r)
{ return write(write(ostr,x), std::forward<R>(r)...); }

inline std::string make_string(const char*text)
{ return {text?text:""}; }

inline std::string make_string(std::string const&text)
{ return {text}; }

template<typename T>
inline auto make_string(T var) -> decltype(std::to_string(var))
{ return std::to_string(var); }

template<class... Args>
inline std::string make_string(Args&&... args)
{
  ostringstream ostr;
  write(ostr,std::forward<Args>(args)...);
  return std::move(ostr.str());
}

Now, this works pretty well and can be used like this

throw std::runtime_error(make_string("offset=",offset," > max_offset =",
                                      max_offset"));

However, there is a problem when printing static constexpr class members, as in

class foo
{
   static constexpr int max_offset=some_value;
   // ...
   void bar(int offset)
   {
     if(offset > max_offset)
     throw std::runtime_error(make_string("offset=",offset," > max_offset=",
                                          max_offset"));
   }
};

This causes an error at link time. The reason is that make_string takes all its arguments by reference, including the static constexpr max_offset. As a result, a reference to foo::max_offset will be required at linking, see also.

How can I avoid this problem without abandoning the idea of make_string()? (Perhaps one could replace the variadic template with a variadic macro, but I would consider this as some sort of regression.) There must be a way for make_string to take its arguments by value or reference, depending on type (so that builtin types can be taken by value). How?

First, I am not sure why you need so much code for make_string. I'd simply define it as

template<class... Args>
inline std::string make_string(Args&&... args)
{
  ostringstream ostr;
  _do{ostr << std::forward<Args>(args)...};
  return std::move(ostr.str());
}

where

struct _do { template <typename... T> _do(T&&...) { } };

is a helper struct that lets you evaluate expressions in the right order (but watch out, GCC incorrectly evaluates right-to-left until 4.9 at least).

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Now, to your question. As I said in my comment, I feel your problem is irrelevant to make_string. In Undefined reference to static class member, in my question passing a static constexpr variable by universal reference?, and in all relevant questions I've seen, the suggested answer is that one defines the variable somewhere out of class:

constexpr int foo::max_offset;

I'm not sure if this is a problem for you. It is a problem for me because in heavily templated code it implies too much duplication (see discussion below my question). Anyhow, if it is a problem, I see a few other simple solutions to ensure call-by-value:

• use make_string(..., int(max_offset)) instead of make_string(..., max_offset)

• as a shortcut, +max_offset does the same job (suggested here)

• define static constexpr int max_offset() { return some_value; }, then use max_offset() instead of max_offset throughout

• let some part of code (function or template) deduce max_offset as a non-type int template parameter, then use it directly

• lastly, define make_string(Args... args) (this is the simplest but does not apply here as you don't want to copy all those strings)

I am not discussing use of make_string in throwing an exception; this is a different problem.