std::vector 的流运算符的参数相关查找

Question

我在我的命名空间中为 std::vector 创建了一个流输出运算符，我能够在我以前的调试代码中使用它：

namespace my_company {

template <typename T>
std::ostream &operator<<(std::ostream &os, std::vector<T> const &vec) {
  os << "{";
  for (int i = 0; i < vec.size(); ++i) {
    if (i != 0) {
      os << ", ";
    }
    os << vec[i];
  }
  os << "}";
  return os;
}

template <typename T>
void debug_impl(T const &var, char const *const expr, char const *const file, int const line) {
  using std::vector;
  std::lock_guard lg(debug_print_mutex);
  std::cout << "DEBUG " << file << ":" << line << " T" << std::this_thread::get_id() << "\n  " << expr << ": "
            << var << std::endl;}

#define DEBUG(var) ::my_company::debug_impl(var, #var, __FILE__, __LINE__)

}

但现在我想使用我的记录器class:

namespace my_company {

class Logger {
 public:
  virtual ~Logger() = default;

  virtual void log_impl(LogLevel log_level, std::string message) = 0;

  template <typename... T>
  void log_bits(LogLevel log_level, T const &...ts) {
    if (log_level >= threshold)
      log_impl(log_level, format_bits(ts...));
  }
  
  template <typename... T>
  void trace(T const &...ts) {
    log_bits(LogLevel::trace, ts...);
  }

 private:
  LogLevel threshold = LogLevel::trace;
};

}

格式化函数是这个：

namespace my_company {

template <typename... T>
std::string format_bits(T const &...ts) {
  std::ostringstream oss;
  // The following is a C++17 [fold expression](https://en.cppreference.com/w/cpp/language/fold).
  (oss << ... << ts);
  return oss.str();
}

}

在 debug_impl 函数中，我现在只委托给记录器：

namespace my_company {

template <typename T>
void debug_impl(T const &var, char const *const expr, char const *const file, int const line) {
  logger->template trace(file, ":", line, " T", std::this_thread::get_id(), "\n  ", expr, ": ", var);
}

}

Clang 不喜欢这样：

logger.h:29:11: error: call to function 'operator<<' that is neither visible in the template definition nor found by argument-dependent lookup
  (oss << ... << ts);
          ^
logger.h:44:27: note: in instantiation of function template specialization 'my_company::format_bits<const char *, char [2], int, char [3], std::thread::id, char [4], const char *, char [3], std::vector<bool, std::allocator<bool>>>' requested here
      log_impl(log_level, format_bits(ts...));
                          ^
logger.h:69:5: note: in instantiation of function template specialization 'my_company::Logger::log_bits<const char *, char [2], int, char [3], std::thread::id, char [4], const char *, char [3], std::vector<bool, std::allocator<bool>>>' requested here
    log_bits(LogLevel::trace, ts...);
    ^
util.h:693:20: note: in instantiation of function template specialization 'my_company::Logger::trace<const char *, char [2], int, char [3], std::thread::id, char [4], const char *, char [3], std::vector<bool, std::allocator<bool>>>' requested here
  logger->template trace(file, ":", line, " T", std::this_thread::get_id(), "\n  ", expr, ": ", var);
                   ^
rotating_buffer.h:40:5: note: in instantiation of function template specialization 'my_company::debug_impl<std::vector<bool, std::allocator<bool>>>' requested here
    DEBUG(used);
    ^
util.h:699:42: note: expanded from macro 'DEBUG'
#define DEBUG(var) ::my_company::debug_impl(var, #var, __FILE__, __LINE__)
                                         ^
logger.h:13:15: note: 'operator<<' should be declared prior to the call site
std::ostream &operator<<(std::ostream &os, std::vector<T> const &vec) {
              ^

我已经在call site之前声明了，都是在logger.h文件里声明的，其他的都包含进去了。 operator<< 是该文件中的第一件事，因此它应该可用。我认为额外的模板层并没有使它变得更容易。

我已经阅读了其他一些问题，但我不明白如何解决这个问题，因为不允许将 operator<< 放入 std，但它不起作用在 my_company 中并将其放入全局命名空间也不起作用。我能做什么？

我们在 Linux.

上使用 Clang 11 和 C++20

有些相关：

Argument Dependent Lookup and stream operators overloading
Output stream operator Argument Dependent Lookup (ADL) for fundamental/STL types/classes

Answer 1

这似乎是一个 clang 11 错误。

Clang 12 接受代码 Demo。

作为解决方法，您可以使用 (oss << ... << ts);:

((oss << ts), ...);

std::vector 的流运算符的参数相关查找

Argument dependent lookup with stream operator for std::vector

c++

templates

argument-dependent-lookup