c++ - std::experimental::source_location 如何实现？

Question

库基础的 C++ 扩展，版本 2 ( N4564 ) 引入了 std::experimental::source_location 类型。

§ 14.1.2 [reflection.src_loc.creation] 说:

static constexpr source_location current() noexcept;

Returns: When invoked by a function call (C++14 § 5.2.2) whose postfix-expression is a (possibly parenthesized) id-expression naming current, returns a source_location with an implementation-defined value. The value should be affected by #line (C++14 § 16.4) in the same manner as for __LINE__ and __FILE__. If invoked in some other way, the value returned is unspecified.

Remarks: When a brace-or-equal-initializer is used to initialize a non-static data member, any calls to current should correspond to the location of the constructor or aggregate initialization that initializes the member.

[ Note: When used as a default argument (C++14 § 8.3.6), the value of the source_location will be the location of the call to current at the call site. — end note ]

如果我没理解错的话，那么这个功能就是这样使用的。

#include <experimental/source_location>  // I don't actually have this header
#include <iostream>
#include <string>
#include <utility>

struct my_exception
{

  std::string message {};
  std::experimental::source_location location {};

  my_exception(std::string msg,
               std::experimental::source_location loc = std::experimental::source_location::current()) :
    message {std::move(msg)},
    location {std::move(loc)}
  {
  }

};

int
do_stuff(const int a, const int b)
{
  if (a > b)
    throw my_exception {"a > b"};  // line 25 of file main.cxx
  return b - a;
}

int
main()
{
  try
    {
      std::cout << do_stuff(2, 1) << "\n";
    }
  catch (const my_exception& e)
    {
      std::cerr << e.location.file_name() << ":" << e.location.line() << ": "
                << "error: " << e.message << "\n";
    }
}

预期输出:

main.cxx:25: error: a > b

如果没有 std::experimental::source_location ，我们可能会使用辅助宏 THROW_WITH_SOURCE_LOCATION，它在内部使用 __FILE__ 和 __LINE__ 宏来正确初始化异常对象。

我想知道图书馆如何实现 std::experimental::source_location 。除非我完全忽略了这一点，否则如果没有特殊的编译器支持，这样做是不可能的。但是需要什么样的神奇编译器功能才能使这项工作正常进行呢？它可以与为 std::initializer_list 部署的技巧相媲美吗？是否有此功能的任何实验性实现可供查看？我检查了 the SVN sources for GCC，但还没有找到任何东西。

Answer 1

实现它需要编译器的支持。例如，with gcc , 你可以使用 built-ins functions喜欢

   int __builtin_LINE()

This function is the equivalent to the preprocessor __LINE__ macro and returns the line number of the invocation of the built-in. In a C++ default argument for a function F, it gets the line number of the call to F.

   const char * __builtin_FUNCTION()

This function is the equivalent to the preprocessor __FUNCTION__ macro and returns the function name the invocation of the built-in is in.

   const char * __builtin_FILE()

This function is the equivalent to the preprocessor __FILE__ macro and returns the file name the invocation of the built-in is in. In a C++ default argument for a function F, it gets the file name of the call to F.