#include ＜exception＞

在 C++ 编程中，异常处理是一种重要的错误处理机制，它允许程序在遇到错误时，能够优雅地处理这些错误，而不是让程序崩溃。

在 C++ 中，异常处理通常使用 try、catch 和 throw 关键字来实现。标准库中提供了 std::exception 类及其派生类来处理异常。

C++ 标准库中的 <exception> 头文件提供了一套异常处理的基础设施，包括异常类、异常处理机制等。

异常是程序运行时发生的一个事件，它中断了正常的指令流程。在C++中，异常可以是任何类型的对象，但通常是一个异常类的对象。C++标准库定义了一些基本的异常类，如 std::exception、std::bad_alloc、std::bad_cast 等。

你可以通过定义自己的异常类来扩展异常处理功能，或者使用标准库中已有的异常类来处理常见的异常情况。

// Exception Handling support header for -*- C++ -*-

// Copyright (C) 1995-2020 Free Software Foundation, Inc.
//
// This file is part of GCC.
//
// GCC is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation; either version 3, or (at your option)
// any later version.
//
// GCC is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/** @file exception
 *  This is a Standard C++ Library header.
 */

#ifndef __EXCEPTION__
#define __EXCEPTION__

#pragma GCC system_header

#pragma GCC visibility push(default)

#include <bits/c++config.h>
#include <bits/exception.h>

extern "C++" {

namespace std
{
  /** @addtogroup exceptions
   *  @{
   */

  /** If an %exception is thrown which is not listed in a function's
   *  %exception specification, one of these may be thrown.  */
  class bad_exception : public exception
  {
  public:
    bad_exception() _GLIBCXX_USE_NOEXCEPT { }

    // This declaration is not useless:
    // http://gcc.gnu.org/onlinedocs/gcc-3.0.2/gcc_6.html#SEC118
    virtual ~bad_exception() _GLIBCXX_TXN_SAFE_DYN _GLIBCXX_USE_NOEXCEPT;

    // See comment in eh_exception.cc.
    virtual const char*
    what() const _GLIBCXX_TXN_SAFE_DYN _GLIBCXX_USE_NOEXCEPT;
  };

  /// If you write a replacement %terminate handler, it must be of this type.
  typedef void (*terminate_handler) ();

  /// If you write a replacement %unexpected handler, it must be of this type.
  typedef void (*unexpected_handler) ();

  /// Takes a new handler function as an argument, returns the old function.
  terminate_handler set_terminate(terminate_handler) _GLIBCXX_USE_NOEXCEPT;

#if __cplusplus >= 201103L
  /// Return the current terminate handler.
  terminate_handler get_terminate() noexcept;
#endif

  /** The runtime will call this function if %exception handling must be
   *  abandoned for any reason.  It can also be called by the user.  */
  void terminate() _GLIBCXX_USE_NOEXCEPT __attribute__ ((__noreturn__));

  /// Takes a new handler function as an argument, returns the old function.
  unexpected_handler set_unexpected(unexpected_handler) _GLIBCXX_USE_NOEXCEPT;

#if __cplusplus >= 201103L
  /// Return the current unexpected handler.
  unexpected_handler get_unexpected() noexcept;
#endif

  /** The runtime will call this function if an %exception is thrown which
   *  violates the function's %exception specification.  */
  void unexpected() __attribute__ ((__noreturn__));

  /** [18.6.4]/1:  'Returns true after completing evaluation of a
   *  throw-expression until either completing initialization of the
   *  exception-declaration in the matching handler or entering @c unexpected()
   *  due to the throw; or after entering @c terminate() for any reason
   *  other than an explicit call to @c terminate().  [Note: This includes
   *  stack unwinding [15.2].  end note]'
   *
   *  2: 'When @c uncaught_exception() is true, throwing an
   *  %exception can result in a call of @c terminate()
   *  (15.5.1).'
   */
  _GLIBCXX17_DEPRECATED
  bool uncaught_exception() _GLIBCXX_USE_NOEXCEPT __attribute__ ((__pure__));

#if __cplusplus >= 201703L || !defined(__STRICT_ANSI__) // c++17 or gnu++98
#define __cpp_lib_uncaught_exceptions 201411L
  /// The number of uncaught exceptions.
  int uncaught_exceptions() _GLIBCXX_USE_NOEXCEPT __attribute__ ((__pure__));
#endif

  // @} group exceptions
} // namespace std

namespace __gnu_cxx
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

  /**
   *  @brief A replacement for the standard terminate_handler which
   *  prints more information about the terminating exception (if any)
   *  on stderr.
   *
   *  @ingroup exceptions
   *
   *  Call
   *   @code
   *     std::set_terminate(__gnu_cxx::__verbose_terminate_handler)
   *   @endcode
   *  to use.  For more info, see
   *  http://gcc.gnu.org/onlinedocs/libstdc++/manual/bk01pt02ch06s02.html
   *
   *  In 3.4 and later, this is on by default.
   */
  void __verbose_terminate_handler();

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace

} // extern "C++"

#pragma GCC visibility pop

#if (__cplusplus >= 201103L)
#include <bits/exception_ptr.h>
#include <bits/nested_exception.h>
#endif

#endif

语法

抛出异常

在 C++ 中，使用 throw 关键字来抛出一个异常，语法如下：

throw exception_object;

捕获异常

使用 try 和 catch 关键字来捕获和处理异常，语法如下：

try {
    // 可能抛出异常的代码
} catch (exception_type e) {
    // 处理异常的代码
}

可以指定捕获的异常类型，也可以使用通用的 catch 块捕获所有类型的异常:

try {
    // 可能会抛出异常的代码
} catch (const std::exception& e) {
    // 处理 std::exception 及其派生类的异常
} catch (...) {
    // 处理所有其他类型的异常
}

std::exception

std::exception 是 C++ 标准库中定义的基类，用于所有标准异常类的基础。它定义了一些虚函数，如 what()，用于返回异常信息的 C 风格字符串。

class exception {
public:
    virtual const char* what() const noexcept;
};

标准异常类

C++ 标准库提供了多个派生自 std::exception 的异常类，如 std::runtime_error、std::logic_error 等，用于表示常见的异常情况。你可以根据具体的异常情况选择合适的类来使用。

throw std::runtime_error("Runtime error occurred");
throw std::logic_error("Logic error occurred");