[Muduo网络库源码分析] (11) base/Types.h_基本类型声明

基本类型声明

功能：为网络库提供基本的类型

---

知识点：

• stirng 的三种实现方式：

---

用途：

可用与类型转换和string的高效使用

---

代码及分析：

Types.h

#ifndef MUDUO_BASE_TYPES_H
#define MUDUO_BASE_TYPES_H

#include 
#ifdef MUDUO_STD_STRING
#include 
#else  // !MUDUO_STD_STRING
#include 
#include 
#endif

#ifndef NDEBUG
#include 
#endif

///
/// The most common stuffs.
///
namespace muduo
{

#ifdef MUDUO_STD_STRING
//无特殊处理的C++实现方式
using std::string;
#else  // !MUDUO_STD_STRING
//利用string的短字符优化实现方式
typedef __gnu_cxx::__sso_string string;
#endif

// Taken from google-protobuf stubs/common.h
//
// Protocol Buffers - Google's data interchange format
// Copyright 2008 Google Inc.  All rights reserved.
// http://code.google.com/p/protobuf/
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
//     * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//     * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following disclaimer
// in the documentation and/or other materials provided with the
// distribution.
//     * Neither the name of Google Inc. nor the names of its
// contributors may be used to endorse or promote products derived from
// this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

// Author: [email protected] (Kenton Varda) and others
//
// Contains basic types and utilities used by the rest of the library.

//
// Use implicit_cast as a safe version of static_cast or const_cast
// for upcasting in the type hierarchy (i.e. casting a pointer to Foo
// to a pointer to SuperclassOfFoo or casting a pointer to Foo to
// a const pointer to Foo).
// When you use implicit_cast, the compiler checks that the cast is safe.
// Such explicit implicit_casts are necessary in surprisingly many
// situations where C++ demands an exact type match instead of an
// argument type convertable to a target type.
//
// The From type can be inferred, so the preferred syntax for using
// implicit_cast is the same as for static_cast etc.:
//
//   implicit_cast(expr)
//
// implicit_cast would have been part of the C++ standard library,
// but the proposal was submitted too late.  It will probably make
// its way into the language in the future.

//类型强转，implicit_cast只能执行up-cast，派生类->基类
//这里模拟了boost库里面的implicit_cast
template<typename To, typename From>
inline To implicit_cast(From const &f)
{
  return f;
}

// When you upcast (that is, cast a pointer from type Foo to type
// SuperclassOfFoo), it's fine to use implicit_cast<>, since upcasts
// always succeed.  When you downcast (that is, cast a pointer from
// type Foo to type SubclassOfFoo), static_cast<> isn't safe, because
// how do you know the pointer is really of type SubclassOfFoo?  It
// could be a bare Foo, or of type DifferentSubclassOfFoo.  Thus,
// when you downcast, you should use this macro.  In debug mode, we
// use dynamic_cast<> to double-check the downcast is legal (we die
// if it's not).  In normal mode, we do the efficient static_cast<>
// instead.  Thus, it's important to test in debug mode to make sure
// the cast is legal!
//    This is the only place in the code we should use dynamic_cast<>.
// In particular, you SHOULDN'T be using dynamic_cast<> in order to
// do RTTI (eg code like this:
//    if (dynamic_cast(foo)) HandleASubclass1Object(foo);
//    if (dynamic_cast(foo)) HandleASubclass2Object(foo);
// You should design the code some other way not to need this.


//进行down_cast 基类->派生类转换，并检查转换是否安全
template<typename To, typename From>     // use like this: down_cast(foo);
inline To down_cast(From* f)                     // so we only accept pointers
{
  // Ensures that To is a sub-type of From *.  This test is here only
  // for compile-time type checking, and has no overhead in an
  // optimized build at run-time, as it will be optimized away
  // completely.
  if (false)
  {
    implicit_cast(0);
  }

#if !defined(NDEBUG) && !defined(GOOGLE_PROTOBUF_NO_RTTI)
  //dynamic_cast检查该类型转换是否安全
  assert(f == NULL || dynamic_cast(f) != NULL);  // RTTI: debug mode only!
#endif
  return static_cast(f);
}

}

#endif