{"id":"58954473","type":"page","status":"current","title":"06.C++类型转换","version":{"by":{"type":"known","username":"ethanliu","userKey":"2c90931b6cfb654f016d5c69becc0001","profilePicture":{"path":"/images/icons/profilepics/default.svg","width":48,"height":48,"isDefault":true},"displayName":"刘浩","_links":{"self":"https://wiki.suncaper.net/rest/api/user?key=2c90931b6cfb654f016d5c69becc0001"},"_expandable":{"status":""}},"when":"2022-07-09T01:51:24.000Z","message":"","number":1,"minorEdit":false,"hidden":false,"_links":{"self":"https://wiki.suncaper.net/rest/experimental/content/58954473/version/1"},"_expandable":{"content":"/rest/api/content/58954473"}},"body":{"storage":{"value":"<ac:layout><ac:layout-section ac:type=\"two_right_sidebar\"><ac:layout-cell><p>类型转换<span>(cast)</span>是将一种数据类型转换成另一种数据类型。例如，如果将一个整型值赋给一个浮点类型的变量，编译器会暗地里将其转换成浮点类型。</p><p>转换是非常有用的，但是它也会带来一些问题，比如在转换指针时，我们很可能将其转换成一个比它更大的类型，但这可能会破坏其他的数据。</p><p>应该小心类型转换，因为转换也就相当于对编译器说：忘记类型检查，把它看做其他的类型。</p><p>一般情况下，尽量少的去使用类型转换，除非用来解决非常特殊的问题。</p><p><span> </span></p><table><tbody><tr><td><p>    无论什么原因，任何一个程序如果使用很多类型转换都值得怀疑<span>.</span></p></td></tr></tbody></table><p><span> </span></p><p>标准<span>c++</span>提供了一个显示的转换的语法，来替代旧的<span>C</span>风格的类型转换。</p><p>使用<span>C</span>风格的强制转换可以把想要的任何东西转换成我们需要的类型。那为什么还需要一个新的<span>C++</span>类型的强制转换呢？</p><p>新类型的强制转换可以提供更好的控制强制转换过程，允许控制各种不同种类的强制转换。<span>C++</span>风格的强制转换其他的好处是，它们能更清晰的表明它们要干什么。程序员只要扫一眼这样的代码，就能立即知道一个强制转换的目的。</p><p><span> </span></p><h2><span>6.1 </span>静态转换<span>(static_cast)</span></h2><ul><li>用于<span><a href=\"http://baike.baidu.com/view/2405425.htm\">类层次结构</a></span>中基类（父类）和<span><a href=\"http://baike.baidu.com/view/535532.htm\">派生类</a></span>（子类）之间指针或引用的转换。</li><li>进行上行转换（把派生类的指针或引用转换成基类表示）是安全的；</li><li>进行下行转换（把基类指针或引用转换成派生类表示）时，由于没有动态类型检查，所以是不安全的。</li><li>用于基本数据类型之间的转换，如把<span>int</span>转换成<span>char</span>，把<span>char</span>转换成<span>int</span>。这种转换的安全性也要开发人员来保证。</li></ul><ac:structured-macro ac:name=\"code\" ac:schema-version=\"1\" ac:macro-id=\"b11d3ea1-fb26-4b3a-a201-86f8dfed7d77\"><ac:parameter ac:name=\"language\">cpp</ac:parameter><ac:plain-text-body><![CDATA[class Animal{};\nclass Dog : public Animal{};\nclass Other{};\n\n//基础数据类型转换\nvoid test01(){\n\tchar a = 'a';\n\tdouble b = static_cast<double>(a);\n}\n\n//继承关系指针互相转换\nvoid test02(){\n\t//继承关系指针转换\n\tAnimal* animal01 = NULL;\n\tDog* dog01 = NULL;\n\t//子类指针转成父类指针,安全\n\tAnimal* animal02 = static_cast<Animal*>(dog01);\n\t//父类指针转成子类指针，不安全\n\tDog* dog02 = static_cast<Dog*>(animal01);\n}\n\n//继承关系引用相互转换\nvoid test03(){\n\n\tAnimal ani_ref;\n\tDog dog_ref;\n\t//继承关系指针转换\n\tAnimal& animal01 = ani_ref;\n\tDog& dog01 = dog_ref;\n\t//子类指针转成父类指针,安全\n\tAnimal& animal02 = static_cast<Animal&>(dog01);\n\t//父类指针转成子类指针，不安全\n\tDog& dog02 = static_cast<Dog&>(animal01);\n}\n\n//无继承关系指针转换\nvoid test04(){\n\t\n\tAnimal* animal01 = NULL;\n\tOther* other01 = NULL;\n\n\t//转换失败\n\t//Animal* animal02 = static_cast<Animal*>(other01);\n}]]></ac:plain-text-body></ac:structured-macro><h2><span>2.2 </span>动态转换<span>(dynamic_cast)</span></h2><ul><li>dynamic_cast主要用于类层次间的上行转换和下行转换；</li><li>在类层次间进行上行转换时，<span>dynamic_cast</span>和<span>static_cast</span>的效果是一样的；</li><li>在进行下行转换时，<span>dynamic_cast</span>具有类型检查的功能，比<span>static_cast</span>更安全；</li></ul><ac:structured-macro ac:name=\"code\" ac:schema-version=\"1\" ac:macro-id=\"f8c3c5c9-48f2-4d7a-8469-a0dfb5529ecb\"><ac:parameter ac:name=\"language\">cpp</ac:parameter><ac:plain-text-body><![CDATA[class Animal {\npublic:\n\tvirtual void ShowName() = 0;\n};\nclass Dog : public Animal{\n\tvirtual void ShowName(){\n\t\tcout << \"I am a dog!\" << endl;\n\t}\n};\nclass Other {\npublic:\n\tvoid PrintSomething(){\n\t\tcout << \"我是其他类!\" << endl;\n\t}\n};\n\n//普通类型转换\nvoid test01(){\n\n\t//不支持基础数据类型\n\tint a = 10;\n\t//double a = dynamic_cast<double>(a);\n}\n\n//继承关系指针\nvoid test02(){\n\n\tAnimal* animal01 = NULL;\n\tDog* dog01 = new Dog;\n\n\t//子类指针转换成父类指针 可以\n\tAnimal* animal02 = dynamic_cast<Animal*>(dog01);\n\tanimal02->ShowName();\n\t//父类指针转换成子类指针 不可以\n\t//Dog* dog02 = dynamic_cast<Dog*>(animal01);\n}\n\n//继承关系引用\nvoid test03(){\n\n\tDog dog_ref;\n\tDog& dog01 = dog_ref;\n\n\t//子类引用转换成父类引用 可以\n\tAnimal& animal02 = dynamic_cast<Animal&>(dog01);\n\tanimal02.ShowName();\n}\n\n//无继承关系指针转换\nvoid test04(){\n\t\n\tAnimal* animal01 = NULL;\n\tOther* other = NULL;\n\n\t//不可以\n\t//Animal* animal02 = dynamic_cast<Animal*>(other);\n}]]></ac:plain-text-body></ac:structured-macro><h2><span>2.3 </span>常量转换<span>(const_cast)</span></h2><p>该运算符用来修改类型的<span>const</span>属性。。</p><ul><li>常量指针被转化成非常量指针，并且仍然指向原来的对象；</li><li>常量引用被转换成非常量引用，并且仍然指向原来的对象；</li></ul><p><span> </span></p><table><tbody><tr><td><p><strong>注意<span>:</span></strong>不能直接对非指针和非引用的变量使用<span>const_cast</span>操作符去直接移除它的<span>const.</span></p></td></tr></tbody></table><ac:structured-macro ac:name=\"code\" ac:schema-version=\"1\" ac:macro-id=\"a726eee3-319d-4acf-9f8c-2b80be07c77a\"><ac:parameter ac:name=\"language\">cpp</ac:parameter><ac:plain-text-body><![CDATA[//常量指针转换成非常量指针\nvoid test01(){\n\t\n\tconst int* p = NULL;\n\tint* np = const_cast<int*>(p);\n\n\tint* pp = NULL;\n\tconst int* npp = const_cast<const int*>(pp);\n\n\tconst int a = 10;  //不能对非指针或非引用进行转换\n\t//int b = const_cast<int>(a); }\n\n//常量引用转换成非常量引用\nvoid test02(){\n\nint num = 10;\n\tint & refNum = num;\n\n\tconst int& refNum2 = const_cast<const int&>(refNum);\n\t\n}]]></ac:plain-text-body></ac:structured-macro><h2><span>2.3 </span>重新解释转换<span>(reinterpret_cast)</span>。。。。。</h2><p>这是最不安全的一种转换机制，最有可能出问题。</p><p>主要用于将一种数据类型从一种类型转换为另一种类型。它可以将一个指针转换成一个整数，也可以将一个整数转换成一个指针<span>.</span></p></ac:layout-cell><ac:layout-cell><p><br 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