AMF0String => AMF0Base
假设析构函数不是virtual:
AMF0String* s = new AMF0String("abc"); delete s; //没有问题,可以调用AMF0String的析构函数释放heap内存。
AMF0String* s = new AMF0String("abc"); delete (AMF0Base*)s; // 内存泄漏,调用的是AMF0Base的析构函数。
看一段代码:
#include <stdlib.h> #include <stdio.h> #include <iostream> using namespace std; class MyStringBase { public: MyStringBase(){ } ~MyStringBase(){ cout << "~MyStringBase" << endl; } }; class MyStringSub : public MyStringBase { public: MyStringSub(){ } ~MyStringSub(){ cout << "~MyStringSub" << endl; } }; void MyFunction(MyStringBase* s){ delete s; // s->~xxx(); } int main(){ MyStringSub* str = new MyStringSub(); MyFunction(str); //sleep(100); return 0; }
[winlin@dev6 temp]$ g++ test.cpp -o test;./test
~MyStringBase
所以子类的析构不会调用。
应该写成:
#include <stdlib.h> #include <stdio.h> #include <iostream> using namespace std; class MyStringBase { public: MyStringBase(){ } virtual ~MyStringBase(){ cout << "~MyStringBase" << endl; } }; class MyStringSub : public MyStringBase { public: MyStringSub(){ } virtual ~MyStringSub(){ cout << "~MyStringSub" << endl; } }; void MyFunction(MyStringBase* s){ delete s; // s->~xxx(); } int main(){ MyStringSub* str = new MyStringSub(); MyFunction(str); //sleep(100); return 0; }
[winlin@dev6 temp]$ g++ test.cpp -o test;./test
~MyStringSub
~MyStringBase
成功调用子类的析构函数。
一个潜在的危险是,在以class定义接口时,必须定义虚析构函数,譬如以下代码可能造成泄漏:
/** * the bytes output stream, which write bytes into. */ class IBytesOutputStream { public: /** * read bytes from stream. */ virtual int32_t Write(int8_t* buffer, u_int32_t size) = 0; };
应该定义为:
/** * the bytes output stream, which write bytes into. */ class IBytesOutputStream { public: IBytesOutputStream(); virtual ~IBytesOutputStream(); public: /** * read bytes from stream. */ virtual int32_t Write(int8_t* buffer, u_int32_t size) = 0; };
举个完整的例子:
/** g++ -g -O0 use-interface.cpp -o use-interface; ./use-interface */ #include <stdio.h> #include <string.h> #include <unistd.h> class IObject { public: virtual const char* ToString() = 0; }; class String : public IObject { private: char* str; public: String(const char* s){ str = NULL; if(s != NULL){ str = new char[strlen(s) + 1]; strcpy(str, s); } } virtual ~String(){ if(str != NULL){ delete[] str; } } public: virtual const char* ToString(){ return str; } }; void destroy(IObject* obj){ delete obj; } int main(int /*argc*/, char** /*argv*/){ while(true){ destroy(new String("winlin")); usleep(1); } return 0; }
command: ./memview summary 24643 for use-interface VMSIZE: 11784 KB RSS: 856 KB total 712 KB shared 4 KB private clean 140 KB private dirty command: ./memview summary 24643 for use-interface VMSIZE: 11784 KB RSS: 884 KB total 712 KB shared 4 KB private clean 168 KB private dirty command: ./memview summary 24643 for use-interface VMSIZE: 11784 KB RSS: 916 KB total 712 KB shared 4 KB private clean 200 KB private dirty command: ./memview summary 24643 for use-interface VMSIZE: 11784 KB RSS: 948 KB total 712 KB shared 4 KB private clean 232 KB private dirty command: ./memview summary 24643 for use-interface VMSIZE: 11916 KB RSS: 980 KB total 712 KB shared 4 KB private clean 264 KB private dirty command: ./memview summary 24643 for use-interface VMSIZE: 11916 KB RSS: 1012 KB total 712 KB shared 4 KB private clean 296 KB private dirty command: ./memview summary 24643 for use-interface VMSIZE: 11916 KB RSS: 1044 KB total 712 KB shared 4 KB private clean 328 KB private dirty command: ./memview summary 24643 for use-interface VMSIZE: 11916 KB RSS: 1076 KB total 712 KB shared 4 KB private clean 360 KB private dirty command: ./memview summary 24643 for use-interface VMSIZE: 11916 KB RSS: 1104 KB total 712 KB shared 4 KB private clean 388 KB private dirty