析构函数，必须是virtual的，否则会内存泄漏
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;
}
输出是：
[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;
}
结果是：
[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;
};
因为它没有定义虚析构函数，当delete (IBytesOutputStream*)os时，子类的析构不会调用。
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;
}
可以看到内存一直涨：
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