Using Memset On Class Objects

It’s common practice in C, to do a memset on structures, in order to initialize all member variables to some default value, usually NULL. Similarly, you can use memset to initialize class objects. But what happens if the class contains some virtual functions?

Let’s take an example:

 class GraphicsObject{protected:	char *m_pcName;	int    m_iId;	//etcpublic:	virtual void Draw() {}	virtual int Area() {}	char* Name() { return m_pcName;}};class Circle: public GraphicsObject{	void Draw() { /*draw something*/ }	int Area() { /*calculate Area*/ }};void main(){	GraphicsObject *obj = new Circle; //Create an object	memset((void *)obj,NULL,sizeof(Circle)); //memset to 0	obj->Name(); //Non virtual function call works fine	obj->Draw(); //Crashes here}


This results in a crash, because every object of a class containing virtual functions contains a pointer to the virtual function table(vtbl). This pointer is used to resolve virtual function calls, at run time and for dynamic type casting. The pointer is hidden, and is not accessible to programmers by normal means. When we do a memset, the value of this pointer also gets overwritten, which, in turn, results in a crash, if a virtual function is called.

To avoid such mysterious crashes, memset should not be used on the objects of a class with virtual functions. Instead use the default constructor or an init routine to initialize member variables.

Share the Post:
Share on facebook
Share on twitter
Share on linkedin

Overview

The Latest

Top 5 B2B SaaS Marketing Agencies for 2023

In recent years, the software-as-a-service (SaaS) sector has experienced exponential growth as more and more companies choose cloud-based solutions. Any SaaS company hoping to stay ahead of the curve in this quickly changing industry needs to invest in effective marketing. So selecting the best marketing agency can mean the difference

technology leadership

Why the World Needs More Technology Leadership

As a fact, technology has touched every single aspect of our lives. And there are some technology giants in today’s world which have been frequently opined to have a strong influence on recent overall technological influence. Moreover, those tech giants have popular technology leaders leading the companies toward achieving greatness.

iOS app development

The Future of iOS App Development: Trends to Watch

When it launched in 2008, the Apple App Store only had 500 apps available. By the first quarter of 2022, the store had about 2.18 million iOS-exclusive apps. Average monthly app releases for the platform reached 34,000 in the first half of 2022, indicating rapid growth in iOS app development.