Accessing a C++ Object in C Code: A Concrete Example

The C++ Standard guarantees that within every instance of class Date, data members are set down in the order of their declarations (static data members are stored outside the object and are therefore ignored). Consider this declaration of the class Date:

 class Date{public:  int day;  int month;  int year;  Date(); //current date  ~Date();  bool isLeap() const;  bool operator == (const Date& other);};

There is no requirement that members be set down in contiguous memory regions; the compiler can insert additional padding bytes between data members to ensure proper alignment. However, this is also the practice in C, so you can safely assume that a Date object has a memory layout that is identical to that of this C struct:

 struct POD_Date/* the following struct has memory layout that is identical to a Date object */{  int day;  int month;  int year;};

Consequently, a Date object can be passed to C code and treated as if it were an instance of POD_Date. You might be surprised that the memory layout in C and C++ is identical in this case; class Date defines member functions in addition to data members, yet there is no trace of these member functions in the object’s memory layout. Where are these member functions stored? C++ treats nonstatic member functions as static functions. In other words, member functions are ordinary functions. They are no different from global functions, except that they take an implicit this argument, which ensures that they are called on an object and that they can access its data members. An invocation of a member function is transformed to a function call, in which the compiler inserts an additional argument that holds the address of the object.

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

Overview

The Latest

microsoft careers

Top Careers at Microsoft

Microsoft has gained its position as one of the top companies in the world, and Microsoft careers are flourishing. This multinational company is efficiently developing popular software and computers with other consumer electronics. It is a dream come true for so many people to acquire a high paid, high-prestige job

your company's audio

4 Areas of Your Company Where Your Audio Really Matters

Your company probably relies on audio more than you realize. Whether you’re creating a spoken text message to a colleague or giving a speech, you want your audio to shine. Otherwise, you could cause avoidable friction points and potentially hurt your brand reputation. For example, let’s say you create a

chrome os developer mode

How to Turn on Chrome OS Developer Mode

Google’s Chrome OS is a popular operating system that is widely used on Chromebooks and other devices. While it is designed to be simple and user-friendly, there are times when users may want to access additional features and functionality. One way to do this is by turning on Chrome OS