Embedded Systems Design - July 2008 - (Page 16)

programming pointers • In C++, it’s the programmer’s responsibility to choose the correct form for each delete-expression (for a single object or array) by including or omitting []. In C, you simply call free(p) whether p points to a single object or to an array. structor to that storage to produce a properly constructed widget object. C and C++ handle dynamic allocation in a fundamentally different way: Whereas malloc just allocates storage of indeterminate value, a new-expression can create a properly initialized object. What about using calloc instead of malloc? Doesn’t calloc initialize objects to zero? Indeed, for some types, setting all bits to zero is a reasonable and useful initial value, but for many class types, it isn’t. In fact, as the C standard notes, all bits zero might not even be the proper representation for a ﬂoating-point zero or a • If C++ really did place a greater premium on type safety than C does, then wouldn’t C++ simply avoid these errors by proAlthough C++ places great viding just one form for emphasis on type safety, in this delete-expressions? case, it trades a little type safety Although C++ places great emphasis on type safefor potentially signiﬁcant ty, it’s not to the exclusion of performance improvements in other concerns, such as permemory allocation. formance. In this case, C++ trades a little type safety for potentially signiﬁcant performance improvements in null pointer. memory allocation. As I’ll explain in an upcoming article, C++ classes use special member functions called dethis design for new- and delete-expressions makes it easistructors to provide automatic resource deallocation. A deer to implement and use efﬁcient customized memory structor’s name is the same as the constructor’s name, but managers. with a ~ in front of the name, as in: INITIALIZING ALLOCATED OBJECTS In C++, a class constructor is a special class member function that initializes objects of its class type. A constructor’s function name is always the same as its class name, as in: class widget { public: widget(); }; class widget { public: widget(); ~widget(); }; // a constructor // a destructor // a constructor Constructors provide guaranteed initialization for class objects. You don’t write calls to constructors—the compiler generates them for you. Whenever you deﬁne an object with a class type, the compiler automatically plants a call to the object’s constructor at the right place in the program. For guaranteed initialization to really be guaranteed, the compiler must generate a call to the appropriate constructor wherever the source code creates an object, including in new-expressions. Thus, for a class type such as widget, a new-expression such as in: p = new widget; Just as new-expressions invoke constructors, delete-expressions invoke destructors. I will elaborate the connection between new-expressions and constructors, and between delete-expressions and destructors, in a forthcoming column. ■ ENDNOTES: 1. Saks, Dan.“The yin and yang of dynamic allocation,” Embedded Systems Design, May, 2008, p. 12. Available online at www.embedded.com/207402546. Saks, Dan.“As Precise as Possible,” Embedded Systems Programming, April, 2002, p. 43. Available online at www.embedded.com/9900563. Saks, Dan.“Why size_t matters,” Embedded Systems Design, July, 2007, p. 13. Available online at www.embedded.com/200900195. Saks, Dan.“Into, but not out of, the void,” Embedded.com, June 2008, www.embedded.com/208403407. Saks, Dan.“Cast with caution,” Embedded Systems Design, July, 2006, p. 15. Available online at www.embedded.com/191600535. 2. 3. 4. 5. doesn’t just allocate storage for a widget; it applies a con- 16 JULY 2008 | embedded systems design | www.embedded.com http://www.embedded.com/207402546 http://www.embedded.com/9900563 http://www.embedded.com/200900195 http://www.embedded.com/208403407 http://www.embedded.com/191600535 http://www.embedded.com

Table of Contents Feed for the Digital Edition of Embedded Systems Design - July 2008

Embedded Systems Design - July 2008 Contents #Include Parity Bit Programming Pointers Interactive C-code Cleaning Tool Supports Multiprocessor SoC Design Building a Power Supply for Discontinuous Transmission Wireless Networks An Exception Primer Advertising Index Break Points Marketplace

Embedded Systems Design - July 2008

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