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In the C programming language, restrict is a keyword, introduced by the C99 standard, [1] that can be used in pointer declarations. By adding this type qualifier , a programmer hints to the compiler that for the lifetime of the pointer, no other pointer will be used to access the object to which it points.
1 Why restrict on all pointers? 2 Alternative working example. 1 comment. 3 "restrict" optional in C++ [but not in C] an important difference? 1 comment.
As of 2014 and C11, there are four type qualifiers in standard C: const , volatile , restrict and _Atomic – the latter has a private name to avoid clashing with user-defined names. [3] The first two of these, const and volatile , are also present in C++, and are the only type qualifiers in C++.
Specifically, C allows a void* pointer to be assigned to any pointer type without a cast, while C++ does not; this idiom appears often in C code using malloc memory allocation, [9] or in the passing of context pointers to the POSIX pthreads API, and other frameworks involving callbacks. For example, the following is valid in C but not C++:
Typical examples of pointers are start pointers, end pointers, and stack pointers. These pointers can either be absolute (the actual physical address or a virtual address in virtual memory ) or relative (an offset from an absolute start address ("base") that typically uses fewer bits than a full address, but will usually require one additional ...
The element pc requires ten blocks of memory of the size of pointer to char (usually 40 or 80 bytes on common platforms), but element pa is only one pointer (size 4 or 8 bytes), and the data it refers to is an array of ten bytes (sizeof * pa == 10).
This kind of type punning is more dangerous than most. Whereas the former example relied only on guarantees made by the C programming language about structure layout and pointer convertibility, the latter example relies on assumptions about a particular system's hardware.
The d-pointer pattern is one of the implementations of the opaque pointer. It is commonly used in C++ classes due to its advantages (noted below). A d-pointer is a private data member of the class that points to an instance of a structure. This method allows class declarations to omit private data members, except for the d-pointer itself. [6]