C/CPP __restrict__ (Implementation Specific) Keyword

from : What does the restrict keyword mean in C++? - Stack Overflow

I was always unsure, what does the restrict keyword mean in C++?

Does it mean the two or more pointer given to the function does not overlap? What else does it mean?

===> a pointer with "restrict" keyword or the implementation/compiler specific version of it, means that the content refered to by the pointer can and only can be accessed through the said pointer.

c++restrict-qualifier

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• 34

  restrict is a c99 keyword. Yes, Rpbert S. Barnes, I know that most compilers support __restrict__. You will note that anything with double underscores is, by definition, implementation specific and thus NOT C++, but a compiler specific version of it. 

  – KitsuneYMG

   Jan 6 '10 at 9:31

• 8

  What? Just because it's implementation specific does not make it not C++; the C++ allows for implementation specific stuff explicitly, and does not disallow it or render it not C++. 

  – Alice

   May 29 '14 at 5:02

• 7

  @Alice KitsuneYMG means that it's not part of ISO C++, and is instead considered a C++ extension. Compiler creators are allowed to make and distribute their own extensions, which coexist with ISO C++ and act as part of a usually-less-or-non-portable unofficial addition to C++. Examples would be MS's old Managed C++, and their more recent C++/CLI. Other examples would be preprocessor directives and macros supplied by some compilers, such as the common #warning directive, or the function signature macros (__PRETTY_FUNCTION__ on GCC, __FUNCSIG__ on MSVC, etc.). 

  – Justin Time - Reinstate Monica

   Mar 17 '16 at 21:57 

• 6

  @Alice To my knowledge, C++11 doesn't mandate full support for all of C99, nor do C++14 or what I know of C++17. restrict isn't considered a C++ keyword (see en.cppreference.com/w/cpp/keyword ), and in fact, the only mention of restrict in the C++11 standard (see open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3337.pdf , a copy of the FDIS with minor editorial changes, §17.2 [library.c], PDF page 413) states that: 

  – Justin Time - Reinstate Monica

   Mar 20 '16 at 17:10

• 6

  @Alice How so? I stated the part that says that restrict is to be omitted from (excluded from, left out of) C standard library function signatures and semantics when those functions are included in the C++ standard library. Or in other words, I stated the fact that says that if a C standard library function's signature contains restrict in C, the restrict keyword must be removed from the C++ equivalent's signature. 

  – Justin Time - Reinstate Monica

(Most Voted Answer)

In his paper, Memory Optimization, Christer Ericson says that while restrict is not part of the C++ standard yet, that it is supported by many compilers and he recommends it's usage when available:

restrict keyword

! New to 1999 ANSI/ISO C standard

! Not in C++ standard yet, but supported by many C++ compilers

! A hint only, so may do nothing and still be conforming

A restrict-qualified pointer (or reference)...

! ...is basically a promise to the compiler that for the scope of the pointer, the target of the pointer will only be accessed through that pointer (and pointers copied from it).

In C++ compilers that support it it should probably behave the same as in C.

See this SO post for details: Realistic usage of the C99 ‘restrict’ keyword?

Take half an hour to skim through Ericson's paper, it's interesting and worth the time.

Edit

I also found that IBM's AIX C/C++ compiler supports the __restrict__ keyword.

g++ also seems to support this as the following program compiles cleanly on g++:

#include <stdio.h>

int foo(int * __restrict__ a, int * __restrict__ b) {
    return *a + *b;
}

int main(void) {
    int a = 1, b = 1, c;

    c = foo(&a, &b);

    printf("c == %d\n", c);

    return 0;
}

I also found a nice article on the use of restrict:

Demystifying The Restrict Keyword

Edit2

I ran across an article which specifically discusses the use of restrict in C++ programs:

Load-hit-stores and the __restrict keyword

Also, Microsoft Visual C++ also supports the __restrict keyword.

The Memory Optimisation paper link is dead, here's a link to the audio from his GDC presentation. gdcvault.com/play/1022689/Memory 

– Grimeh

 Dec 8 '16 at 20:21

• 1

  @EnnMichael: Obviously if you're going to use it in a portable C++ project, you should #ifndef __GNUC__ #define __restrict__ /* no-op */ or similar. And define it to __restrict if _MSC_VER is defined. 

  – Peter Cordes

   Apr 27 '17 at 22:43