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?
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.