What does the restrict keyword mean in C++?

Question

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?

Answer 1

Nothing. It was added to the C99 standard.

Answer 2

This is the original proposal to add this keyword. As dirkgently pointed out though, this is a C99 feature, it has nothing to do with C++.

Answer 3

Since header files from some C libraries use the keyword, the C++ language will have to do something about it.. at the minimum, ignoring the keyword, so we don't have to #define the keyword to a blank macro to suppress the keyword.

Answer 4

In his paper, Memory Optimization, Christer Ericson (G-d of War) 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.

Answer 5

There's no such keyword in C++. List of C++ keywords can be found in section 2.11/1 of C++ language standard. restrict is a keyword in C99 version of C language, but in in C++.