What does the restrict keyword mean in C++?

In the C++ standard, the restrict keyword is not defined. restrict is a keyword that exists in the C language (introduced in the C99 standard), used to inform the compiler that a pointer is the sole means of accessing the data. This helps the compiler optimize, as it knows that no other pointers may point to the same data.

In C++, although restrict is not present, some compilers (such as GCC and MSVC) support similar functionality, typically through extensions, like GCC's __restrict__ or MSVC's __restrict.

An example of using restrict is when performing array operations; if you can ensure that two arrays do not overlap, you can use the restrict keyword to inform the compiler of this, allowing the compiler to potentially generate more optimized code.

c
void add(int n, float *restrict result, float *restrict a, float *restrict b) {
    for (int i = 0; i < n; ++i) {
        result[i] = a[i] + b[i];
    }
}

In this example, the arrays result, a, and b are marked with restrict, meaning that the memory regions they point to do not overlap, and the compiler can optimize under this assumption. In C++, although you cannot directly use restrict, if you use a compiler that supports similar functionality, you can use the corresponding extension keywords to achieve a similar effect.