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# __restrict
Like the **__declspec ( [restrict](../cpp/restrict.md) )** modifier, the `__restrict` keyword indicates that a symbol is not aliased in the current scope. The `__restrict` keyword differs from the `__declspec ( restrict )` modifier in the following ways:  
  
-   The `__restrict` keyword is valid only on variables, and `__declspec ( restrict )` is only valid on function declarations and definitions.  
  
-   `__restrict` is similar to `restrict` from the C99 spec, but `__restrict` can be used in C++ or C programs.  
  
-   When `__restrict` is used, the compiler will not propagate the no-alias property of a variable. That is, if you assign a `__restrict` variable to a non-`__restrict` variable, the compiler will still allow the non-__restrict variable to be aliased. This is different from the behavior of the `restrict` keyword from the C99 specification.  
  
 Generally, if you affect the behavior of an entire function, it is better to use `__declspec ( restrict )` than the keyword.  
  
 In Visual Studio 2015 and later, `__restrict` can be used on C++ references.  
  
> [!NOTE]
>  When used on a variable that also has the [volatile](../cpp/volatile-cpp.md) keyword, `volatile` will take precedence.  
  
## Example  
  
```  
// __restrict_keyword.c  
// compile with: /LD  
// In the following function, declare a and b as disjoint arrays  
// but do not have same assurance for c and d.  
void sum2(int n, int * __restrict a, int * __restrict b,   
          int * c, int * d) {  
   int i;  
   for (i = 0; i < n; i++) {  
      a[i] = b[i] + c[i];  
      c[i] = b[i] + d[i];  
    }  
}  
  
// By marking union members as __restrict, tell compiler that  
// only z.x or z.y will be accessed in any given scope.  
union z {  
   int * __restrict x;  
   double * __restrict y;  
};  
```  
  
## See Also  
 [Keywords](../cpp/keywords-cpp.md)