# Extra Clang Tools 21.1.6 documentation

## clang-tidy - readability-non-const-parameter

# readability-non-const-parameter

The check finds function parameters of a pointer type that could be changed to point to a constant type instead.

When `const` is used properly, many mistakes can be avoided. Advantages when using `const` properly:

- prevent unintentional modification of data;

- get additional warnings such as using uninitialized data;

- make it easier for developers to see possible side effects.

This check is not strict about constness, it only warns when the constness will make the function interface safer.

    // warning here; the declaration "const char *p" would make the function
    // interface safer.
    char f1(char *p) {
      return *p;
    }

    // no warning; the declaration could be more const "const int * const p" but
    // that does not make the function interface safer.
    int f2(const int *p) {
      return *p;
    }

    // no warning; making x const does not make the function interface safer
    int f3(int x) {
      return x;
    }

    // no warning; Technically, *p can be const ("const struct S *p"). But making
    // *p const could be misleading. People might think that it's safe to pass
    // const data to this function.
    struct S { int *a; int *b; };
    int f3(struct S *p) {
      *(p->a) = 0;
    }

    // no warning; p is referenced by an lvalue.
    void f4(int *p) {
      int &x = *p;
    }