How to raise warning if return value is disregarded?

Question

I\'d like to see all the places in my code (C++) which disregard return value of a function. How can I do it - with gcc or static code analysis tool?

Bad code exampl

Answer 1

For C++17 the answer to this question changes since we now have the [[nodiscard]] attribute. Covered in [dcl.attr.nodiscard]:

The attribute-token nodiscard may be applied to the declarator-id in a function declaration or to the declaration of a class or enumeration. It shall appear at most once in each attribute-list and no attribute-argument-clause shall be present.

and

[ Example:

struct [[nodiscard]] error_info { /* ... */ };
error_info enable_missile_safety_mode();
void launch_missiles();
void test_missiles() {
  enable_missile_safety_mode(); // warning encouraged
  launch_missiles();
}
error_info &foo();
void f() { foo(); }             // warning not encouraged: not a nodiscard call, because neither
                                // the (reference) return type nor the function is declared nodiscard

 — end example ]

So modifying your example (see it live):

[[nodiscard]] int f(int z) {
    return z + (z*2) + z/3 + z*z + 23;
}


int main()
{
  int i = 7;
  f(i); // now we obtain a diagnostic

  return 1;
}

We now obtain a diagnostic with both gcc and clang e.g.

warning: ignoring return value of function declared with 'nodiscard' attribute [-Wunused-result]
  f(i); // now we obtain a diagnostic
  ^ ~