PETSC_NODISCARD

Mark the return value of a function as non-discardable Not available in Fortran

Notes

Hints to the compiler that the return value of a function must be captured. A diagnostic may (but is not required to) be emitted if the value is discarded. It is safe to use this in both C and C++ source files.

In this context “captured” means assigning the return value of a function to a named variable or casting it to void. Between the two, assigning to a named variable is the most portable way of silencing any warnings, since PETSC_NODISCARD may expand to GCC’s __attribute__((warn_unused_result)) which will still emit warnings when casting results to void.

Example Usage

  class Foo
  {
    int x;

  public:
    PETSC_NODISCARD Foo(int y) : x(y) { }
  };

  PETSC_NODISCARD int factorial(int n)
  {
    return n <= 1 ? 1 : (n * factorial(n - 1));
  }

  factorial(10);          // Warning: ignoring return value of function declared 'nodiscard'
  auto x = factorial(10); // OK, capturing return value
  (void)factorial(10);    // Maybe OK, casting to void
  auto y = factorial(10); // OK, capturing in y (and casting y to void to silence
  (void)y;                // set-but-not-used warnings)

  Foo(x);          // Warning: Ignoring temporary created by a constructor declared 'nodiscard'
  auto f = Foo(x); // OK, capturing constructed object
  (void)Foo(x);    // Maybe OK, casting to void
  auto g = Foo(x); // OK, capturing in g (and casting g to void to silence set-but-not-used
  (void)g;         // warnings)

See Also

PETSC_NULLPTR, PETSC_CONSTEXPR_14

Level

beginner

Location

include/petscmacros.h

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