Can a C++ function be declared such that the return value cannot be ignored?

To summarize from other answers & comments, basically you have 3 choices:

1. Get C++17 to be able to use [[nodiscard]]
2. In g++ (also clang++), use compiler extensions like __wur (defined
   as __attribute__ ((__warn_unused_result__))), or the more portable (C++11 and up only) [[gnu::warn_unused_result]] attribute.
3. Use runtime checks to catch the problem during unit testing

If all of these 3 are not possible, then there is one more way, which is kind of “Negative compiling”. Define your Unignorable as below:

struct Unignorable {
  Unignorable () = default;
#ifdef NEGATIVE_COMPILE
  Unignorable (const Unignorable&) = delete;  // C++11
  Unignorable& operator= (const Unignorable&) = delete;
  //private: Unignorable (const Unignorable&); public:  // C++03
  //private: Unignorable& operator= (const Unignorable&); public: // C++03
  /* similar thing for move-constructor if needed */
#endif
};

Now compile with -DNEGATIVE_COMPILE or equivalent in other compilers like MSVC. It will give errors at wherever the result is Not ignored:

auto x = foo();  // error

However, it will not give any error wherever the result is ignored:

foo(); // no error

Using any modern code browser (like eclipse-cdt), you may find all the occurrences of foo() and fix those places which didn’t give error. In the new compilation, simply remove the pre-defined macro for “NEGATIVE_COMPILE”.

This might be bit better compared to simply finding foo() and checking for its return, because there might be many functions like foo() where you may not want to ignore the return value.

This is bit tedious, but will work for all the versions of C++ with all the compilers.