Google Test Framework does not print std::optional

user_3074280 0
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I have a data structure which is being notified to a listener. Among its many variables, it has an optional double which represents an angle:

struct Data
{
    // ...other data fields
    std::optional<double> angle;
    // ...other data fields
};

I want to check that the listener is being called with the right angle. I am using the Google Test Framework, so I have defined my own matcher here:

MATCHER_P( HasAngle, angle, "" )
{
    if ( !arg.angle.has_value( ) )
    {
        return false;
    }

    return arg.angle.value( ) == angle;
}

And checked if my listener is being called with the expected value.

EXPECT_CALL( listener, on_event( HasAngle( 45.0 ) ) ).Times( AtLeast( 1 ) );

It ended up that the listener is not called with the expected value, but the printouts are really unreadable:

  Expected arg #0: has angle 45
       Actual: 128-byte object <75-85 4D-52 FB-21 F9-BF 47-D9 48-53 FB-21 09-C0 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 01-17 23-83 FD-7F 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00>

The actual value is unintelligible. I was not expecting the framework to not support C++17 types by default, so I started reading the Google Test Framework Cookbook and I tried to add my own PrintTo overload like this:

void
PrintTo( const std::optional< double >& opt, std::ostream* os )
{
    if ( !opt )
    {
        *os << "std::nullopt";
    }
    else
    {
        *os << *opt;
    }
}

Needless to say, it did not help. What I am pretty much sure is that since the output states 128-byte object it is trying to print the sole std::optional<double> and not the entire struct Data, which is much bigger than that, but it is not succeeding despite the overloaded PrintTo.

googlemock c++17 googletest c++
4 Answers

GoogleTest does print std::optional values, if you use the Optional(m) matcher.

Example test:

TEST_F(MyClassTests, myFunctionTest)
{
    std::optional<std::string> actual_value = "foo";
    std::string expected_value = "bar";

    EXPECT_THAT(actual_value, testing::Optional(expected_value));
}

The output will be similar, but after the cryptic 128-byte object <75-85 4D-52 FB-21 F9-BF... text, the contained value will be printed additionally:

...
[ RUN      ] MyClassTests.myFunctionTest
myclasstests.cpp:123: Failure
Value of: actual_value
Expected: value is equal to "bar"
  Actual: 40-byte object <DE-AD BE-EF CA-FE BA-BE ...>, whose value "foo" doesn't match
[  FAILED  ] MyClassTests.myFunctionTest (0 ms)
...

If you expect a std::optional to be std::nullopt in a testcase, you will most probably do it with:

EXPECT_EQ(actual_value, std::nullopt);

The test will fail, if actual_value actually has a value. But again, the actual value will not be printed. As a workaround, you can implement the test like this:

if (actual_value) {
    FAIL() << "actual_value is not std::nullopt, but: " << actual_value.value();
}
user_119422680

Your attempt does not work as it is a std:: type. You would have to define the PrintTo in std namespace according to documentation

// If you can't declare the function in the class it's important that PrintTo() // is defined in the SAME namespace that defines Bar. C++'s look-up rules rely // on that.

So I would expect the following to work (if included before the google test framework):

namespace std {
void
PrintTo( const optional< double >& opt, std::ostream* os )
{
    if ( !opt )
    {
        *os << "std::nullopt";
    }
    else
    {
        *os << *opt;
    }
}
}
user_22615380

You're worrying about the wrong type. Your struct Data is the result being tested, not a std::optional<double>. That's why your code has arg.angle, because arg is a Data. Define a stream insertion operator for Data, then gtest will use it.

user_17359220

Definition of standard output operator for your type in the test should help and be used in gmock error messages

template <class T>
std::ostream & operator<<(std::ostream & os, std::optional<T> const & opt)
{
  if (opt) return os << opt.value();
  return os << "EMPTY";
}
user_16541580
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