Describe a google mock matcher as std::string

Question

My question (for the inpatient)

Given a google-mock matcher, I would like to describe it to a string. For example:

std::string description = DescribeMatcher(Ge(0)) // puts "size is > 0" in the string

Does anybody know an easy way of doing that? Did not find anything in googlemock documentation. I did it myself like this:

template<typename T, typename S>
std::string DescribeMatcher(S matcher)
{
    Matcher<T> matcherCast = matcher;
    std::ostringstream os;
    matcherCast.DescribeTo(&os);
    return os.str();
}

Background

I want to write my own matcher that is based on a different one. My matcher matches a string that represents a name of a file having the specified size.

MATCHER_P(FileSizeIs, sizeMatcher, std::string("File size ") + DescribeMatcher(sizeMatcher))
{
    auto fileSize = fs::file_size(arg);
    return ExplainMatchResult(sizeMatcher, fileSize, result_listener);
}

Here is an example for its usage:

EXPECT_THAT(someFileName, FileSizeIs(Ge(100)); // the size of the file is at-least 100 bytes
EXPECT_THAT(someFileName, FileSizeIs(AllOf(Ge(200), Le(1000)); // the size of the file is between 200 and 1000 bytes

The problem is in the last argument of the MATCHER_P macro. I want the description of FileSizeIs to be based on the description of sizeMatcher. However, I did not find any such function inside googlemock and had to write it myself.

Answer 1

I have had a similar issue creating nested matchers. My solution uses MatcherInterface instead of MATCHER_P. For your case this would be something like:

template <typename InternalMatcher>
class FileSizeIsMatcher : public MatcherInterface<fs::path> {
public:
    FileSizeIsMatcher(InternalMatcher internalMatcher)
            : mInternalMatcher(SafeMatcherCast<std::uintmax_t>(internalMatcher))
    {
    }
    bool MatchAndExplain(fs::path arg, MatchResultListener* listener) const override {
        auto fileSize = fs::file_size(arg);
        *listener << "whose size is " << PrintToString(fileSize) << " ";
        return mInternalMatcher.MatchAndExplain(fileSize, listener);
    }

    void DescribeTo(std::ostream* os) const override {
        *os << "file whose size ";
        mInternalMatcher.DescribeTo(os);
    }

    void DescribeNegationTo(std::ostream* os) const override {
        *os << "file whose size ";
        mInternalMatcher.DescribeNegationTo(os);
    }
private:
    Matcher<std::uintmax_t> mInternalMatcher;
};

template <typename InternalMatcher>
Matcher<fs::path> FileSizeIs(InternalMatcher m) {
    return MakeMatcher(new FileSizeIsMatcher<InternalMatcher>(m));
};

Example:

TEST_F(DetectorPlacementControllerTests, TmpTest) {
    EXPECT_THAT("myFile.txt", FileSizeIs(Lt(100ul)));
}

Gives output:

Value of: "myFile.txt"
Expected: file whose size is < 100
Actual: "myFile.txt" (of type char [11]), whose size is 123