matchers.md

Matchers

Matchers are an alternative way to do assertions which are easily extensible and composable. This makes them well suited to use with more complex types (such as collections) or your own custom types. Matchers were first popularised by the Hamcrest family of frameworks.

In use

Matchers are introduced with the REQUIRE_THAT or CHECK_THAT macros, which take two arguments. The first argument is the thing (object or value) under test. The second part is a match expression, which consists of either a single matcher or one or more matchers combined using &&, || or ! operators.

For example, to assert that a string ends with a certain substring:

using Catch::Matchers::EndsWith; // or Catch::EndsWith
std::string str = getStringFromSomewhere();
REQUIRE_THAT( str, EndsWith( "as a service" ) );

The matcher objects can take multiple arguments, allowing more fine tuning. The built-in string matchers, for example, take a second argument specifying whether the comparison is case sensitive or not:

REQUIRE_THAT( str, EndsWith( "as a service", Catch::CaseSensitive::No ) );

And matchers can be combined:

REQUIRE_THAT( str,
    EndsWith( "as a service" ) ||
    (StartsWith( "Big data" ) && !Contains( "web scale" ) ) );

The combining operators do not take ownership of the matcher objects. This means that if you store the combined object, you have to ensure that the matcher objects outlive its last use. What this means is that code like this leads to a use-after-free and (hopefully) a crash:

TEST_CASE("Bugs, bugs, bugs", "[Bug]"){
    std::string str = "Bugs as a service";

    auto match_expression = Catch::EndsWith( "as a service" ) ||
        (Catch::StartsWith( "Big data" ) && !Catch::Contains( "web scale" ) );
    REQUIRE_THAT(str, match_expression);
}

Built in matchers

Catch2 provides some matchers by default. They can be found in the Catch::Matchers::foo namespace and are imported into the Catch namespace as well.

There are two parts to each of the built-in matchers, the matcher type itself and a helper function that provides template argument deduction when creating templated matchers. As an example, the matcher for checking that two instances of std::vector are identical is EqualsMatcher<T>, but the user is expected to use the Equals helper function instead.

String matchers

The string matchers are StartsWith, EndsWith, Contains, Equals and Matches. The first four match a literal (sub)string against a result, while Matches takes and matches an ECMAScript regex. Do note that Matches matches the string as a whole, meaning that "abc" will not match against "abcd", but "abc.*" will.

Each of the provided std::string matchers also takes an optional second argument, that decides case sensitivity (by-default, they are case sensitive).

Vector matchers

Catch2 currently provides 5 built-in matchers that work on std::vector. These are

• Contains which checks whether a specified vector is present in the result
• VectorContains which checks whether a specified element is present in the result
• Equals which checks whether the result is exactly equal (order matters) to a specific vector
• UnorderedEquals which checks whether the result is equal to a specific vector under a permutation
• Approx which checks whether the result is "approx-equal" (order matters, but comparison is done via Approx) to a specific vector

Approx matcher was introduced in Catch 2.7.2.

Floating point matchers

Catch2 provides 3 matchers for working with floating point numbers. These are WithinAbsMatcher, WithinUlpsMatcher and WithinRelMatcher.

The WithinAbsMatcher matcher accepts floating point numbers that are within a certain distance of target. It should be constructed with the WithinAbs(double target, double margin) helper.

The WithinUlpsMatcher matcher accepts floating point numbers that are within a certain number of ULPs of the target. Because ULP comparisons need to be done differently for floats and for doubles, there are two overloads of the helpers for this matcher, WithinULP(float target, int64_t ULPs), and WithinULP(double target, int64_t ULPs).

The WithinRelMatcher matcher accepts floating point numbers that are approximately equal with the target number with some specific tolerance. In other words, it checks that |lhs - rhs| <= epsilon * max(|lhs|, |rhs|), with special casing for INFINITY and NaN. There are 4 overloads of the helpers for this matcher, WithinRel(double target, double margin), WithinRel(float target, float margin), WithinRel(double target), and WithinRel(float target). The latter two provide a default epsilon of machine epsilon * 100.

WithinRel matcher was introduced in Catch 2.10.0

Generic matchers

Catch also aims to provide a set of generic matchers. Currently this set contains only a matcher that takes arbitrary callable predicate and applies it onto the provided object.

Because of type inference limitations, the argument type of the predicate has to be provided explicitly. Example:

REQUIRE_THAT("Hello olleH",
             Predicate<std::string>(
                 [] (std::string const& str) -> bool { return str.front() == str.back(); },
                 "First and last character should be equal")
);

The second argument is an optional description of the predicate, and is used only during reporting of the result.

Exception matchers

Catch2 also provides an exception matcher that can be used to verify that an exception's message exactly matches desired string. The matcher is ExceptionMessageMatcher, and we also provide a helper function Message.

The matched exception must publicly derive from std::exception and the message matching is done exactly, including case.

ExceptionMessageMatcher was introduced in Catch 2.10.0

Example use:

REQUIRE_THROWS_MATCHES(throwsDerivedException(),  DerivedException,  Message("DerivedException::what"));

Custom matchers

It's easy to provide your own matchers to extend Catch or just to work with your own types.

You need to provide two things:

1. A matcher class, derived from Catch::MatcherBase<T> - where T is the type being tested. The constructor takes and stores any arguments needed (e.g. something to compare against) and you must override two methods: match() and describe().
2. A simple builder function. This is what is actually called from the test code and allows overloading.

Here's an example for asserting that an integer falls within a given range (note that it is all inline for the sake of keeping the example short):

// The matcher class
class IntRange : public Catch::MatcherBase<int> {
    int m_begin, m_end;
public:
    IntRange( int begin, int end ) : m_begin( begin ), m_end( end ) {}

    // Performs the test for this matcher
    bool match( int const& i ) const override {
        return i >= m_begin && i <= m_end;
    }

    // Produces a string describing what this matcher does. It should
    // include any provided data (the begin/ end in this case) and
    // be written as if it were stating a fact (in the output it will be
    // preceded by the value under test).
    virtual std::string describe() const override {
        std::ostringstream ss;
        ss << "is between " << m_begin << " and " << m_end;
        return ss.str();
    }
};

// The builder function
inline IntRange IsBetween( int begin, int end ) {
    return IntRange( begin, end );
}

// ...

// Usage
TEST_CASE("Integers are within a range")
{
    CHECK_THAT( 3, IsBetween( 1, 10 ) );
    CHECK_THAT( 100, IsBetween( 1, 10 ) );
}

Running this test gives the following in the console:

/**/TestFile.cpp:123: FAILED:
  CHECK_THAT( 100, IsBetween( 1, 10 ) )
with expansion:
  100 is between 1 and 10

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