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" ) ) ); 

Built in matchers

Catch currently provides some matchers, they are in the Catch::Matchers and Catch namespaces.

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

The vector matchers are Contains, VectorContains and Equals. VectorContains looks for a single element in the matched vector, Contains looks for a set (vector) of elements inside the matched vector.

Floating point matchers

The floating point matchers are WithinULP and WithinAbs. WithinAbs accepts floating point numbers that are within a certain margin of target. WithinULP performs an ULP-based comparison of two floating point numbers and accepts them if they are less than certain number of ULPs apart.

Do note that ULP-based checks only make sense when both compared numbers are of the same type and WithinULP will use type of its argument as the target type. This means that WithinULP(1.f, 1) will expect to compare floats, but WithinULP(1., 1) will expect to compare doubles.

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.

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
    virtual 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 {
        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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