catch2/docs/matchers.md

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# Matchers

**Contents**<br>
[Using Matchers](#using-matchers)<br>
[Built-in matchers](#built-in-matchers)<br>
[Writing custom matchers (old style)](#writing-custom-matchers-old-style)<br>
[Writing custom matchers (new style)](#writing-custom-matchers-new-style)<br>

Matchers, as popularized by the [Hamcrest](https://en.wikipedia.org/wiki/Hamcrest)
framework are an alternative way to write assertions, useful for tests
where you work with complex types or need to assert more complex
properties. Matchers are easily composable and users can write their
own and combine them with the Catch2-provided matchers seamlessly.


## Using Matchers

Matchers are most commonly used in tandem with the `REQUIRE_THAT` or
`CHECK_THAT` macros. The `REQUIRE_THAT` macro takes two arguments,
the first one is the input (object/value) to test, the second argument
is the matcher itself.

For example, to assert that a string ends with the "as a service"
substring, you can write the following assertion

```cpp
using Catch::Matchers::EndsWith;

REQUIRE_THAT( getSomeString(), EndsWith("as a service") );
```

Individual matchers can also be combined using the C++ logical
operators, that is `&&`, `||`, and `!`, like so:

```cpp
using Catch::Matchers::EndsWith;
using Catch::Matchers::Contains;

REQUIRE_THAT( getSomeString(),
              EndsWith("as a service") && Contains("web scale"));
```

The example above asserts that the string returned from `getSomeString`
_both_ ends with the suffix "as a service" _and_ contains the string
"web scale" somewhere.


Both of the string matchers used in the examples above live in the
`catch_matchers_string.hpp` header, so to compile the code above also
requires `#include <catch2/matchers/catch_matchers_string.hpp>`.

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

```cpp
#include <catch2/catch_test_macros.hpp>
#include <catch2/matchers/catch_matchers_string.h>

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

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


## Built-in matchers

Every matcher provided by Catch2 is split into 2 parts, a factory
function that lives in the `Catch::Matchers` namespace, and the actual
matcher type that is in some deeper namespace and should not be used by
the user. In the examples above, we used `Catch::Matchers::Contains`.
This is the factory function for the
`Catch::Matchers::StdString::ContainsMatcher` type that does the actual
matching.

Out of the box, Catch2 provides the following matchers:


### `std::string` matchers

Catch2 provides 5 different matchers that work with `std::string`,
* `StartsWith(std::string str, CaseSensitive)`,
* `EndsWith(std::string str, CaseSensitive)`,
* `Contains(std::string str, CaseSensitive)`,
* `Equals(std::string str, CaseSensitive)`, and
* `Matches(std::string str, CaseSensitive)`.

The first three should be fairly self-explanatory, they succeed if
the argument starts with `str`, ends with `str`, or contains `str`
somewhere inside it.

The `Equals` matcher matches a string if (and only if) the argument
string is equal to `str`.

Finally, the `Matches` matcher performs an ECMASCript regex match using
`str` against the argument string. It is important to know that
the match is performed against the string as a whole, meaning that
the regex `"abc"` will not match input string `"abcd"`. To match
`"abcd"`, you need to use e.g. `"abc.*"` as your regex.

The second argument sets whether the matching should be case-sensitive
or not. By default, it is case-sensitive.

> `std::string` matchers live in `catch2/matchers/catch_matchers_string.hpp`


### Vector matchers

_Vector matchers have been deprecated in favour of the generic
range matchers with the same functionality._

Catch2 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](https://github.com/catchorg/Catch2/issues/1499) in Catch2 2.7.2.

An example usage:
```cpp
    std::vector<int> some_vec{ 1, 2, 3 };
    REQUIRE_THAT(some_vec, Catch::Matchers::UnorderedEquals(std::vector<int>{ 3, 2, 1 }));
```

This assertions will pass, because the elements given to the matchers
are a permutation of the ones in `some_vec`.

> vector matchers live in `catch2/matchers/catch_matchers_vector.hpp`


### Floating point matchers

Catch2 provides 3 matchers that target floating point numbers. These
are:

* `WithinAbs(double target, double margin)`,
* `WithinUlps(FloatingPoint target, uint64_t maxUlpDiff)`, and
* `WithinRel(FloatingPoint target, FloatingPoint eps)`.

> `WithinRel` matcher was introduced in Catch2 2.10.0


`WithinAbs` creates a matcher that accepts floating point numbers whose
difference with `target` is less than the `margin`.

`WithinULP` creates a matcher that accepts floating point numbers that
are no more than `maxUlpDiff`
[ULPs](https://en.wikipedia.org/wiki/Unit_in_the_last_place)
away from the `target` value. The short version of what this means
is that there is no more than `maxUlpDiff - 1` representeable floating
point numbers between the argument for matching and the `target` value.

`WithinRel` creates a matcher that accepts floating point numbers that
are _approximately equal_ with the `target` with tolerance of `eps.`
Specifically, it matches if
`|arg - target| <= eps * max(|arg|, |target|)` holds. If you do not
specify `eps`, `std::numeric_limits<FloatingPoint>::epsilon * 100`
is used as the default.


In practice, you will often want to combine multiple of these matchers,
together for an assertion, because all 3 options have edge cases where
they behave differently than you would expect. As an example, under
the `WithinRel` matcher, a `0.` only ever matches a `0.` (or `-0.`),
regardless of the relative tolerance specified. Thus, if you want to
handle numbers that are "close enough to 0 to be 0", you have to combine
it with the `WithinAbs` matcher.

For example, to check that our computation matches known good value
within 0.1%, or is close enough (no different to 5 decimal places)
to zero, we would write this assertion:
```cpp
    REQUIRE_THAT( computation(input),
        Catch::Matchers::WithinRel(expected, 0.001)
     || Catch::Matchers::WithinAbs(0, 0.000001) );
```


> floating point matchers live in `catch2/matchers/catch_matchers_floating.hpp`


### Miscellaneous matchers

Catch2 also provides some matchers and matcher utilities that do not
quite fit into other categories.

The first one of them is the `Predicate(Callable pred, std::string description)`
matcher. It creates a matcher object that calls `pred` for the provided
argument. The `description` argument allows users to set what the
resulting matcher should self-describe as if required.

Do note that you will need to explicitly specify the type of the
argument, like in this example:

```cpp
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 predicate matcher lives in `catch2/matchers/catch_matchers_predicate.hpp`


The other miscellaneous matcher utility is exception matching.


#### Matching exceptions

Catch2 provides an utility macro for asserting that an expression
throws exception of specific type, and that the exception has desired
properties. The macro is `REQUIRE_THROWS_MATCHES(expr, ExceptionType, Matcher)`.

> `REQUIRE_THROWS_MATCHES` macro lives in `catch2/matchers/catch_matchers.hpp`


Catch2 currently provides only one matcher for exceptions,
`Message(std::string message)`. `Message` checks that the exception's
message, as returned from `what` is exactly equal to `message`.

Example use:
```cpp
REQUIRE_THROWS_MATCHES(throwsDerivedException(),  DerivedException,  Message("DerivedException::what"));
```

Note that `DerivedException` in the example above has to derive from
`std::exception` for the example to work.

> the exception message matcher lives in `catch2/matchers/catch_matchers_exception.hpp`


### Generic range Matchers

> Generic range matchers were introduced in Catch2 X.Y.Z

Catch2 also provides some matchers that use the new style matchers
definitions to handle generic range-like types. These are:

* `IsEmpty()`
* `SizeIs(size_t target_size)`
* `SizeIs(Matcher size_matcher)`
* `Contains(T&& target_element, Comparator = std::equal_to<>{})`
* `Contains(Matcher element_matcher)`

`IsEmpty` should be self-explanatory. It successfully matches objects
that are empty according to either `std::empty`, or ADL-found `empty`
free function.

`SizeIs` checks range's size. If constructed with `size_t` arg, the
matchers accepts ranges whose size is exactly equal to the arg. If
constructed from another matcher, then the resulting matcher accepts
ranges whose size is accepted by the provided matcher.

`Contains` accepts ranges that contain specific element. There are
again two variants, one that accepts the desired element directly,
in which case a range is accepted if any of its elements is equal to
the target element. The other variant is constructed from a matcher,
in which case a range is accepted if any of its elements is accepted
by the provided matcher.


## Writing custom matchers (old style)

The old style of writing matchers has been introduced back in Catch
Classic. To create an old-style matcher, you have to create your own
type that derives from `Catch::Matchers::MatcherBase<ArgT>`, where
`ArgT` is the type your matcher works for. Your type has to override
two methods, `bool match(ArgT const&) const`,
and `std::string describe() const`.

As the name suggests, `match` decides whether the provided argument
is matched (accepted) by the matcher. `describe` then provides a
human-oriented description of what the matcher does.

We also recommend that you create factory function, just like Catch2
does, but that is mostly useful for template argument deduction for
templated matchers (assuming you do not have CTAD available).

To combine these into an example, let's say that you want to write
a matcher that decides whether the provided argument is a number
within certain range. We will call it `IsBetweenMatcher<T>`:

```c++
#include <catch2/catch_test_macros.hpp>
#include <catch2/matchers/catch_matchers.h>
// ...


template <typename T>
class IsBetweenMatcher : public Catch::Matchers::MatcherBase<T> {
    T m_begin, m_end;
public:
    IsBetweenMatcher(T begin, T end) : m_begin(begin), m_end(end) {}

    bool match(T const& in) const override {
        return in >= m_begin && in <= m_end;
    }

    std::string describe() const override {
        std::ostringstream ss;
        ss << "is between " << m_begin << " and " << m_end;
        return ss.str();
    }
};

template <typename T>
IsBetweenMatcher<T> IsBetween(T begin, T end) {
    return { begin, end };
}

// ...

TEST_CASE("Numbers are within range") {
    // infers `double` for the argument type of the matcher
    CHECK_THAT(3., IsBetween(1., 10.));
    // infers `int` for the argument type of the matcher
    CHECK_THAT(100, IsBetween(1, 10));
}
```

Obviously, the code above can be improved somewhat, for example you
might want to `static_assert` over the fact that `T` is an arithmetic
type... or generalize the matcher to cover any type for which the user
can provide a comparison function object.

Note that while any matcher written using the old style can also be
written using the new style, combining old style matchers should
generally compile faster. Also note that you can combine old and new
style matchers arbitrarily.

> `MatcherBase` lives in `catch2/matchers/catch_matchers.hpp`


## Writing custom matchers (new style)

> New style matchers were introduced in Catch2 X.Y.Z

To create a new-style matcher, you have to create your own type that
derives from `Catch::Matchers::MatcherGenericBase`. Your type has to
also provide two methods, `bool match( ... ) const` and overriden
`std::string describe() const`.

Unlike with old-style matchers, there are no requirements on how
the `match` member function takes its argument. This means that the
argument can be taken by value or by mutating reference, but also that
the matcher's `match` member function can be templated.

This allows you to write more complex matcher, such as a matcher that
can compare one range-like (something that responds to `begin` and
`end`) object to another, like in the following example:

```cpp
#include <catch2/catch_test_macros.hpp>
#include <catch2/matchers/catch_matchers_templated.hpp>
// ...

template<typename Range>
struct EqualsRangeMatcher : Catch::Matchers::MatcherGenericBase {
    EqualsRangeMatcher(Range const& range):
        range{ range }
    {}

    template<typename OtherRange>
    bool match(OtherRange const& other) const {
        using std::begin; using std::end;

        return std::equal(begin(range), end(range), begin(other), end(other));
    }

    std::string describe() const override {
        return "Equals: " + Catch::rangeToString(range);
    }

private:
    Range const& range;
};

template<typename Range>
auto EqualsRange(const Range& range) -> EqualsRangeMatcher<Range> {
    return EqualsRangeMatcher<Range>{range};
}

TEST_CASE("Combining templated matchers", "[matchers][templated]") {
    std::array<int, 3> container{{ 1,2,3 }};

    std::array<int, 3> a{{ 1,2,3 }};
    std::vector<int> b{ 0,1,2 };
    std::list<int> c{ 4,5,6 };

    REQUIRE_THAT(container, EqualsRange(a) || EqualsRange(b) || EqualsRange(c));
}
```

Do note that while you can rewrite any matcher from the old style to
a new style matcher, combining new style matchers is more expensive
in terms of compilation time. Also note that you can combine old style
and new style matchers arbitrarily.

> `MatcherGenericBase` lives in `catch2/matchers/catch_matchers_templated.hpp`


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