catch2/docs/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](https://en.wikipedia.org/wiki/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:
 
 ```c++
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:

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

And matchers can be combined:

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

## Built in matchers
Currently only a few string matchers are built-in: `StartsWith`, `EndsWith`, and `Contains` and `Equals`.
These each take an optional second argument for case sensitivity (defaulting to case sensitive).
More matchers will be coming - for example for testing elements in a vector.

## 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):

```c++
// 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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