Matcher documentation

docs/assertions.md

@@ -105,13 +105,11 @@ REQUIRE_NOTHROW([&](){
 
 ## Matcher expressions
 
-To support Matchers a slightly different form is used. Matchers will be more fully documented elsewhere. *Note that Matchers are still at early stage development and are subject to change.*
+To support Matchers a slightly different form is used. Matchers have [their own documentation](matchers.md).
 
 * **REQUIRE_THAT(** _lhs_, _matcher expression_ **)** and  
 * **CHECK_THAT(** _lhs_, _matcher expression_ **)**  
 
-Currently only string matchers are implemented and consist of: `Contains`, `Equals`, `StartsWith` and `EndsWith`.
-
 Matchers can be composed using `&&`, `||` and `!` operators.
 
 ---

docs/matchers.md

@@ -0,0 +1,102 @@
+# 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
+```
+
+---
+
+[Home](Readme.md)