[Type parametrised test cases](#type-parametrised-test-cases)<br>
[Signature based parametrised test cases](#signature-based-parametrised-test-cases)<br>
While Catch fully supports the traditional, xUnit, style of class-based fixtures containing test case methods this is not the preferred style.
Instead Catch provides a powerful mechanism for nesting test case sections within a test case. For a more detailed discussion see the [tutorial](tutorial.md#test-cases-and-sections).
Test cases and sections are very easy to use in practice:
Tags allow an arbitrary number of additional strings to be associated with a test case. Test cases can be selected (for running, or just for listing) by tag - or even by an expression that combines several tags. At their most basic level they provide a simple way to group several related tests together.
The tag expression, ```"[widget]"``` selects A, B & D. ```"[gadget]"``` selects C & D. ```"[widget][gadget]"``` selects just D and ```"[widget],[gadget]"``` selects all four test cases.
For more detail on command line selection see [the command line docs](command-line.md#specifying-which-tests-to-run)
Tag names are not case sensitive and can contain any ASCII characters.
This means that tags `[tag with spaces]` and `[I said "good day"]`
are both allowed tags and can be filtered on. However, escapes are not
supported however and `[\]]` is not a valid tag.
The same tag can be specified multiple times for a single test case,
but only one of the instances of identical tags will be kept. Which one
is kept is functionally random.
### Special Tags
All tag names beginning with non-alphanumeric characters are reserved by Catch. Catch defines a number of "special" tags, which have meaning to the test runner itself. These special tags all begin with a symbol character. Following is a list of currently defined special tags and their meanings.
*`[.]` - causes test cases to be skipped from the default list (i.e. when no test cases have been explicitly selected through tag expressions or name wildcards). The hide tag is often combined with another, user, tag (for example `[.][integration]` - so all integration tests are excluded from the default run but can be run by passing `[integration]` on the command line). As a short-cut you can combine these by simply prefixing your user tag with a `.` - e.g. `[.integration]`.
*`[!throws]` - lets Catch know that this test is likely to throw an exception even if successful. This causes the test to be excluded when running with `-e` or `--nothrow`.
*`[!mayfail]` - doesn't fail the test if any given assertion fails (but still reports it). This can be useful to flag a work-in-progress, or a known issue that you don't want to immediately fix but still want to track in your tests.
*`[!shouldfail]` - like `[!mayfail]` but *fails* the test if it *passes*. This can be useful if you want to be notified of accidental, or third-party, fixes.
*`[!nonportable]` - Indicates that behaviour may vary between platforms or compilers.
*`[@<alias>]` - tag aliases all begin with `@` (see below).
*`[!benchmark]` - this test case is actually a benchmark. Currently this only serves to hide the test case by default, to avoid the execution time costs.
## Tag aliases
Between tag expressions and wildcarded test names (as well as combinations of the two) quite complex patterns can be constructed to direct which test cases are run. If a complex pattern is used often it is convenient to be able to create an alias for the expression. This can be done, in code, using the following form:
Now when `[@nhf]` is used on the command line this matches all tests that are tagged `[failing]`, but which are not also hidden.
## BDD-style test cases
In addition to Catch's take on the classic style of test cases, Catch supports an alternative syntax that allow tests to be written as "executable specifications" (one of the early goals of [Behaviour Driven Development](http://dannorth.net/introducing-bdd/)). This set of macros map on to ```TEST_CASE```s and ```SECTION```s, with a little internal support to make them smoother to work with.
This macro maps onto ```TEST_CASE``` and works in the same way, except that the test case name will be prefixed by "Scenario: "
* **GIVEN(** _something_**)**
* **WHEN(** _something_**)**
* **THEN(** _something_**)**
These macros map onto ```SECTION```s except that the section names are the _something_ texts prefixed by
"given: ", "when: " or "then: " respectively. These macros also map onto the AAA or A<sup>3</sup> test pattern
(standing either for [Assemble-Activate-Assert](http://wiki.c2.com/?AssembleActivateAssert) or
[Arrange-Act-Assert](http://wiki.c2.com/?ArrangeActAssert)), and in this context, the macros provide both code
documentation and reporting of these parts of a test case without the need for extra comments or code to do so.
Semantically, a `GIVEN` clause may have multiple _independent_`WHEN` clauses within it. This allows a test
to have, e.g., one set of "given" objects and multiple subtests using those objects in various ways in each
of the `WHEN` clauses without repeating the initialisation from the `GIVEN` clause. When there are _dependent_
clauses -- such as a second `WHEN` clause that should only happen _after_ the previous `WHEN` clause has been
executed and validated -- there are additional macros starting with `AND_`:
* **AND_GIVEN(** _something_**)**
* **AND_WHEN(** _something_**)**
* **AND_THEN(** _something_**)**
These are used to chain ```GIVEN```s, ```WHEN```s and ```THEN```s together. The `AND_*` clause is placed
_inside_ the clause on which it depends. There can be multiple _independent_ clauses that are all _dependent_
on a single outer clause.
```cpp
SCENARIO( "vector can be sized and resized" ) {
GIVEN( "An empty vector" ) {
auto v = std::vector<std::string>{};
// Validate assumption of the GIVEN clause
THEN( "The size and capacity start at 0" ) {
REQUIRE( v.size() == 0 );
REQUIRE( v.capacity() == 0 );
}
// Validate one use case for the GIVEN object
WHEN( "push_back() is called" ) {
v.push_back("hullo");
THEN( "The size changes" ) {
REQUIRE( v.size() == 1 );
REQUIRE( v.capacity() >= 1 );
}
}
}
}
```
This code will result in two runs through the scenario:
```
Scenario : vector can be sized and resized
Given : An empty vector
Then : The size and capacity start at 0
Scenario : vector can be sized and resized
Given : An empty vector
When : push_back() is called
Then : The size changes
```
See also [runnable example on godbolt](https://godbolt.org/z/eY5a64r99),
with a more complicated (and failing) example.
> `AND_GIVEN` was [introduced](https://github.com/catchorg/Catch2/issues/1360) in Catch2 2.4.0.
When any of these macros are used the console reporter recognises them and formats the test case header such that the Givens, Whens and Thens are aligned to aid readability.
Other than the additional prefixes and the formatting in the console reporter these macros behave exactly as ```TEST_CASE```s and ```SECTION```s. As such there is nothing enforcing the correct sequencing of these macros - that's up to the programmer!
## Type parametrised test cases
In addition to `TEST_CASE`s, Catch2 also supports test cases parametrised
