catch2/docs/test-fixtures.md

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# Test fixtures

**Contents**<br>
[Non-Templated test fixtures](#non-templated-test-fixtures)<br>
[Templated test fixtures](#templated-test-fixtures)<br>
[Signature-based parameterised test fixtures](#signature-based-parametrised-test-fixtures)<br>
[Template fixtures with types specified in template type lists](#template-fixtures-with-types-specified-in-template-type-lists)<br>

## Non-Templated test fixtures

Although Catch2 allows you to group tests together as 
[sections within a test case](test-cases-and-sections.md), it can still 
be convenient, sometimes, to group them using a more traditional test. 
Catch2 fully supports this too with 3 different macros for 
non-templated test fixtures. They are: 

| Macro    | Description |
|----------|-------------|
|1. `TEST_CASE_METHOD(className, ...)`| Creates a uniquely named class which inherits from the class specified by `className`. The test function will be a member of this derived class. An instance of the derived class will be created for every partial run of the test case. |
|2. `METHOD_AS_TEST_CASE(member-function, ...)`| Uses `member-function` as the test function. An instance of the class will be created for each partial run of the test case. |
|3. `TEST_CASE_PERSISTENT_FIXTURE(className, ...)`| Creates a uniquely named class which inherits from the class specified by `className`. The test function will be a member of this derived class. An instance of the derived class will be created at the start of the test run. That instance will be destroyed once the entire test case has ended. |

### 1. `TEST_CASE_METHOD`


You define a `TEST_CASE_METHOD` test fixture as a simple structure:

```c++
class UniqueTestsFixture {
  private:
   static int uniqueID;
  protected:
   DBConnection conn;
  public:
   UniqueTestsFixture() : conn(DBConnection::createConnection("myDB")) {
   }
  protected:
   int getID() {
     return ++uniqueID;
   }
 };

 int UniqueTestsFixture::uniqueID = 0;

 TEST_CASE_METHOD(UniqueTestsFixture, "Create Employee/No Name", "[create]") {
   REQUIRE_THROWS(conn.executeSQL("INSERT INTO employee (id, name) VALUES (?, ?)", getID(), ""));
 }
 TEST_CASE_METHOD(UniqueTestsFixture, "Create Employee/Normal", "[create]") {
   REQUIRE(conn.executeSQL("INSERT INTO employee (id, name) VALUES (?, ?)", getID(), "Joe Bloggs"));
 }
```

The two test cases here will create uniquely-named derived classes of 
UniqueTestsFixture and thus can access the `getID()` protected method 
and `conn` member variables. This ensures that both the test cases 
are able to create a DBConnection using the same method 
(DRY principle) and that any ID's created are unique such that the 
order that tests are executed does not matter. 

### 2. `METHOD_AS_TEST_CASE`

`METHOD_AS_TEST_CASE` lets you register a member function of a class 
as a Catch2 test case. The class will be separately instantiated 
for each method registered in this way.

```cpp
class TestClass {
    std::string s;

public:
    TestClass()
        :s( "hello" )
    {}

    void testCase() {
        REQUIRE( s == "hello" );
    }
};


METHOD_AS_TEST_CASE( TestClass::testCase, "Use class's method as a test case", "[class]" )
```

This type of fixture is similar to [TEST_CASE_METHOD](#1-test_case_method) except in this 
case it will directly use the provided class to create an object rather than a derived 
class.

### 3. `TEST_CASE_PERSISTENT_FIXTURE`

> [Introduced](https://github.com/catchorg/Catch2/pull/2885) in Catch2 3.7.0

`TEST_CASE_PERSISTENT_FIXTURE` behaves in the same way as
[TEST_CASE_METHOD](#1-test_case_method) except that there will only be
one instance created throughout the entire run of a test case. To 
demonstrate this have a look at the following example:

```cpp
class ClassWithExpensiveSetup {
public:
    ClassWithExpensiveSetup() {
        // expensive construction
        std::this_thread::sleep_for( std::chrono::seconds( 2 ) );
    }

    ~ClassWithExpensiveSetup() noexcept {
        // expensive destruction
        std::this_thread::sleep_for( std::chrono::seconds( 1 ) );
    }

    int getInt() const { return 42; }
};

struct MyFixture {
    mutable int myInt = 0;
    ClassWithExpensiveSetup expensive;
};

TEST_CASE_PERSISTENT_FIXTURE( MyFixture, "Tests with MyFixture" ) {

    const int val = myInt++;

    SECTION( "First partial run" ) {
        const auto otherValue = expensive.getInt();
        REQUIRE( val == 0 );
        REQUIRE( otherValue == 42 );
    }

    SECTION( "Second partial run" ) { REQUIRE( val == 1 ); }
}
```

This example demonstrates two possible use-cases of this fixture type:
1. Improve test run times by reducing the amount of expensive and 
redundant setup and tear-down required.
2. Reusing results from the previous partial run, in the current
partial run.

This test case will be executed twice as there are two leaf sections.
On the first run `val` will be `0` and on the second run `val` will be 
`1`. This demonstrates that we were able to use the results of the
previous partial run in subsequent partial runs.

Additionally, we are simulating an expensive object using 
`std::this_thread::sleep_for`, but real world use-cases could be:
1. Creating a D3D12/Vulkan device
2. Connecting to a database
3. Loading a file.

The fixture object (`MyFixture`) will be constructed just before the
test case begins, and it will be destroyed just after the test case 
ends. Therefore, this expensive object will only be created and 
destroyed once during the execution of this test case. If we had used 
`TEST_CASE_METHOD`, `MyFixture` would have been created and destroyed 
twice during the execution of this test case.

NOTE: The member function which runs the test case is `const`. Therefore 
if you want to mutate any member of the fixture it must be marked as
`mutable` as shown in this example. This is to make it clear that
the initial state of the fixture is intended to mutate during the
execution of the test case.

## Templated test fixtures

Catch2 also provides `TEMPLATE_TEST_CASE_METHOD` and
`TEMPLATE_PRODUCT_TEST_CASE_METHOD` that can be used together
with templated fixtures and templated template fixtures to perform
tests for multiple different types. Unlike `TEST_CASE_METHOD`,
`TEMPLATE_TEST_CASE_METHOD` and `TEMPLATE_PRODUCT_TEST_CASE_METHOD` do
require the tag specification to be non-empty, as it is followed by
further macro arguments.

Also note that, because of limitations of the C++ preprocessor, if you
want to specify a type with multiple template parameters, you need to
enclose it in parentheses, e.g. `std::map<int, std::string>` needs to be
passed as `(std::map<int, std::string>)`.
In the case of `TEMPLATE_PRODUCT_TEST_CASE_METHOD`, if a member of the
type list should consist of more than single type, it needs to be enclosed
in another pair of parentheses, e.g. `(std::map, std::pair)` and
`((int, float), (char, double))`.

Example:
```cpp
template< typename T >
struct Template_Fixture {
    Template_Fixture(): m_a(1) {}

    T m_a;
};

TEMPLATE_TEST_CASE_METHOD(Template_Fixture,
                          "A TEMPLATE_TEST_CASE_METHOD based test run that succeeds",
                          "[class][template]",
                          int, float, double) {
    REQUIRE( Template_Fixture<TestType>::m_a == 1 );
}

template<typename T>
struct Template_Template_Fixture {
    Template_Template_Fixture() {}

    T m_a;
};

template<typename T>
struct Foo_class {
    size_t size() {
        return 0;
    }
};

TEMPLATE_PRODUCT_TEST_CASE_METHOD(Template_Template_Fixture,
                                  "A TEMPLATE_PRODUCT_TEST_CASE_METHOD based test succeeds",
                                  "[class][template]",
                                  (Foo_class, std::vector),
                                  int) {
    REQUIRE( Template_Template_Fixture<TestType>::m_a.size() == 0 );
}
```

_While there is an upper limit on the number of types you can specify
in single `TEMPLATE_TEST_CASE_METHOD` or `TEMPLATE_PRODUCT_TEST_CASE_METHOD`,
the limit is very high and should not be encountered in practice._

## Signature-based parameterised test fixtures

> [Introduced](https://github.com/catchorg/Catch2/issues/1609) in Catch2 2.8.0.

Catch2 also provides `TEMPLATE_TEST_CASE_METHOD_SIG` and `TEMPLATE_PRODUCT_TEST_CASE_METHOD_SIG` to support
fixtures using non-type template parameters. These test cases work similar to `TEMPLATE_TEST_CASE_METHOD` and `TEMPLATE_PRODUCT_TEST_CASE_METHOD`,
with additional positional argument for [signature](test-cases-and-sections.md#signature-based-parametrised-test-cases).

Example:
```cpp
template <int V>
struct Nttp_Fixture{
    int value = V;
};

TEMPLATE_TEST_CASE_METHOD_SIG(
    Nttp_Fixture,
    "A TEMPLATE_TEST_CASE_METHOD_SIG based test run that succeeds",
    "[class][template][nttp]",
    ((int V), V),
    1, 3, 6) {
    REQUIRE(Nttp_Fixture<V>::value > 0);
}

template<typename T>
struct Template_Fixture_2 {
    Template_Fixture_2() {}

    T m_a;
};

template< typename T, size_t V>
struct Template_Foo_2 {
    size_t size() { return V; }
};

TEMPLATE_PRODUCT_TEST_CASE_METHOD_SIG(
    Template_Fixture_2,
    "A TEMPLATE_PRODUCT_TEST_CASE_METHOD_SIG based test run that succeeds",
    "[class][template][product][nttp]",
    ((typename T, size_t S), T, S),
    (std::array, Template_Foo_2),
    ((int,2), (float,6))) {
    REQUIRE(Template_Fixture_2<TestType>{}.m_a.size() >= 2);
}
```

## Template fixtures with types specified in template type lists

Catch2 also provides `TEMPLATE_LIST_TEST_CASE_METHOD` to support template fixtures with types specified in
template type lists like `std::tuple`, `boost::mpl::list` or `boost::mp11::mp_list`. This test case works the same as `TEMPLATE_TEST_CASE_METHOD`,
only difference is the source of types. This allows you to reuse the template type list in multiple test cases.

Example:
```cpp
using MyTypes = std::tuple<int, char, double>;
TEMPLATE_LIST_TEST_CASE_METHOD(Template_Fixture,
                               "Template test case method with test types specified inside std::tuple",
                               "[class][template][list]",
                               MyTypes) {
    REQUIRE( Template_Fixture<TestType>::m_a == 1 );
}
```

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