D Programming Language
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# Unit Tests

    UnitTest:
        unittest BlockStatement

Unit tests are a series of test cases applied to a module to determine if it is working properly. Ideally, unit tests should be run every time a program is compiled.

Unit tests are a special function defined like:

    unittest
    {
        ...test code...
    }

There can be any number of unit test functions in a module, including within struct, union and class declarations. They are executed in lexical order. Stylistically, a unit test for a function should appear immediately following it.

A compiler switch, such as -unittest for dmd, will cause the unittest test code to be compiled and incorporated into the resulting executable. The unittest code gets run after static initialization is run and before the main() function is called.

Additionally, this switch enables the conditional version(unittest) statement, enabling your code to take a different path depending on whether you're compiling with unittests or not.

For example, given a class Sum that is used to add two values:

    class Sum
    {
        int add(int x, int y) { return x + y; }

        unittest
        {
            Sum sum = new Sum;
            assert(sum.add(3,4) == 7);
            assert(sum.add(-2,0) == -2);
        }
    }

#### Attributed Unittests

A unittest may be attributed with any of the global function attributes. Such unittests are useful in verifying the given attribute(s) on a template function:

    void myFunc(T)(T[] data)
    {
        if (data.length > 2)
            data[0] = data[1];
    }

    @safe nothrow unittest
    {
        auto arr = [1,2,3];
        myFunc(arr);
        assert(arr == [2,2,3]);
    }

This unittest verifies that myFunc contains only @safe, nothrow code. Although this can also be accomplished by attaching these attributes to myFunc itself, that would prevent myFunc from being instantiated with types T that have @system or throwing code in their opAssign method, or other methods that myFunc may call. The above idiom allows myFunc to be instantiated with such types, yet at the same time verify that the @system and throwing behaviour is not introduced by the code within myFunc itself.

#### Documented Unittests

Documented unittests allow the developer to deliver code examples to the user, while at the same time automatically verifying that the examples are valid. This avoids the frequent problem of having outdated documentation for some piece of code.

If a declaration is followed by a documented unittest, the code in the unittest will be inserted in the example section of the declaration:

    /// Math class
    class Math
    {
        /// add function
        static int add(int x, int y) { return x + y; }

        ///
        unittest
        {
            assert(add(2, 2) == 4);
        }
    }

    ///
    unittest
    {
        auto math = new Math();
        auto result = math.add(2, 2);
    }

The above will generate the following documentation:

class Math;
  Math class
  Example:

      auto math = new Math;
      auto result = math.add(2, 2);

  int add(int x, int y);
    add function
    Example:

        assert(add(2, 2) == 4);

A unittest which is not documented, or is marked as private will not be used to generate code samples.

There can be multiple documented unittests and they can appear in any order. They will be attached to the last non-unittest declaration:

    /// add function
    int add(int x, int y) { return x + y; }

    /// code sample generated
    unittest
    {
        assert(add(1, 1) == 2);
    }

    /// code sample not generated because the unittest is private
    private unittest
    {
        assert(add(2, 2) == 4);
    }

    unittest
    {
        /// code sample not generated because the unittest isn't documented
        assert(add(3, 3) == 6);
    }

    /// code sample generated, even if it only includes comments (or is empty)
    unittest
    {
        /** assert(add(4, 4) == 8); */
    }

The above will generate the following documentation:

int add(int x, int y);
  add function
  Examples:
  code sample generated

      assert(add(1, 1) == 2);

  Examples:
  code sample generated, even if it is empty or only includes comments

      /** assert(add(4, 4) == 8); */

### Versioning

The version identifier unittest is predefined if the compilation is done with unit tests enabled.

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