assertions

    ## Assertion macros

    Most test frameworks have a large collection of assertion macros to capture all possible conditional forms (```_EQUALS```, ```_NOTEQUALS```, ```_GREATER_THAN``` etc).

    **doctest** is different (but it's like [**Catch**](https://github.com/catchorg/Catch2) in this regard). Because it decomposes comparison expressions most of these forms are reduced to one or two that you will use all the time. That said, there is a rich set of auxiliary macros as well.

    There are 3 levels of assert severity for all assertion macros:

    - ```REQUIRE``` - this level will immediately quit the test case if the assert fails and will mark the test case as failed.
    - ```CHECK``` - this level will mark the test case as failed if the assert fails but will continue with the test case.
    - ```WARN``` - this level will only print a message if the assert fails but will not mark the test case as failed.

    The ```CHECK``` level is mostly useful if you have a series of essentially orthogonal assertions and it is useful to see all the results rather than stopping at the first failure.

    All asserts evaluate the expressions only once and if they fail - the values are [**stringified**](stringification.html) properly.

    Since **doctest** is [**thread-safe**](faq.html#is-doctest-thread-aware) all asserts and [**logging**](logging.html) macros can be used in threads spawned from test cases.

    Note that the ```REQUIRE``` level of asserts uses exceptions to end the current test case. It might be dangerous to use this level of asserts inside destructors of user-defined classes - if a destructor is called during stack unwinding due to an exception and a ```REQUIRE``` assert fails then the program will terminate. Also since C++11 all destructors are by default ```noexcept(true)``` unless specified otherwise so such an assert will lead to ```std::terminate()``` being called.

    ## Expression decomposing asserts

    These are of the form ```CHECK(expression)```  (Same for ```REQUIRE``` and ```WARN```).

    ```expression``` can be a binary comparison like ```a == b``` or just a single thing like ```vec.isEmpty()```.

    If an exception is thrown it is caught, reported, and counted as a failure (unless the assert is of level ```WARN```).

    Examples:

    ```
    CHECK(flags == state::alive | state::moving);
    CHECK(thisReturnsTrue());
    REQUIRE(i < 42);
    ```

    - Negating asserts - ```<LEVEL>_FALSE(expression)``` - evaluates the expression and records the _logical NOT_ of the result.

    These forms exist as a workaround for the fact that ```!``` prefixed expressions cannot be decomposed properly.

    Example:

    ```
    REQUIRE_FALSE(thisReturnsFalse());
    ```

    - Using the [**```DOCTEST_CONFIG_SUPER_FAST_ASSERTS```**](configuration.html#doctest_config_super_fast_asserts) config option can make compilation of asserts up to [**31-63%**](benchmarks.html#cost-of-an-assertion-macro) faster!
    - These asserts also have a ```_MESSAGE``` form - like ```CHECK_MESSAGE(expression, message)``` which is basically a code block ```{}``` with a scoped [**```INFO()```**](logging.html#info) logging macro together with the ```CHECK``` macro - that way the message will be relevant only to that assert. The binary/unary asserts don't have this variation yet.

    Examples:

    ```
    INFO("this is relevant to all asserts, and here is some var: ", local);

    CHECK_MESSAGE(a < b, "relevant only to this assert ", other_local, " more text!");

    CHECK(b < c); // here only the first INFO() will be relevant
    ```

    For more information about the ```INFO()``` macro visit the [logging page](logging.html).

    ## Binary and unary asserts

    These asserts don't use templates to decompose the comparison expressions for the left and right parts.

    These have the same guarantees as the expression decomposing ones but [**57-68% faster**](benchmarks.html#cost-of-an-assertion-macro) for compilation.

    ```<LEVEL>``` is one of 3 possible: ```REQUIRE```/```CHECK```/```WARN```.

    - ```<LEVEL>_EQ(left, right)``` - same as ```<LEVEL>(left == right)```
    - ```<LEVEL>_NE(left, right)``` - same as ```<LEVEL>(left != right)```
    - ```<LEVEL>_GT(left, right)``` - same as ```<LEVEL>(left >  right)```
    - ```<LEVEL>_LT(left, right)``` - same as ```<LEVEL>(left <  right)```
    - ```<LEVEL>_GE(left, right)``` - same as ```<LEVEL>(left >= right)```
    - ```<LEVEL>_LE(left, right)``` - same as ```<LEVEL>(left <= right)```
    - ```<LEVEL>_UNARY(expr)``` - same as ```<LEVEL>(expr)```
    - ```<LEVEL>_UNARY_FALSE(expr)``` - same as ```<LEVEL>_FALSE(expr)```

    Using the [**```DOCTEST_CONFIG_SUPER_FAST_ASSERTS```**](configuration.html#doctest_config_super_fast_asserts) config option can make the binary asserts to compile up to [**84-91%**](benchmarks.html#cost-of-an-assertion-macro) faster!

    ## Exceptions

    ```<LEVEL>``` is one of 3 possible: ```REQUIRE```/```CHECK```/```WARN```.

    - ```<LEVEL>_THROWS(expression)```

    Expects that an exception (of any type) is thrown during evaluation of the expression.

    - ```<LEVEL>_THROWS_AS(expression, exception_type)```

    Expects that an exception of the _specified type_ is thrown during evaluation of the expression.

    Note that ```const``` and ```&``` are added to the exception type if missing (users shouldn't care) - the standard practice for exceptions in C++ is ```Throw by value, catch by (const) reference```.

    ```
    CHECK_THROWS_AS(func(), const std::exception&);
    CHECK_THROWS_AS(func(), std::exception); // same as above
    ```

    - ```<LEVEL>_THROWS_WITH(expression, c_string)```

    Expects that an exception is thrown during evaluation of the expression and is successfully translated to the _specified c string_ (see [**translating exceptions**](stringification.html#translating-exceptions)).

    ```
    CHECK_THROWS_WITH(func(), "invalid operation!");
    ```

    - ```<LEVEL>_THROWS_WITH_AS(expression, c_string, exception_type)```

    This is a combination of ```<LEVEL>_THROWS_WITH``` and ```<LEVEL>_THROWS_AS```.

    ```
    CHECK_THROWS_WITH_AS(func(), "invalid operation!", std::runtime_error);
    ```

    - ```<LEVEL>_NOTHROW(expression)```

    Expects that no exception is thrown during evaluation of the expression.

    Note that these asserts also have a ```_MESSAGE``` form - like ```CHECK_THROWS_MESSAGE(expression, message)``` - these work identically to the ```_MESSAGE``` form of the normal macros (```CHECK_MESSAGE(a < b, "this shouldn't fail")```) described earlier.

    One may use the [**```DOCTEST_CONFIG_VOID_CAST_EXPRESSIONS```**](configuration.html#doctest_config_void_cast_expressions) config identifier to cast the expression in these asserts to void to avoid warnings or other issues - for example nodiscard statements whose result isn't checked. This will however limit the ability to write entire ```{}``` blocks of code as the expression (or multiple statements) but in that case a simple lambda can be used. This should have been the default behavior from day 1 of the framework...

    ## Using asserts out of a testing context

    Asserts can be used outside of a testing context (in code not called from a ```TEST_CASE()```) instead of [```assert()```](https://en.cppreference.com/w/cpp/error/assert).

    A ```doctest::Context``` object still has to be created somewhere and set as the default one using the ```setAsDefaultForAssertsOutOfTestCases()``` method - and then asserts will work. A handler can be registered by calling the ```setAssertHandler()``` method on the context object. If no handler is set then ```std::abort()``` is called on failure.

    The results would be best when using the [**```DOCTEST_CONFIG_SUPER_FAST_ASSERTS```**](configuration.html#doctest_config_super_fast_asserts) config identifier.

    Checkout the [**example**](../../examples/all_features/asserts_used_outside_of_tests.cpp) showcasing how that is done. For more information see the [**issue for the feature request**](https://github.com/onqtam/doctest/issues/114).

    Currently [**logging macros**](logging.html) cannot be used for extra context for asserts outside of a test run. That means that the ```_MESSAGE``` variants of asserts are also not usable - since they are just a packed ```INFO()``` with an assert right after it.

    ## Floating point comparisons

    When comparing floating point numbers - especially if at least one of them has been computed - great care must be taken to allow for rounding errors and inexact representations.

    **doctest** provides a way to perform tolerant comparisons of floating point values through the use of a wrapper class called ```doctest::Approx```. ```doctest::Approx``` can be used on either side of a comparison expression. It overloads the comparisons operators to take a relative tolerance into account. Here's a simple example:

    ```
    REQUIRE(performComputation() == doctest::Approx(2.1));
    ```

    By default a small epsilon value (relative - in percentages) is used that covers many simple cases of rounding errors. When this is insufficient the epsilon value (the amount within which a difference either way is ignored) can be specified by calling the ```epsilon()``` method on the ```doctest::Approx``` instance. e.g.:

    ```
    REQUIRE(22.0/7 == doctest::Approx(3.141).epsilon(0.01)); // allow for a 1% error
    ```

    When dealing with very large or very small numbers it can be useful to specify a scale, which can be achieved by calling the ```scale()``` method on the ```doctest::Approx``` instance.

    --------

    - Check out the [**example**](../../examples/all_features/assertion_macros.cpp) which shows many of these macros
    - Do not wrap assertion macros in ```try```/```catch``` - the REQUIRE macros throw exceptions to end the test case execution!

    ---------------

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