# FuzzTest Overview

Consider a typical unit test for a function that parses the leading digits of a
string into an integer:

```c++
TEST(ParseLeadingDigitsTest, ParsesIntCorrectly) {
  std::string input = "42IsTheAnswer";
  std::optional<int> output = ParseLeadingDigits(input);
  EXPECT_THAT(output, Optional(Eq(42)));
}
```

This single input/output example is great, but what if there is some tricky edge
case with some other input? Wouldn’t it be great to have a test that is not
specific to `42`?

With FuzzTest, you can write tests that generalize to a wider set of inputs:

```c++
void ParsesIntCorrectly(int number, const std::string& suffix) {
  std::string input = absl::StrCat(number, suffix);
  std::optional<int> output = ParseLeadingDigits(input);
  EXPECT_THAT(output, Optional(Eq(number)));
}
FUZZ_TEST(ParseLeadingDigitsTest, ParsesIntCorrectly)
  .WithDomains(/*number=*/Arbitrary<int>(), /*suffix=*/InRegexp("[^0-9].*"));
```

In the fuzz test version of the above test, `ParsesIntCorrectly` runs
`ParseLeadingDigits` with many different inputs, specified by abstract input
domains. The test has parameters (`number` and `suffix`), and it verifies that
the output matches the input parameter number. We call `ParsesIntCorrectly` the
*property function*, which we instantiate with the `FUZZ_TEST` macro.

FuzzTest will run a test with many different parameter values from the specified
input domains and find tricky edge cases that invalidate your assertions.

Writing fuzz tests requires you to shift focus from providing interesting
specific inputs to specifying EXPECT-ations (properties) that must hold for a
given set of inputs. This doesn't mean that you always need to write explicit
ASSERT/EXPECT statements in the property function (the test body). You might
simply check that the ASSERT-ions in your code under test don't get invalidated
with any inputs. If you can identify properties that generalize to all or a set
of inputs, your can write additional assertions too and let FuzzTest find the
tricky edge cases for you. This sort of testing is commonly known as
"property-based testing".

The most typical property to check is that your code has no undefined behavior
(e.g., buffer overflows, use-after-frees, integer overflows, uninitialized
memory, etc). This doesn't need any explicit assertions either, because you can
rely on the implicit assertions of sanitizers
(https://clang.llvm.org/docs/AddressSanitizer.html)
to test for such undefined behavior:

```c++
void HasNoUndefinedBehavior(const std::string& input) {
  ParseLeadingDigits(input);
}
FUZZ_TEST(ParseLeadingDigitsTest, HasNoUndefinedBehavior); // Uses Arbitrary<T> as input domain for each parameter by default.
```