An assertion library for C++
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An assertion library for C++

Snowhouse is a stand-alone assertion framework for C++.

It is a header-only library. You can simply use the headers-only branch as a submodule:

git submodule add -b headers-only snowhouse
git submodule update --init --recursive

As an alternative, CMake >= 3.0 users can use Snowhouse with the provided library target. Assuming you have cloned the master branch into a snowhouse subdirectory, your CMakeLists.txt might contain lines like the following:

add_executable(app main.cpp)
target_link_libraries(app snowhouse)


#include <snowhouse/snowhouse.h>

using namespace snowhouse;

int main()
  std::cout << "Testing that 23 is 23" << std::endl;
  AssertThat(23, Is().EqualTo(23));

    AssertThat(12, Is().LessThan(11).And().GreaterThan(99));
  catch(const AssertionException& ex)
    std::cout << "Apparently this failed:" << std::endl;
    std::cout << ex.GetMessage() << std::endl;

  return 0;


Snowhouse uses a constraint-based assertion model that is heavily inspired by the model used in NUnit. An assertion in Snowhouse is written using the following format:

AssertThat(actual_value, <constraint expression>);

where <constraint expression> is an expression that actual_value is evaluated against when the test is executed.

Constraint expressions come in two basic forms: composite and fluent expressions.

Composite Expressions

With composite expressions, you can create compact, powerful expressions that combine a set of predefined constraints with ones that you provide yourself.


AssertThat(length, IsGreaterThan(4) && !Equals(10));

Composite expressions can be any combination of constraints and the standard logical C++ operators.

You can also add your own constraints to be used within composite expressions.

Fluent Expressions

With fluent expressions, you can create assertions that better convey the intent of a test without exposing implementation-specific details. Fluent expressions aim to help you create tests that are not just by developers for developers, but rather can be read and understood by domain experts.

Fluent expressions also have the ability to make assertions on the elements in a container in a way you cannot achieve with composite expressions.


AssertThat(length, Is().GreaterThan(4).And().Not().EqualTo(10));

Basic Constraints

Equality Constraint

Used to verify equality between actual and expected.

AssertThat(x, Equals(12));
AssertThat(x, Is().EqualTo(12));

EqualityWithDelta Constraint

Used to verify equality between actual and expected, allowing the two to differ by a delta.

AssertThat(2.49, EqualsWithDelta(2.5, 0.1));
AssertThat(2.49, Is().EqualToWithDelta(2.5, 0.1));

GreaterThan Constraint

Used to verify that actual is greater than a value.

AssertThat(x, IsGreaterThan(4));
AssertThat(x, Is().GreaterThan(4));

LessThan Constraint

Used to verify that actual is less than a value.

AssertThat(x, IsLessThan(3));
AssertThat(x, Is().LessThan(3));

GreaterThanOrEqualTo Constraint

Used to verify that actual is greater than or equal to a value.

AssertThat(x, IsGreaterThanOrEqualTo(5));
AssertThat(x, Is().GreaterThanOrEqualTo(5));

LessThanOrEqualTo Constraint

Used to verify that actual is less than or equal to a value.

AssertThat(x, IsLessThanOrEqualTo(6));
AssertThat(x, Is().LessThanOrEqualTo(6));

Pointer Constraints

Used to check for nullptr equality.

AssertThat(x, IsNull());
AssertThat(x, Is().Null());

Note that this feature is only available for C++11-compliant compilers. In this case, the SNOWHOUSE_HAS_NULLPTR macro is defined.

String Constraints

String assertions in Snowhouse are used to verify the values of STL strings (std::string).

Equality Constraints

Used to verify that actual is equal to an expected value.

AssertThat(actual_str, Equals("foo"));
AssertThat(actual_str, Is().EqualTo("foo"));

Contains Constraint

Used to verify that a string contains a substring.

AssertThat(actual_str, Contains("foo"));
AssertThat(actual_str, Is().Containing("foo"));

EndsWith Constraint

Used to verify that a string ends with an expected substring.

AssertThat(actual_str, EndsWith("foo"));
AssertThat(actual_str, Is().EndingWith("foo"));

StartsWith Constraint

Used to verify that a string starts with an expected substring.

AssertThat(actual_str, StartsWith("foo"));
AssertThat(actual_str, Is().StartingWith("foo"));

HasLength Constraint

Used to verify that a string is of the expected length.

AssertThat(actual_str, HasLength(5));
AssertThat(actual_str, Is().OfLength(5));

Constraints on Multiline Strings

If you have a string that contains multiple lines, you can use the collection constraints to make assertions on the content of that string. This may be handy if you have a string that, for instance, represents the resulting content of a file or a network transmission.

Snowhouse can handle both Windows (CR+LF) and Unix (LF) line endings.

std::string lines = "First line\r\nSecond line\r\nThird line";
AssertThat(lines, Has().Exactly(1).StartingWith("Second"));

Container Constraints

The following constraints can be applied to containers in the standard template library.

Contains Constraint

Used to verify that a container contains an expected value.

AssertThat(container, Contains(12));
AssertThat(container, Is().Containing(12));

HasLength Constraint

Used to verify that a container has the expected length.

AssertThat(container, HasLength(3));
AssertThat(container, Is().OfLength(3));

IsEmpty Constraint

Used to verify that a container is empty.

AssertThat(container, IsEmpty());
AssertThat(container, Is().Empty());


Used to verify that all elements of a STL sequence container matches an expectation.

AssertThat(container, Has().All().LessThan(5).Or().EqualTo(66));


Used to verify that at least a specified amount of elements in a STL sequence container matches an expectation.

AssertThat(container, Has().AtLeast(3).StartingWith("foo"));


Used to verify that at most a specified amount of elements in a STL sequence container matches an expectation.

Assert:That(container, Has().AtMost(2).Not().Containing("failed"));


Used to verify that a STL sequence container has exactly a specified amount of elements that matches an expectation.

AssertThat(container, Has().Exactly(3).GreaterThan(10).And().LessThan(20));


Used to verify that two STL sequence containers are equal.

AssertThat(container1, EqualsContainer(container2));
AssertThat(container1, Is().EqualToContainer(container2));
Predicate functions

You can supply a predicate function or a functor to EqualsContainer to customize how to compare the elements in the two containers.

With a predicate function:

static bool are_my_types_equal(const my_type& lhs, const my_type& rhs)
  return lhs.my_val_ == rhs.my_val_;

AssertThat(container1, EqualsContainer(container2, are_my_types_equal));

With a functor as predicate:

struct within_delta
  within_delta(int delta) : delta_(delta) {}

  bool operator()(const my_type& lhs, const my_type& rhs) const
    return abs(lhs.my_val_ - rhs.my_val_) <= delta_;

  int delta_;

AssertThat(container1, Is().EqualToContainer(container1, within_delta(1));


Exception constraints can be used to verify that your code throws the correct exceptions.


AssertThrows succeeds if the exception thrown by the call is of the supplied type (or one of its subtypes).

AssertThrows(std::logic_error, myObject.a_method(42));

Making Assertions on the Thrown Exceptions

If AssertThrows succeeds, it will store the thrown exception so that you can make more detailed assertions on it.

AssertThrows(std::logic_error, myObject.a_method(42));
AssertThat(LastException<std::logic_error>().what(), Is().Containing("logic failure"));

The LastException<> is available in the scope of the call to AssertThrows. An exception is not available between specs in order to avoid the result of one spec contaminating another.

Custom Constraints

You can add your own constraints to Snowhouse to create more expressive specifications.

Fulfills Constraints

By defining the following matcher

struct IsEvenNumber
  bool Matches(const int actual) const
    return (actual % 2) == 0;

  friend std::ostream& operator<<(std::ostream& stm, const IsEvenNumber& );

std::ostream& operator<<(std::ostream& stm, const IsEvenNumber& )
  stm << "An even number";
  return stm;

You can create the following constraints in Snowhouse:

AssertThat(42, Fulfills(IsEvenNumber()));
AssertThat(42, Is().Fulfilling(IsEvenNumber()));

Your custom matcher should implement a method called Matches() that takes a parameter of the type you expect and returns true if the passed parameter fulfills the constraint.

To get more expressive failure messages, you should also implement the streaming operator as in the example above.

Getting better output for your types

Whenever Snowhouse prints an error message for a type, it will use the stream operator for that type, otherwise it will print "[unsupported type]" as a placeholder.

struct MyType { int x; char c; };

AssertThat(myType, Fulfills(MyConstraint());

Will output the following if the constraint fails:

Expected: To fulfill my constraint
Actual: [unsupported type]

If we add a stream operator:

std::ostream& operator<<(std::ostream& stream, const MyType& a)
  stream << a.c << a.x;
  return stream;

the output will be a bit more readable:

Expected: To fulfill my constraint
Actual: f23

If it is necessary to print an object in a different manner than the usual output stream operator provides, for example, to output more detailed information, we can use a specialization of the Stringizer class template:

namespace snowhouse
  struct Stringizer<MyType>
    static std::string ToString(const MyType& a)
      std::stringstream stream;
      stream << "MyType(x = " << a.x << ", c = " << int(a.c) << "('" << a.c << "'))";
      return stream.str();

with output:

Expected: To fulfill my constraint
Actual: MyType(x = 23, c = 102('f'))

Configurable Failure Handlers

You can provide Snowhouse with custom failure handlers, for example to call std::terminate instead of throwing an exception. See DefaultFailureHandler for an example of a failure handler. You can derive your own macros with custom failure handlers using SNOWHOUSE_ASSERT_THAT and SNOWHOUSE_ASSERT_THROWS. See the definitions of AssertThat and AssertThrows for examples of these. Define SNOWHOUSE_NO_MACROS to disable the unprefixed macros AssertThat and AssertThrows.

Example Use Cases

Assert Program State

Log an error immediately as we may crash if we try to continue. Do not attempt to unwind the stack as we may be inside a destructor or nothrow function. We may want to call std::terminate, or attempt to muddle along with the rest of the program.

Assert Program State in Safe Builds

As above, but only in debug builds.

Test Assert

Assert that a test behaved as expected. Throw an exception and let our testing framework deal with the test failure.


Snowhouse uses Semantic Versioning 2.0.0 since version 3.0.0. The macros SNOWHOUSE_MAJOR, SNOWHOUSE_MINOR and SNOWHOUSE_PATCH are defined accordingly and SNOWHOUSE_VERSION contains the version string. Note that in prior versions SNOWHOUSE_VERSION was the only defined macro.


The development of Snowhouse takes place on GitHub.

Snowhouse is licensed under the Boost Software License. See LICENSE_1_0.txt for further information.

By making available code for inclusion into Snowhouse (e.g., by opening a pull request on GitHub), you guarantee that the code is licensed under the same license as Snowhouse.

Please make sure to be consistent with the project's coding style. The .clang-format file allows easy checking and implementation of the coding style.

C++ code should comply to C++98, C++03- and C++11. Please use __cplusplus guards if you want to use language features of a certain C++ version.


Snowhouse was originally developed as part of the Igloo testing framework by Joakim Karlsson. It has been extracted to be usable in other contexts, for example, Bandit.

Snowhouse is maintained by Stephan Beyer since October 2016.