Author

Dominic Koepke
Mail: DNKpp2011@gmail.com

Introduction

mimic++ is a c++20 mocking framework, which aims to offer a natural end expressive syntax, without constantly resorting to macros. To be honest, macros cannot be completely avoided, but they can at least be reduced to a very minimum.

The one thing, that this framework does different than all other (or at least all I am aware of) mocking framework is, that mock objects explicitly are function objects, thus directly callable. It's simple and straight forward.

As I'm mainly working on template or functional-style code, I wanted something simpler than always creating explicit mock types for the simplest of use cases. So, mimicpp::Mock objects can directly be used as functional objects, but they can also be used as member objects and thus serve as actual member functions.

If you are curious, have a look at the documentation, investigate the examples folder or play around online at godbolt.org.

Core Design

The framework is designed with two core concepts in mind: Mocks and Expectations. Mocks can be used to define behaviour on a per test-case basis, without the necessity of creating dozens of types. The go-to example is, if you have a custom type, which somehow makes a connection to a concrete database, you do not want to setup an actual database connection during your test runs. You then simply install a database mock, which then yields the exact replies as it were defined for that particular case: the so called "Expectations".

So, Mocks and Expectations are going together hand in hand.

Examples

Mocks as function objects

As already said, mimicpp::Mocks are already function objects.

#include <mimic++/mimic++.hpp>
 
// it's recommended to pull these sub-namespaces out, because then the expectation setup becomes much more readable
namespace expect = mimicpp::expect;
namespace matches = mimicpp::matches;
namespace finally = mimicpp::finally;
namespace then = mimicpp::then;
 
using matches::_;   // That's the wildcard matcher, which matches everything
 
TEST_CASE("Mocks are function objects.")
{
    mimicpp::Mock<int(std::string, std::optional<int>)> mock{};     // actually enables just `int operator ()(std::string, std::optional<int>)`
    SCOPED_EXP mock.expect_call("Hello, World", _)                  // requires the first argument to match the string "Hello, World"; the second has no restrictions
                and expect::at_least(1)                             // controls, how often the whole expectation must be matched
                and expect::arg<0>(!matches::range::is_empty())     // addtionally requires the first argument to be not empty (note the preceeding !)
                and expect::arg<1>(matches::ne(std::nullopt))       // requires the second argument to compare unequal to "std::nullopt"
                and expect::arg<1>(matches::lt(1337))               // and eventually to be less than 1337
                and then::apply_arg<0>(                             // That's a side-effect, which get's executed, when a match has been made.
                    [](std::string_view str) { std::cout << str; }) //     This one writes the content of the first argument to std::cout.
                and finally::returns(42);                           // And, when matches, returns 42
 
    int result = mock("Hello, World", 1336);                        // matches
    REQUIRE(42 == result);
}
 
TEST_CASE("Mocks can be overloaded.")
{
    mimicpp::Mock<
        int(std::string, std::optional<int>),                   // same as previous test
        void() const                                            // enables `void operator ()() const` (note the const specification)
    > mock{};
 
    SCOPED_EXP mock.expect_call()                               // setup an expectation for the void() overload
                and expect::twice();                            // should be matched twice
 
    mock();                                                     // first match
 
    // you can always create new expectations as you need them, even if the object is already in use
    SCOPED_EXP mock.expect_call(!matches::range::is_empty(), 42)        // you can always apply matches directly; if just a value is provided, defaults to matches::eq
                and expect::once()                                      // once() is the default, but you can state that explicitly
                and finally::throws(std::runtime_error{"some error"});  // when matches, throws an exception
 
    REQUIRE_THROWS(mock("Test", 42));                           // ok, matched
 
    // still a pending expectation for void() overload
    std::as_const(mock)();                                      // explicitly call from a const object
}

Mocks as member functions

mimicpp::Mocks can also serve as member functions. Sure, there are some limitations, but for the most cases it works well.

#include <mimic++/mimic++.hpp>
 
namespace finally = mimicpp::finally;
 
// let's build a function, which actually expects an object and requires .get() member function,
// This member function then should return something printable
inline void foo(const auto& obj)
{
    std::cout << obj.get();
}
 
TEST_CASE("Mocks can be used as member functions.")
{
    struct Mock
    {
        mimicpp::Mock<int() const> get{}; // that serves as the .get() member function
    };
 
    Mock mock{};
    SCOPED_EXP mock.get.expect_call()
                and finally::returns(42);
 
    foo(mock);      // fine, foo calls the get() member-function
}

Mocking interfaces

mimic++ also provides helpers for interface mocking.

#include <mimic++/mimic++.hpp>
 
namespace finally = mimicpp::finally;
 
// let's say, we have the following interface
class Interface
{
public:
    virtual ~Interface() = default;
    virtual int get() const = 0;
};
 
// and a function, which this time actually requires an interface.
inline void foo(const Interface& obj)
{
    std::cout << obj.get();
}
 
TEST_CASE("Interfaces can be mocked.")
{
    class Derived
        : public Interface
    {
    public:
        ~Derived() override = default;
 
        // this generates the override method and a mock object named get_
        MOCK_METHOD(get, int, (), const);
    };
 
    Derived mock{};
    SCOPED_EXP mock.get_.expect_call()      // note the _ suffix. That's the name of the mock object.
                and finally::returns(42);
 
    foo(mock);        // fine, foo calls the get() member-function, which forwards the call to the mock object get_.
}

Sometimes, an interface method may also have several overloads. mimic++ directly supports overriding overload-sets.

#include <mimic++/mimic++.hpp>
 
namespace finally = mimicpp::finally;
 
// let's say, we have the following interface, with an overload-set
class Interface
{
public:
    virtual ~Interface() = default;
    virtual int& get() = 0;
    virtual const int& get() const = 0;
};
 
// and a function, using the const overload of that interface
inline void foo(const Interface& obj)
{
    std::cout << obj.get();
}
 
TEST_CASE("Interface overload-sets are directly supported.")
{
    class Derived
        : public Interface
    {
    public:
        ~Derived() override = default;
 
        // this generates two overloads of get and a single mock object named get_
        MOCK_OVERLOADED_METHOD(
            get,                                    // the name of the overload-set
            ADD_OVERLOAD(int&, ()),                 // enables "int& operator ()()" on the mock
            ADD_OVERLOAD(const int&, (), const));   // enables "const int& operator ()() const" on the mock
    };
 
    Derived mock{};
    SCOPED_EXP std::as_const(mock).get_.expect_call()   // As we expect the const overload to be used, we must explicitly select that overload.
                and finally::returns(42);               // The returned reference is valid, as long as the expectation is alive.
 
    foo(mock);        // fine, foo calls the get() const member-function, which forwards the call to the mock object get_ as before.
}

Other Choices

Always Stay Within The Language Definition

There are a lot of mocking frameworks, which utilize clever tricks and apply some compiler specific instructions to make the work more enjoyable. mimic++ does not! This framework will never touch the ground of undefined behaviour or tricks, which will only work under some circumstances, as this is nothing I want to support and maintain over a set of compilers or configurations. Unfortunatle this often leads to a less elegant syntax for users. If you need that, than this framework is probably not the right for you. Pick your poison :)

Special Acknowledgement

This framework is highly inspired by the well known trompeloeil, which I have used myself for several years now. It's definitly not bad, but sometimes feels a little bit dated and some macros do not play very well with formatting tools and the like. If you need a pre-c++20 mocking-framework, you should definitly give it a try.

Fun fact: mimic++ uses trompeloeil for it's own test suite :D

Customizability

A framework should be a useful tool that can be used in a variety of ways. However, it should not be a foundation that limits the house to be built on it. For this reason mimic++ offers various ways for customization: E.g. users may create their own expectation policies and integrate them seamlessly, without changing any line of the mimic++ code-base.

Stringification

mimic++ can not provide stringification for any type out there, but it's often very useful to see a proper textual reprensentation of an object, when a test fails. mimic++ will use std::format for formattable types, but sometimes that is not, what we want, as users for example want to have an alternative stringification just for testing purposes. Users can therefore add their specializations of the mimicpp::custom::Printer type and thus tell mimic++ how a given type shall be printed.

Custom specializations will always be prefered over any pre-existing printing methods, thus users may even override the stringification of the internal report types.

Matchers

Matchers are used to check, whether arguements satisfy some requirements. There are already many existing matchers available, but users often have special needs. mimic++ provides a very generic mimicpp::PredicateMatcher, which is often already sufficient. But, if one needs full control, just start with a fresh type (without any inheritance) and start building your own. They just have to match the mimicpp::matcher_for concept. For more information, head to the documentation.

Policies

There are multiple types of policie, depending on the task they shall fulfill. The expectation policy has full control, whether a match can be made or shall be rejected, while the finalize policy determines, what a mock shall do, when it actually matched (like returning a value or throwing an exception). They can implement arbitrary logic, so feel free to experiment. There is no base type requirement, they simply have to satisfy either the mimicpp::expectation_policy_for, mimicpp::control_policy or mimicpp::finalize_policy_for.

Documentation

The documenation is generated via doxygen. Users can do this locally by enabling both, the MIMICPP_CONFIGURE_DOXYGEN and MIMICPP_ENABLE_GENERATE_DOCS, cmake options and building the target mimicpp-generate-docs manually.

The documentation for the main branch is always available on the github-pages; for the development branch it is also available on the dev-gh-pages branch, but unfortunatly not directly viewable on the browser. Every release has the generated documentation attached.

Integration

mimic++ is a head-only library. Users can easily enjoy all features by simply including the mimic++/mimic++.hpp header. Of course one can be more granular and include just what's necessary. The choice is yours.

Installation

CMake

This framework is header-only and completely powered by cmake, thus the integration into a cmake project is straight-forward.

target_link_libraries(
    <your_target_name>
    PUBLIC
    mimicpp::mimicpp
)

Users can either pick a commit in the main branch or a version tag and utilize the cmake FetchContent module:

include(FetchContent)
 
FetchContent_Declare(
    mimicpp
    GIT_REPOSITORY https://github.com/DNKpp/mimicpp
    GIT_TAG        <any_commit_hash_or_tag>
)
 
FetchContent_MakeAvailable(mimicpp)
# do not forget linking via target_link_libraries as shown above

As an alternative, I recommend the usage of CPM, which is a featureful wrapper around the FetchContent functionality:

include(CPM.cmake) # or include(get_cpm.cmake)
 
CPMAddPackage("gh:DNKpp/mimicpp#<any_commit_hash_or_tag>")
# do not forget linking via target_link_libraries as shown above

Single-Header

As an alternative each release has a header file named mimic++-amalgamated.hpp attached, which contains all definitions (except for the specific test-framework adapters) and can be simply dropped into any c++20 project (or used on godbolt.org. After that, users may also just pick the appropriate adapter header for their desired test-framework and put that into their project aswell.

Test Framework

Mocking frameworks usually do not exist for their own, as they are in fact just an advanced technice for creating tests. Instead, they should work together with any existing test framework out there. mimic++ provides the IReporter interface, which in fact serves as a bridge from mimic++ into the utilized test framework. mimic++ provides some concrete reporter implementations for well known test frameworks, but users may create custom adapters for any test framework or simply use the default reporter. For more details have a look into the reporting section in the documentation.

Official adapters exist for the following frameworks:

Testing

mimic++ utilizes a strict testing policy, thus each official feature is well tested. The effect of those test-cases are always tracked by the extensive ci, which checks the compilation success, test cases outcomes and coverage on dozens of different os, compiler and build configurations.

The coverage is generated via gcov and evaluated by codacy, codecov and coveralls.

Nevertheless, even with very high effort, the 100% code-coverage is without reach. I do my best to cover each branch, but even now the coverage percentage varies between the individual tools. The goal is to be close to 100% as possible.

On the other hand-side, there is a whole range of code which doesn't even get investigated by these tools: templates (and macros). mimic++ has a lot of templating code at the very heart, which is at least of equal effort to get right (and tested). So, treat the coverage percentage with a grain of salt.

CI Tests

The listed configurations are explicitly tested, but other do probably work, too. As new compilers become available, they will be added to the workflow, but older compilers will probably never be supported.

Symbol	Description
x	works
*	works with caveats
-	does not work
?	not tested

Windows

OS	Compiler	c++-20	c++-23	std-format	fmt
Windows 2022	msvc	x	x	x	x
Windows 2022	clangCl	x	x	x	x

Linux

Compiler	libstdc++	libc++	c++-20	c++-23	std-format	fmt
clang-16	x	x	x	x	-	x
clang-17	x	x	x	x	x	x
clang-18	x	*	x	x	x	x
gcc-12	x	?	x	x	-	x
gcc-13	x	?	x	x	x	x
gcc-14	x	?	x	x	x	x

macOS

Compiler	libstdc++	libc++	c++-20	c++-23	std-format	fmt
AppleClang-16.0.6^1^	?	x	x	x	x	x
AppleClang-17.0.6	?	x	x	x	x	x
AppleClang-18.1.6	?	x	x	x	x	x

^1^ There is an issue with AppleClang-16 and boost::filesystem on the current macos-latest, but this is just for the boost-test-adapter. Everything else works fine.

License

BSL-1.0 (free, open source)

         Copyright Dominic "DNKpp" Koepke 2024 - 2024
Distributed under the Boost Software License, Version 1.0.
   (See accompanying file LICENSE_1_0.txt or copy at
         https://www.boost.org/LICENSE_1_0.txt)

Known Issues

Clang-18.1 + libc++

Date: 25.09.2024

This combination introduced a regression regarding the std::invocable concept and a default parameter of type std::source_location. On this version, all invocable checks will fail, but the std::is_invocable trait still works as expected. Unfortunatly this can not solved easily by this lib, sorry for that.

Clang-17 and Clang-19 do not suffer from this issue. For more information have a look here.