Mocksmith

Mocksmith is a command line utility and a Rust library to automatically generate C++ mock classes and header files for the Google Mock (gMock) framework, part of Google Test. This documentation mainly focuses on the command line program.

Installation

Mocksmith uses libclang to parse the source C++ code provided to it, so you need to install clang first. On Linux, you typically install clang with your package manager, e.g., apt in Debian-based distributions like Ubuntu:

$ sudo apt install clang

For Windows, you can find binaries to install here, e.g., LLVM-20.1.7. Make sure to add the LLVM bin folder to your PATH environment variable, which is an option when installing, otherwise Mocksmith will not be able to find libclang.

When you have libclang in place, you can install the mocksmith program with cargo:

$ cargo install mocksmith

Currently, no pre-built binaries for mocksmith are distributed.

Usage

To generate mocks to stdout, just pass paths to files with classes to mock as arguments to mocksmith.

$ cat IMyItem.h
#pragma once
#include <string>
class IMyItem {
    virtual ~IMyItem() = default;
    virtual int do_stuff(const std::string& text) = 0;
};

$ mocksmith IMyItem.h
class MockMyItem : public IMyItem
{
public:
  MOCK_METHOD(int, do_stuff, (const std::string & text), (override));
};

Generate Header Files

To generate a complete header file, you pass an option with the name of the header file. With this option, a single header file is generated, with all mocks for classes from all source files.

$ mocksmith -o MockMyItem.h IMyItem.h

$ cat MockMyItem.h
// Automatically generated by Mocksmith
#pragma once

#include "IMyItem.h"
#include <gmock/gmock.h>

class MockMyItem : public IMyItem
{
public:
  MOCK_METHOD(int, do_stuff, (const std::string & text), (override));
};

If you prefer to create one header file per source file, pass an option with the output directory instead.

$ mocksmith -d . IMyItem.h

$ ls MockMyItem.h
MockMyItem.h

Note that mocksmith does not overwrite existing files if the content would not change. This is useful to avoid triggering rebuilds of code which includes the mock header file, if the build system depends on file timestamps.

Naming Mocks

If you don't like the way mocksmith names the mock classes, you can use a regex-based approach, similar to how sed works. The input to the regex part is the name of the class to mock.

$ mocksmith -n "s/I(.*)/Fake\1/" IMyItem.h
class FakeMyItem : public IMyItem
[...]

Naming Output Files

The output files in an output directory can be named similarly to the way mocks can be named.

$ mocksmith -d . -f "s/I(.*).h/Fake\1.hpp/" IMyItem.h

$ ls FakeMyItem.hpp
FakeMyItem.hpp

Include Paths

Most of the time, you will have to tell mocksmith include search paths with the -I option, similar to compilers like clang and gcc. If clang can't locate an included header, mocksmith will stop and report an error.

Let's say we have a directory structure like this:

$ tree code/
code/
|-- itemcomponent
|   |-- IMyItem.h
|   `-- mocks
`-- subcomponent
    `-- MyEnum.h

$ cat code/itemcomponent/IMyItem.h
#pragma once
#include "subcomponent/MyEnum.h"
class IMyItem {
    virtual ~IMyItem() = default;
    virtual void do_stuff(MyEnum) = 0;
};

$ cat code/subcomponent/MyEnum.h
#pragma once
enum class MyEnum { One=1, Two=2 };

If you use mocksmith without include search paths to locate included headers, you get an error.

$ mocksmith code/itemcomponent/IMyItem.h
Error: Could not create mocks for file code/itemcomponent/IMyItem.h

Caused by:
    Parse error in file code/itemcomponent/IMyItem.h at line 2, column 10: 'subcomponent/MyEnum.h' file not found

Adding -I code/ solves this:

$ mocksmith -I code/ code/itemcomponent/IMyItem.h
class MockMyItem : public IMyItem
{
public:
  MOCK_METHOD(void, do_stuff, (MyEnum), (override));
};

The specified include search paths are also used to decide how to include the header file of the source class when generating header files with mocks. The shortest relative path from the given include search paths is used.

$ mocksmith -I code -d code/itemcomponent/mocks/ code/itemcomponent/IMyItem.h

$ cat code/itemcomponent/mocks/MockMyItem.h
// Automatically generated by Mocksmith
#pragma once

#include "itemcomponent/IMyItem.h"
#include <gmock/gmock.h>
[...]

It is possible to ignore errors from libclang with the --ignore-errors option but it is not recommended since clang will report unknown types as int. It also fails to identify methods as virtual when returning unknown types. Errors will still be logged, unless the --silent option is used.

Additional Options

See the help text, using the -h or --help option, for a complete list of options.

Limitations

You should have LLVM/Clang 19 or newer. Intermittent problems have been seen when executing tests using Clang 18.x, indicating possible issues when using the program.

On Windows, there is a bug in the underlying clang-rs Rust wrapper library which makes it impossible to use the "runtime" feature. Thus, libclang must be in the PATH since it cannot be located at runtime automatically.

If you have problems with Mocksmith not finding your libclang installation, consult the clang-sys documentation.

Alternatives

Up until, and including, Google Test 1.11.0, there was a Python script included with Google Mock to generate mocks. The script was based on a simple C++ parser written in Python which had problems with modern constructs and advanced features of C++.