Metatest is a unit testing framework for C++ template metaprograms. It can be integrated into other C++ unit testing frameworks and has its own reporting capabilities as well. There are no special steps in the compilation of unit tests compared to other C++ code.

Table of contents

Getting the library


The library is published under the Boost Software License.


The basic principle of Metatest is to compile the source, evaluate test cases at compile time and generate an executable that outputs a detailed and customizable report on the results.

The exact procedure is as follows: Test cases are written as nullary metafunctions returning wrapped boolean values. These metafunctions are evaluated and, depending on the fail/success statuses, report items are generated utilizing the framework's own type pretty-printing facility. Pretty-printing is needed to display detailed error messages. For example when a test case compares the result of a metafunction evaluation with the expected value and they differ, the test framework should display the result and the expected value in the report. Our solution achieves this by generating a string at compile-time containing all these details. In order for classes - data, metaprograms operate on - to be usable with the framework, we need to be able to convert them into strings.

Test results are then collected in STL containers in a test suite hierarchy, the root of which is located in a global test driver object. This hierarchy is accessible through an iterator interface. Test result objects provide their own methods via which fail/success statuses, test names and detailed descriptions can be accessed.

The test cases are collected into test suites. A test suite can contain test cases and further test suites, thus the tests are collected in a tree structure of nested suites. This hierarchy is built at runtime, by the constructors of global objects. This tree can be processed by regular C++ code, the root can be accessed as a singleton object. The tree provides STL-like iterators to process the children of each node. This data-structure can be used to either generate a report directly or to integrate compile-time tests into runtime testing frameworks. In our implementation we provide a number of report generators and a tool that adds compile-time tests to the Boost unit testing framework's test suite hierarchy.

Test cases are registered in the driver by constructors of global objects. Our implementation provides a convenient macro for creating these global objects. It takes two parameters - an object representing the location of the test case in the test suite hierarchy and the name of the metafunction.

Basic usage

First, the following header must be included.

#include <metatest/test.hpp>

Individual tests are organized in a test suite hierarchy. To create a suite, first, a suite path must be declared by creating an instance of suite_path. Using operator() suite paths can be constructed the following way:

const suite_path my_suite_path = suite_path("a")("b")("c");

Test cases are written as nullary template metafunctions, each returning a wrapped boolean value. They are added to the test suites by registering them using the supplied METATEST_ADD_TEST macros.

The root of the test suite hierarchy is added to a global test driver singleton object through which the hierarchy can be traversed at runtime. A number of default main functions are provided building different (plain text, xml, Boost test, etc.) reports.

A default main generating plain text report on the standard output is available by including the following header.

#include <metatest/main.hpp>


In this section we assume that we have a template metafunction, my_reverse, that reverses boost::mpl::vector values. With the above applied, we get the following test suite for my_reverse.

const suite_path suite = suite_path("my_suite")("my_reverse");

typedef vector<> empty;
typedef vector<char> one;
typedef vector<char, short, int, long> four;

typedef vector<long, int, short, char> four_r;

typedef equal< my_reverse<empty>::type, empty > empty_test;
typedef equal< my_reverse<one>::type, one > one_test;
typedef equal< my_reverse<four>::type, four_r > four_test;

METATEST_ADD_TEST(suite, empty_test)
METATEST_ADD_TEST(suite, one_test)
METATEST_ADD_TEST(suite, four_test)

By compiling it, the unit tests are executed. Assuming that our my_reverse implementation is correct, the default plain text reporter outputs the following report.

The following tests have been executed:
  my_suite::my_reverse::empty_test: OK
  my_suite::my_reverse::one_test: OK
  my_suite::my_reverse::four_test: OK
Number of tests: 3
Number of failures: 0

To simulate library bugs, we modify the suite.

const suite_path suite = suite_path("my_suite")("my_reverse");

typedef vector<> empty;
typedef vector<char> one;
typedef vector<char, short, int, long> four;

typedef vector<long, int, short, char> four_r;

typedef equal< my_reverse<empty>::type, one > empty_test;
typedef equal< my_reverse<one>::type, empty > one_test;
typedef equal< my_reverse<four>::type, four > four_test;

METATEST_ADD_TEST(suite, empty_test)
METATEST_ADD_TEST(suite, one_test)
METATEST_ADD_TEST(suite, four_test)

When run, this yields the following summary.

The following tests have been executed:
  my_suite::my_reverse::empty_test: FAIL (test.cpp:31)
        equal<mpl::vector<>, mpl::vector<char>, is_same<_, _>>
  my_suite::my_reverse::one_test: FAIL (test.cpp:32)
        equal<mpl::vector<char>, mpl::vector<>, is_same<_, _>>
  my_suite::my_reverse::four_test: FAIL (test.cpp:33)
        equal<mpl::vector<long, int, short, char>, mpl::vector<char, short, int, long>, is_same<_, _>>
Number of tests: 3
Number of failures: 3

Besides being concise, this output does not depend on the compiler. The format is standard, but also customizable.

This example can be found in the example/reverse directory of the library.

Customizing test reports

The framework offers facilities for customization in two ways - first, report formats can be customized, and second, custom types can be registered for pretty printing.

Customizing reports

Test results are collected by a test_driver singleton object. This object provides means to access the root of the test suite hierarchy from which test suites, and individual test results can be acquired. This whole system comprises run-time structures that provide iterators for traversal.

A test_suite object represents one node of the test suite hierarchy. It contains zero or more nested test_suite objects and zero or more test_result objects, representing test results.

A test_result object represents the result of one compile-time test. It either succeeds or fails, and in case it fails, the test_result object will provide a reason in the form of an std::string.

A location object represents a location in the source code. It can used to display in the report, where a specific test case comes from.

As an example, test results in the first tier of the test suite hierarchy can be printed as follows:

int main()
  using std::cout;
  const test_suite &results = test_driver::instance().suite().results();

  for (test_suite::result_list::const_iterator it =
        it != results.end();
    if (it->success())
      cout << "OK";
      cout << "FAIL (" << it->get_location() << ")";
      if (it->has_reason())
        cout << "\n\t" << it->get_reason();

Pretty printing custom types

For pretty printing types, the framework provides a metafunction, to_stream, which, as a class, provides a static run method taking an output stream as its parameter and pretty printing the type to the stream:

template <class T>
struct to_stream
  // unspecified

  static std::ostream& run(std::ostream&);

To use to_stream with a custom type, one has to specialise this template directly and make use of pattern matching through template specialisation. We demonstrate the specialisation for for user-defined types in the following example.

struct UDT {};

struct to_stream<UDT>
  typedef to_stream type;

  static std::ostream& run(std::ostream& os)
  { return os << "UDT"; }

The to_stream metafunction is defined in the following header:

#include <metatest/to_stream.hpp>

This header defines to_stream and adds pretty printing to metafunctions, metafunction classes and data types of Boost mpl. Unfortunately, in order to do this, it has to include every public header of mpl. This feature can be disabled (and re-enabled when needed) by defining the METATEST_NO_TO_STREAM_MPL macro before including the to_stream.hpp header.

To include the declaration of to_stream only, the following header is provided:

#include <metatest/to_stream_fwd.hpp>

A number of convenience utilities are provided for pretty printing. After including to_stream_fwd.hpp the following macros can be used to define specializations of to_stream for classes and template instances:

These macros expect that they are not used in any namespace.

When pretty-printing compile-time data-types and metafunctions, a recurring task is pretty-printing arguments of a templates recursively. A metafunction called to_stream_argument_list is provided for taking-care of pretty-printing template arguments. It uses to_stream to write <T1, ..., Tn> to the stream. Here is an example usage:

template <class A, class B>
struct plus
  // implementation of the plus metafunction
  struct to_stream
    // display "plus<A, B>"
    static std::ostream& run(std::ostream& o)
      o << "plus<";
      to_stream_argument_list<A, B>::run(o);
      return o << ">";

This code displays the name of the metafunction and the <, > characters and uses to_stream_argument_list to display the arguments.

Pre-defined reporting tools

The library provides the following report generating functions:

bool plaintext_report(std::ostream &os) generates a plain-text report.
bool xml_report(std::ostream &os) generates an xml report.
bool html_report(std::ostream &os) generates an html/javascript report.

The library provides different headers that can be included in a compilation unit to define a main function doing some sort of pretty printing. Using them, the contents of the main.cpp can be one include. The following headers are available:

<metatest/main.hpp> calls plaintext_report
<metatest/main_html.hpp> calls html_report
<metatest/main_boost.hpp> calls build_metatest_test_suite

Integration with other unit testing libraries

Compile-time assertions can be turned into runtime ones by passing the result of the compile-time assertion to a runtime testing framework as a constant value. Metatest provides the meta_warn, meta_check and meta_require utilities for passing the result of compile-time assertions to the Boost.Test framework. In case of failures, the error report provided by Boost.Test contains the pretty-printed expression as well.

Best practices



A number of existing unit tests for Boost.MPL ported to use metatest instead of static assertions. The output was generated using the HTML output generator capability of metatest.

source code


Demonstarting the use of meta_check, meta_warn and meta_require. The output was generated by Boost.Test. Using these assertion functions, one can write Boost.Test test cases checking both the compile-time and runtime parts of the code.

source code


This example shows how to use the plain text reporter provided by metatest. It prints the report as plain text to the standard output. This is ideal for integrating with a command-line build system, such as Make.

source code


This example contains macros making it possible to write metatest test cases without using an extra typedef per test case. Such macros were not added to metatest, because when the test cases contain syntax errors, the compiler can give better error messages when no macros are used. This example can be used to experiment with the interface and compare error messages in both cases. The output contains no errors.

source code


This example demonstrates the difference between the error report using different testing tools. The same tests (containing failures) can be run using metatest and another solution based on static assertions.

source code


Example demonstrating how the type pretty-printing utility, to_stream can be used.

source code

Ábel Sinkovics, Endre Sajó, Zoltán Porkoláb:
Towards more reliable C++ template metaprograms,
In Jaan Penjam: 12th Symposium on Programming Languages and Software Tools (SPLST'11), Tallinn, Estonia, 5-7 October, 2011., pp. 260-271.

Ábel Sinkovics:
Unit testing of C++ template metaprograms,
In Proceedings of the 8th Joint Conference on Matematics and Computer Science, MaCS 2010, Selected papers, pp. 375-384


Test case registration tools

Classes building up the test suite hierarchy at runtime

Type pretty-printing tools

Report generators provided by the library:

Integration with Boost.Test:

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