Getting started

This section presents how to use Metashell to do simple debugging.

Trying Metashell online

You can try Metashell in your browser without installing anything. You can find an online demo here.

Evaluating simple expressions

Let's look at how to evaluate the expression 6 + 7. If you have access to python, ghci or erl, start it and type 6 + 7 (6 + 7. in Erlang) and press enter. You will see 13, the result of this expression immediately.

Let's do the same in template metaprogramming. Start metashell. In template metaprogramming the integer numbers are represented by instances of the int_ template class - get it by running the following command:

> #include <boost/mpl/int.hpp>

Addition is implemented by the plus metafunction - get it by running the following command:

> #include <boost/mpl/plus.hpp>

Note that you can use the arrow keys to re-run earlier commands or slightly updated versions of them. The following command is not necessary, but makes life easier:

> using namespace boost::mpl;

Once you have set your environment up, you can evaluate the expression 6 + 7 by running the following:

> plus<int_<6>, int_<7>>::type

You get mpl_::integral_c<int, 13>, which is the representation of the value 13.

How about Fibonacci?

You have seen how to do simple arithmetic using Boost.MPL's integral types but you haven't been able to run the classical Fibonacci example of template metaprogramming. Let's write it first:

> template <int N> struct fib { static constexpr int value = fib<N - 1>::value + fib<N - 2>::value; };
> template <> struct fib<0> { static constexpr int value = 1; };
> template <> struct fib<1> { static constexpr int value = 1; };

Note that Metashell supports multiline definitions as well. You can enter the above in the shell the following way as well:

> template <int N> \
...> struct fib \
...> { \
...>   static constexpr int value = fib<N - 1>::value + fib<N - 2>::value; \
...> };
> template <> \
...> struct fib<0> \
...> { \
...>   static constexpr int value = 1; \
...> };
> template <> \
...> struct fib<1> \
...> { \
...>   static constexpr int value = 1; \
...> };

What is important is that the last character of a line has to be \. That is how Metashell knows that there is more to come.

If you type the above commands into the shell you define the fib template class and its necessary specialisations. If you try to evaluate it, you get an error:

> fib<6>::value
<stdin>:6:26: error: template argument for template type parameter must be a type

The problem is that Metashell assumes, that the result of a template metaprogram is a type. And fib<6>::value is not a type. It is an integer value. You need some boxing solution, such as boost::mpl::int_ to turn it into a type:

> #include <boost/mpl/int.hpp>
> boost::mpl::int_<fib<6>::value>

In this case we knew that the result is an int value. For other values other boxing classes have to be used. Keeping it all in mind is not convenient. Metashell offers a tool for working with scalar results in the shell. Run the following command to get it:

> #include <metashell/scalar.hpp>

The above header defines the SCALAR macro which can be used to work with expressions evaluating to scalar results. For example:

> SCALAR(fib<6>::value)
std::integral_constant<int, 13>

The SCALAR macro instantiates std::integral_constant with the right arguments.

Using the Metadebugger

Metadebugger lets you inspect step by step how the compiler runs your metaprograms.

Let's debug the fibonacci metaprogram seen earlier. Start the metadebugger by entering:

> #msh mdb boost::mpl::int_<fib<6>::value>
For help, type "help".
Metaprogram started

You'll see, that the prompt has changed to (mdb). Now you can enter metadebugger commands.


Metadebugger provides an interface similar to gdb. For example you can step the metaprogram forward three steps:

(mdb) step 3
fib<4> (TemplateInstantiation)

As you can see, metadebugger tells you that in this step fib<4> is getting instantiated in a TemplateInstantiation event.

Stepping backwards is also trivial in a template metaprogram:

(mdb) step -1
fib<5> (TemplateInstantiation)


You can check the current backtrace:

(mdb) bt
#0 fib<5> (TemplateInstantiation)
#1 fib<6> (TemplateInstantiation)
#2 int_<fib<6>::value>

This shows us that:

  • We started the template metaprogram execution by evaluating int_<fib<6>::value>.
  • The evaluation of this expression has (at some point) called fib<6>.
  • The fib metafunction has (at some point) called fib<5>. This is where we are in the execution of the metaprogram.


Metadebugger can also see into the future, and print the forwardtrace from any step:

(mdb) ft
fib<5> (TemplateInstantiation)
+ fib<4> (TemplateInstantiation)
| + fib<3> (TemplateInstantiation)
| | + fib<2> (TemplateInstantiation)
| | | + fib<1> (Memoization)
| | | ` fib<0> (Memoization)
| | ` fib<1> (Memoization)
| ` fib<2> (Memoization)
` fib<3> (Memoization)

This shows us what metafunctions the metaprogram will call after the current location. As you can see the output shows the relations between the function calls: which metafunction calls which other metafunctions. The events in the output of forwardtrace happen in that order from the top down.

You probably noticed that there are two kinds of events metadebugger shows you:

  • TemplateInstantiation event happens when the compiler first encounters and instantiates a new template type. During a TemplateInstantiation event the compiler will instantiate every subtype it needs to get to the result.
  • Memoization event happens when a compiler encounters a type, which it had already instantiated before. It won't go through the instantiation process again, instead it uses technique called memoization to speed up the compilation. This basically means that the compiler remembers every type it had instantiated, and reuses them when it encounters them again.

Full template specializations (e.g. fib<0> and fib<1>) also appear as Memoization events.

For example, in the above forwardtrace output, you can see that fib<5> creates fib<4> in a TemplateInstantiation event, which in turn instantiates fib<3> also in a TemplateInstantiation event and so on. You can also see, that when fib<5> gets to the point to instantiate fib<3> it has already been instantiated by fib<4>, so only a Memoization event happens.

There are a few more type of template events that we haven't seen so far:

  • ExplicitTemplateArgumentSubstitution: event happens when the compiler substitutes explicit template arguments provided for a function template.
  • DeducedTemplateArgumentSubstitution: event happens when the compiler substitutes template arguments determined as part of template argument deduction for either a class template partial specialization or a function template.

Check out the section about How to debug expressions involving SFINAE? for more information about these event types.

Breakpoints and continue

You can also create breakpoints:

(mdb) rbreak fib<3>
Breakpoint "fib<3>" will stop the execution on 3 locations

Now let's continue the execution until the first breakpoint:

(mdb) continue
Breakpoint 1: regex("fib<3>") reached
fib<3> (TemplateInstantiation)

Commands can be abbreviated if unambiguous. For example you can use just c instead of continue:

(mdb) c
Breakpoint 1: regex("fib<3>") reached
fib<3> (Memoization)

You can repeat the last command by simply hitting enter again and again:

Breakpoint 1: regex("fib<3>") reached
fib<3> (Memoization)
Metaprogram finished

Evaluating failing metaprograms

Sometimes you want to debug metaprograms which don't compile for some reason. Metadebugger allows this and will behave similarly to how runtime debuggers stop the execution when some runtime failure is detected (like a Segmentation Fault). To demonstrate this, consider the following faulty version of the fibonacci metaprogram:

> template <int N> struct fib { static constexpr int value = fib<N - 1>::value + fib<N - 2>::value; };
> template <> struct fib<0> { static constexpr int value = 1; };
> template <> struct fib<1> {};
> template <int N> struct int_;

Let's try to evaluate int_<fib<5>::value>:

> int_<fib<5>::value>

You'll see a bunch of error messages appear. Let's debug out metaprogram to see what's going on:

> #msh mdb int_<fib<5>::value>
For help, type "help".
Metaprogram started

Continue the execution until we hit the error:

> continue
Metaprogram finished

You should see the same error messages as above, but this time we can actually check out where the error happened:

(mdb) bt
#0 fib<1> (Memoization)
#1 fib<2> (TemplateInstantiation)
#2 fib<3> (TemplateInstantiation)
#3 fib<4> (TemplateInstantiation)
#4 fib<5> (TemplateInstantiation)
#5 int_<fib<5>::value>

Since the top frame (#0) is the Memoization of fib<1> we can suspect that there is something wrong with that. Indeed, there is no value member defined in the fib<1> specialization.

Reevaluation and precompiled headers

Metadebugger never uses precompiled headers. This is because instantiation events which happened in the precompiled code, doesn't appear again, when a metaprogram is compiled using precompiled headers. This is expected, since this is one of the reasons PCH can speed up compilation.

This might seem like bad news, but it has a very useful side effect. If you included files while you set up the compilation environment, you can actually modify those files on the fly without restarting Metadebugger and using #msh environment reload. The modified file will be included and reparsed every time you evaluate a metaprogram.

This technique can be used outside Metadebugger, but you might have to explicitly turn off precompiled headers. See this section for more information: What happens to files included to the environment?

To reevaulate the last metaprogram, you can simply enter evaluate (or e for short) without giving any expression as an argument:

(mdb) e
Metaprogram started


Metadebugger can be used to profile template metaprograms. To enable profiling call evaluate or #msh mdb with the -profile flag:

(mdb) evaluate -profile int_<fib<4>::value>
Metaprogram started
(mdb) ft
+ [0.41ms, 76.89%] fib<4> (TemplateInstantiation)
| + [0.14ms, 26.70%] fib<2> (TemplateInstantiation)
| | + [0.01ms, 2.44%] fib<1> (Memoization)
| | ` [0.01ms, 2.06%] fib<0> (Memoization)
| ` [0.06ms, 11.84%] fib<3> (TemplateInstantiation)
|   + [0.01ms, 1.12%] fib<1> (Memoization)
|   ` [0.01ms, 0.94%] fib<2> (Memoization)
+ [0.05ms, 8.84%] int_<5> (TemplateInstantiation)
` [0.01ms, 0.94%] fib<4> (Memoization)

Some extra information is printed every time a frame is printed by any command. You can see the absolute time taken by an instantiation and all of it's sub instantiations as well as an approximate ratio of the time taken by an instantiation and the full template instantiation process as a percentage. The sum of the percentages on the top level will never add up to 100% since the compiler does other things which are not shown in the trace.

One more improtant difference in profile mode, is that the instantiations and sub instantiations are not ordered by the order which they were actually instantiated by the compiler, but the time they took the longest one being the first. This means that you can usually find the bottleneck just by looking at the first few steps of the metaprogram.

Profiling a translation unit

When trying to find out why a specific translation unit takes a long time to compile, you may want to see the trace for the full file, not just a single expression. You can do this with Metadebugger as well:

> #include "file_which_takes_a_long_time_to_compile.cpp"
> #msh mdb -profile -
Metaprogram started

The - means, that the final trace you can traverse is not constrained to a single expression, but to whole translation unit. From this point, you can use the usual commands to traverse and inspect the instantiation tree.

Full mode

There are three modes which Metadebugger can operate in. The normal mode and profiling mode which was shown in the previous chapters, and the full mode. To demonstrate the difference let's evaluate a metaprogram in full mode and print the forwardtrace:

(mdb) evaluate -full int_<fib<4>::value>
Metaprogram started
(mdb) ft
+ fib<4>
| + fib<3>
| | + fib<2>
| | | + fib<1>
| | | ` fib<0>
| | ` fib<1>
| ` fib<2>
|   + fib<1>
|   ` fib<0>
` mpl_::int_<5>

Full mode doesn't try to follow what the real compiler does, but instead it tries to simulate a dumb compiler, which doesn't use memoizations to speed up compilation. For example, fib<2> and its full sub call tree is visible two times. This mode can be useful, when the part of the trace you're interested in is hidden under multiple layers of Memoizations in normal mode.

Please note, that traces in full mode can get extremely long.

Using step over

A special qualifier, over can be passed to the step command. Using step over will jump over sub instantiations. Let's take a look at an example:

(mdb) evaluate int_<fib<5>::value>
Metaprogram started
(mdb) ft
+ fib<5> (TemplateInstantiation)
| + fib<4> (TemplateInstantiation)
| | + fib<3> (TemplateInstantiation)     A
| | | + fib<2> (TemplateInstantiation)
| | | | + fib<1> (Memoization)
| | | | ` fib<0> (Memoization)
| | | ` fib<1> (Memoization)             B
| | ` fib<2> (Memoization)               C
| ` fib<3> (Memoization)
+ fib<5> (Memoization)
` int_<5> (TemplateInstantiation)

Three instantiations are marked, so it is easier to talk about them. First, let's simply step forward to A:

(mdb) step 3
fib<3> (TemplateInstantiation)       A

Here, step over will jump over the types instantiated by A to C:

(mdb) step over
fib<2> (Memoization)       C

Of course, step over -1 will do the reverse and bring us back to A:

(mdb) step over -1
fib<3> (TemplateInstantiation)       A

Be careful though, step over -1 is not always the inverse of step over. Let's step forward to B to demonstrate this:

(mdb) step 4
fib<1> (Memoization)       B

Since there are no more sub instantiations to step over, from here step over has no other choice but to behave like a normal step command and jump out to C.

(mdb) step over
fib<2> (Memoization)       C

But from C, as we've seen earlier, step over -1 will bring us back to A and not B.

(mdb) step over -1
fib<3> (TemplateInstantiation)       A

Quit Metadebugger

To exit from metadebugger use Ctrl+D or the quit command.

(mdb) quit

Data structures of Boost.MPL

Let's play with the vector implementation Boost.MPL provides. It is defined in the following header:

#include <boost/mpl/vector.hpp>

By including it, we get the vector template class. Again, to make life easier let's run:

> using namespace boost::mpl;

Now let's create a vector:

> vector<int, double, char>

The shell's response is different than what we'd expect:

boost::mpl::vector<int, double, char, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mp
l_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mp
l_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>

vector uses default template arguments for simulating variadic templates and Metashell displays them as well. mpl_::na means there is no such element.

Let's add a new element to the vector. There is a push_front operator for this defined in the following header:

> #include <boost/mpl/push_front.hpp>

To add a new element (void in this example) to this vector, let's run the following command:

> push_front<vector<int, double, char>, void>::type

This calls the push_front metafunction to add void to the beginning of the vector. The result is something unexpected again:

boost::mpl::v_item<void, boost::mpl::vector<int, double, char, mpl_::na, mpl_::n
a, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::n
a, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, 1>

This is not an instance of the vector template class. Boost.MPL's vector implementation uses tag dispatching for identifying vector values.

Metashell provides a header file that can be used to display Boost.MPL's containers in a way we'd expect to see them. Let's include this header:

> #include <metashell/formatter.hpp>

This header defines the formatting rules for the Boost.MPL containers. Let's try running the push_front operation again:

> push_front<vector<int, double, char>, void>::type
boost_::mpl::vector<void, int, double, char>

This time the result is what we'd expect. Note that it is in the boost_ namespace instead of boost. The vector template class in boost_ is a helper template class used for pretty-printing.

The header metashell/formatter.hpp loads all formatters that come with Metashell which may be more than what you need. Among other dependencies the formatters include the data structures they format, which may slow the shell down. To get only the formatters (and related data structures) you really need, you can include them one by one. For example to get the formatter of mpl::vector you need to include metashell/formatter/vector.hpp.

Writing custom formatters

The previous section has presented how to make Metashell format the Boost.MPL containers properly. Metashell needs to be prepared for other libraries using tag dispatching as well. This can be done by using standard C++ code. Note that the content of mpl_formatter.hpp that did the magic for the Boost.MPL containers is also standard C++ code.

Metashell declares the following metafunction:

namespace metashell
  template <class T>
  struct format;

When Metashell evaluates a template metaprogramming expression, such as push_front<vector<int, double, char>, void>::type it passes the result of it to this metafunction. The result of this metafunction call is displayed. The default implementation of this metafunction returns its argument unchanged but it can be overridden. This is what mpl_formatter.hpp does.

Using specialisation

One way of overriding it is by specialising it. As an example override, let's make the shell display the type int as double:

> namespace metashell { template <> struct format<int> { typedef double type; }; }

Now let's try evaluating the expression int:

> int

The above specialisation of metashell::format made Metashell display double instead of int.

Using tag dispatching

Another way of overriding the formatter is using tag dispatching. The default implementation of metashell::format evaluates metashell::format_impl<T::tag>::apply<T>::type where T is format's argument. When T::tag is not defined, void is used instead.

Specialising metashell::format_impl for a new tag adds support for formatting values using that tag. These specialisations can be part of the implementation of a data-type to avoid Metashell users having to include extra headers for formatting.

Embedding custom formatters

Custom formatters can be part of a data type's implementation. Metashell defines the __METASHELL macro, thus the implementation of a data type (eg. a typelist) can detect if it is used inside Metashell. For example:

 * typelist.hpp

// The definition of the typelist
struct empty_list;

template <class Head, class Tail>
struct cons;

// Metashell formatter
#ifdef __METASHELL
namespace metashell
  template <>
  struct format<empty_list>
    // ...

  template <class Head, class Tail>
  struct format<cons<typename format<Head>::type, Tail>>
    // ...

This example defines the formatter only when the header file is used inside Metashell. This is useful for debugging and has no impact on the regular users of the header.

Note that it is a formatter for a container, which contains elements. Those elements might also need formatting, therefore it formats the elements as well by calling format recursively on each element (see format<Head>::type).

The version of the shell is also available for the headers. Metashell defines the __METASHELL_MAJOR, __METASHELL_MINOR and __METASHELL_PATCH macros.

Testing macros

Metashell can be used as an interactive shell for testing macros. The easiest way to get started with it is to run the following command in Metashell:

> #msh preprocessor mode

This updates the shell's settings to turn it into an ideal environment for testing macros. You can start Metashell in this mode by using the --preprocessor command line argument.

Evaluating simple expressions

Let's look at how to evaluate the expression 6 + 7. If you have access to python, ghci or erl, start it and type 6 + 7 (6 + 7. in Erlang) and press enter. You will see 13, the result of this expression immediately.

Let's do the same in preprocessor metaprogramming. Start Metashell and run #msh preprocessor mode to turn it into a preprocessor shell. Addition is implemented by the BOOST_PP_ADD macro - get it by running the following command:

> #include <boost/preprocessor/arithmetic/add.hpp>

You will see a number of empty lines and lines starting with # printed on your screen. The reason behind it is that Metashell displays the result of preprocessing the included file. The preprocessor directives are processed (and produce no output) but all the empty lines in the included file are displayed. The lines starting with # provide source file name and line information to any C++ compiler processing the precompiled code, so the compiler can provide useful source locations in the error messages.

Once you have set your environment up, you can evaluate the expression 6 + 7 by running the following:

> BOOST_PP_ADD(6, 7)

You get 13, which is the result of this addition. You can try adding other numbers as well.

Note that you can add arbitrarily large numbers using BOOST_PP_ADD. The arguments (and the result) have to be in the range 0 .. BOOST_PP_LIMIT_MAG where BOOST_PP_LIMIT_MAG is a macro. Let's find out its value. It is defined in the following header:

> #include <boost/preprocessor/config/limits.hpp>

The above command makes the BOOST_PP_LIMIT_MAG macro available, so it is easy to check its value:


Checking which macros are available

Preshell can be used to get the list of available macros. The following command lists the names of the macros:

> #msh macro names

The result of it is the list of macros defined at point where the pragma was used. The following command displays the definition of the macros as well:

> #msh macros

The output of this command can be long, since it displays all macros and their actual definition.