C++ *-inl.h files

This section explains the “what, why, and how” of Drake’s use of the *-inl.h pattern for templated C++ code. (The word inl here is an abbreviation for inline.)

Explicit template instantiation

Compiling templated code can take a long time, but in many cases only a small set of concrete types are ever instantiated. By moving template method bodies out of the header file and using explicit template instantiation, we can drastically speed up compile times.

(Note that implicit template instantiation is still allowed, and indeed encouraged when appropriate; this document only covers the rules for explicit instantiation.)

Drake uses two approaches for explicit template instantiation; each one is described immediately below. We prefer the first “traditional” approach when possible, because it is simpler. The second “*-.inl pattern” approach may be used when required.

Two-file, traditional approach

The traditional approach to explicit template instantiation uses only an *.h file and *.cc file. The declarations go in the *.h file, and their definitions and instantiations go in the *.cc file.

This two-file solution is straightforward and well-known, so if it meets your needs you should prefer it.

Three-file, -inl.h pattern approach

The case for *-inl.h files appears when we wish to provide the opportunity for calling code to use template types other than the ones instantiated in our *.cc file. The problem with the traditional approach is that method bodies only exist in the *.cc file, so there is no mechanism for calling code to instantiate them with different types.

The *-inl.h pattern works around this problem by placing the method bodies in a third file named *-inl.h. Code that only uses standard types should only include the *.h file. Code that requires a non-standard type should instead include only the *-inl.h file.

Example of the *-inl.h pattern

Here is a minimal example.

my_class.h

The header file declares and documents the class:

#pragma once

/// MyClass ... (write an overview) ...
///
/// @tparam T must be ... (document the requirements) ...
///
/// This class uses Drake's `-inl.h` pattern.  When seeing linker errors from
/// this class, please refer to https://drake.mit.edu/cxx_inl.html.
///
/// Instantiated templates for the following kinds of T's are provided:
/// - double
///
/// They are already available to link against in the containing library.
template <typename T>
class MyClass {
 public:
  T get_random_number() const;
};

my_class-inl.h

The inl file defines the templated methods:

#pragma once

/// @file
/// Template method implementations for my_class.h.
/// Most users should only include that file, not this one.
/// For background, see https://drake.mit.edu/cxx_inl.html.

#include "my_class.h"

template <typename T>
T MyClass::get_random_number() const {
  return static_cast<T>(4.0);  // chosen by fair dice roll
}

my_class.cc

The implementation file explicitly instantiates the templates (it instructs the compiler to emit object code for some pre-defined types):

#include "my_class-inl.h"

template class MyClass<double>;

main.cc

Calling code uses the *.h header, not the *-inl.h header:

#include "my_class.h"

int main() {
  MyClass<double> dice;
  std::cerr << dice.get_random_number() << std::endl;
}

This works for all of the supported types listed in the my_class.h header.

non-standard_main.cc

Unusual calling code uses the *-inl.h header, not the *.h header:

#include "symbolic.h"
#include "my_class-inl.h"

int main() {
  MyClass<Expression> dice;
  std::cerr << dice.get_random_number() << std::endl;
}

This is used for types not already listed in the my_class.h header. This performs implicit instantiation (so is often slower to compile). Whenever possible, prefer to add more explicitly-supported types to MyClass’s API for everyone to use, rather than use this implicit form.

Rules

  1. The file names must end with .h and -inl.h and .cc.
  2. The comments in each of the two header files must cite this page.
  3. The class must document which types are pre-instantiated.