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The Kaldi coding style

When starting to code the final version of the Kaldi toolkit, we had decided to use OpenFst as a C++ library.

For consistency with OpenFst, we decided to use the same coding style in most respects.

Many aspects of the Kaldi coding style will be obvious from viewing the code. Key points include:

  • Rules on naming of tokens, e.g. MyTypeName, MyFunction, my_class_member_var_, my_struct_member, KALDI_MY_DEFINE, kGlobalConstant, kEnumMember, g_my_global_variable, my_namespace, my-file.h, my-file.cc, my-implementation-inl.h
  • Rules governing function arguments: no non-const references; inputs precede outputs.
  • Rules governing whitespace and formatting: 80 characters per line max (except where necessary), open-brace on same line as function; see code for other whitespace conventions.
  • I/O: we use C++ style I/O, with specific conventions on I/O routines for objects (see Kaldi I/O mechanisms).
  • Function arguments: we don't allow reference arguments to be non-const (use pointers), with an exception for iostreams. Inputs must precede outputs in a function parameter list.
  • Error status is mostly indicated by exceptions (see Kaldi logging and error-reporting for specific mechanisms).
  • For "normal" integers, we try to use int32. This is because Kaldi's binary I/O mechanisms (Kaldi I/O mechanisms) are easiest to use when the binary size of integer types is known.
  • For "normal" floating-point values, we use BaseFloat which is a typedef (if you compile with KALDI_DOUBLEPRECISION=1 it's double, otherwise float). This makes it easier to test algorithms in double precision and check for differences. However, we always use double for accumulators.
  • We prepend all our #defines with KALDI_, to avoid possible future conflicts with other codebases (since #defines are not protected by namespaces). All the kaldi code is in namespace kaldi, except for OpenFst extensions which are in namespace fst.
  • Class constructors taking one argument must use the "explicit" keyword (this prevents unwanted type conversions).
  • We generally avoid copy constructors and assignment operators (there is a KALDI_DISALLOW_COPY_AND_ASSIGN that disables the "default" ones that C++ provides).
  • We avoid operator overloading, except where required by STL algorithms.
  • We generally avoid function overloading, preferring distinct names.
  • We use C++-style casts like static_cast<int>, rather than C-style ones like (int)
  • We use const wherever possible.

Exceptions to the Google C++ style guide include:

  • We make use of iostreams, and we allow non-const references to iostreams to be passed to functions (this violates the no-non-const-references rule).
  • For get/set methods, suppose the class member is called x_, the Google-style get and set methods would be x() and set_x(). However, following the OpenFst coding style we call them X() and SetX(): for example, Mean() and SetMean(). This particular rule is new; in the past we had an inconsistent approach, and we will be changing the code to conform.