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Coding Style

C++11 features

We make extensive use of new features in the C++ 11 standard. Developers must be familiar with the standard to be able to understand the code. Here are some of the features that are extensively used.

  • Type inference (aka auto keyword).
  • Initializer lists.
  • Lambda functions and expressions.
  • nullptr constant.
  • Strongly typed enumerations.
  • Right angle bracket.
  • Thread local storage.
  • Threading facilities.
  • Tuple types.

Comments

The comments in the Lean codebase contain Doxygen commands. Doxygen is the de facto standard tool for generating documentation from annotated C++ sources.

Namespaces

All code is in the lean namespace. Each frontend is stored in a separate nested namespace. For example, the SMT 2.0 frontend is stored in the lean::smt namespace.

Exception: some debugging functions are stored outside of the lean namespace. These functions are called print and are meant to be used when debugging Lean using gdb.

Smart pointers

We only use std::shared_ptr template for class C only if we expect to create only a few objects (< 1000) of class C. Otherwise, we implement our own intrusive smart pointer. For example, the class expr is an intrusive smart pointer to expr_cell. We may have millions of expr objects. We say it is intrusive because the reference counter is stored in expr_cell.

We use std::unique_ptr to make sure unique resources will be freed correctly.

Template

We organize template source code using the approach described at http://www.codeproject.com/Articles/3515/How-To-Organize-Template-Source-Code

Idioms

We use some popular C++ idioms:

  • Pimpl
  • RAII Resource Acquisition Is Initialization

Formatting

  • We use 4 spaces for indentation.

  • Class, method, and function names are lower case We use _ for composite names. Example: type_checker.

  • Class/struct fields should start with the prefix m_.

Example:

 class point {
     int m_x;
     int m_y;
 public:
     ...
 };

  • We do not use the #ifndef-#define-#endif idiom for header files. Instead we use #pragma once.

  • We write type const & v instead of const type & v.

  • We use const extensively.

  • if-then-else

The following forms are acceptable:

 if (cond) {
     ...
 } else {
     ...
 }

and

 if (cond)
    statement1;
 else
    statement2;

In exceptional cases, we also use

 if (cond) statement;

and

 if (cond) statement1; else stament2;
  • if-then-else-if-else

The following forms are acceptable:

 if (cond) {
     ...
 } else if (cond) {
     ...
 } else {
     ...
 }

and

 if (cond)
     statement1;
 else if (cond)
     statement2;
 else
     statement3;
  • We frequently format code using extra spaces

For example, we write

environment const & m_env;
cache               m_cache;
normalizer          m_normalizer;
volatile bool       m_interrupted;

instead of

environment const & m_env;
cache m_cache;
normalizer m_normalizer;
volatile bool m_interrupted;

  • We use the macro lean_assert for assertions. The macro lean_assert is extensively used when writing unit tests.

  • Spaces in expressions We write a == b instead of a==b. Similarly, we write x < y + 1 instead of x<y+1.

Google's C++ Style Guide

We are using a modified version of Google's C++ Style Guide. We also have our version of Google's style checker cpplint.py. You can run the checker over the codebase by typing:

make style

If you use Ninja, you can check by ninja style. It is also a part of testcases and can be run by

ctest -R style_check

Disabled Features:

  • Namespace should be terminated with "namespace"
  • At least two spaces is best between code and comments
  • Do not use dynamic_cast<>. If you need to cast within a class hierarchy, use static_cast<> to upcast. Google doesn't support RTTI.
  • "public:" should be preceded by a blank line
  • Missing space before {
  • Found C system header after C++ system header. Should be: environment.h, c system, c++ system, other.
  • Labels should always be indented at least one space. If this is a member-initializer list in a constructor or the base class list in a class definition, the colon should be on the following line.
  • You don't need a ; after a }
  • No #ifndef header guard found
  • Streams are highly discouraged.
  • Extra space before ( in function call
  • Else clause should never be on same line as else
  • Extra space before )
  • Is this a non-const reference? If so, make const or use a pointer.
  • All parameters should be named in a function

Modified Features:

  • Add #include <list> for list<>

    => Check std::list instead of list because we do have our own lean::list type.

  • Add #include <algorithm> for copy

    => Check std::copy instead of copy because we do have our own lean::copy method.

  • Do not use namespace using-directives. Use using-declarations instead.

    => Allow this if filename contains "tests/"

  • Small and focused functions are preferred: foo() has xxx non-comment lines (error triggered by exceeding 500 lines).

    => Allow this if filename contains "tests/"

  • Include the directory when naming .h files

    => Allow this if the included filename is "version.h" which is generated by cmake.

Git pre-push hook

Since git 1.8.2, git introduced pre-push hook which is executed before actual push operation is performed. Using this, we can automatically run the style checker over the changed files before we push commits to repositories. This is useful because it prevents us from accidentally pushing the commits which contain style problems.

  • Create <PROJECT_ROOT>/.git/hooks/pre-push file with the following contents:
#!/usr/bin/env bash
IFS=' '
DIR="$( cd "$( dirname "$0" )" && pwd )"
CHECKER=$DIR/../../src/cmake/Modules/cpplint.py
while read local_ref local_sha remote_ref remote_sha;
do
    CHANGED_FILES=`git diff --name-only $local_sha $remote_sha | grep '\(cpp\|h\)$'`
    if [ ! -z "$CHANGED_FILES" -a "$CHANGED_FILES" != " " ]; then
        echo $CHANGED_FILES | xargs $CHECKER
        RET=$?
        if [ $RET -ne 0 ]; then
            echo "There is error(s) from style-check. Please fix them before push to the repo."
            exit $RET
        fi
    fi
done
exit 0
  • Give "exec" permission to the file
chmod +x <PROJECT_ROOT>/.git/hooks/pre-push

Note that you need to change CHECKER variable if you want to use other checkers.