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.

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. Here is a list of checks that we disabled:

Namespace should be terminated with "namespace" [readability/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 [whitespace/blank_line]
Missing space before { [whitespace/braces]
Found C system header after C++ system header. Should be: environment.h, c system, c++ system, other. [build/include_order] [4]
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. [whitespace/labels]
You don't need a ; after a } [readability/braces]
No #ifndef header guard found
Streams are highly discouraged.
Extra space before ( in function call [whitespace/parens]
Else clause should never be on same line as else [whitespace/newline]
Extra space before ) [whitespace/parens]
Is this a non-const reference? If so, make const or use a pointer. [runtime/references]

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