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Flang C++ Style Guide

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This document captures the style guide rules that are followed in the Flang codebase.

In brief:

Use clang-format from llvm 7 on all C++ source and header files before every merge to main. All code layout should be determined by means of clang-format.
Where a clear precedent exists in the project, follow it.
Otherwise, where LLVM's C++ style guide is clear on usage, follow it.
Otherwise, where a good public C++ style guide is relevant and clear, follow it. Google's is pretty good and comes with lots of justifications for its rules.
Reasonable exceptions to these guidelines can be made.
Be aware of some workarounds for known issues in older C++ compilers that should still be able to compile f18. They are listed at the end of this document.

In particular:

Use serial commas in comments, error messages, and documentation unless they introduce ambiguity.

Error messages

Messages should be a single sentence with few exceptions.
Fortran keywords should appear in upper case.
Names from the program appear in single quotes.
Messages should start with a capital letter.
Messages should not end with a period.

Files

File names should use dashes, not underscores. C++ sources have the extension ".cpp", not ".C" or ".cc" or ".cxx". Don't create needless source directory hierarchies.
Header files should be idempotent. Use the usual technique:

#ifndef FORTRAN_header_H_
#define FORTRAN_header_H_
// code
#endif  // FORTRAN_header_H_

#include every header defining an entity that your project header or source file actually uses directly. (Exception: when foo.cpp starts, as it should, with #include "foo.h", and foo.h includes bar.h in order to define the interface to the module foo, you don't have to redundantly #include "bar.h" in foo.cpp.)
In the source file "foo.cpp", put its corresponding #include "foo.h" first in the sequence of inclusions. Then #include other project headers in alphabetic order; then C++ standard headers, also alphabetically; then C and system headers.
Don't use #include <iostream>. If you need it for temporary debugging, remove the inclusion before committing.

Naming

C++ names that correspond to well-known interfaces from the STL, LLVM, and Fortran standard can and should look like their models when the reader can safely assume that they mean the same thing -- e.g., clear() and size() member functions in a class that implements an STL-ish container. Fortran intrinsic function names are conventionally in ALL CAPS.
Non-public data members should be named with leading miniscule (lower-case) letters, internal camelCase capitalization, and a trailing underscore, e.g. DoubleEntryBookkeepingSystem myLedger_;. POD structures with only public data members shouldn't use trailing underscores, since they don't have class functions from which data members need to be distinguishable.
Accessor member functions are named with the non-public data member's name, less the trailing underscore. Mutator member functions are named set_... and should return *this. Don't define accessors or mutators needlessly.
Other class functions should be named with leading capital letters, CamelCase, and no underscores, and, like all functions, should be based on imperative verbs, e.g. HaltAndCatchFire().
It is fine to use short names for local variables with limited scopes, especially when you can declare them directly in a for()/while()/if() condition. Otherwise, prefer complete English words to abbreviations when creating names.

Commentary

Use // for all comments except for short /*notes*/ within expressions.
When // follows code on a line, precede it with two spaces.
Comments should matter. Assume that the reader knows current C++ at least as well as you do and avoid distracting her by calling out usage of new features in comments.

Layout

Always run clang-format on your changes before committing code. LLVM has a git-clang-format script to facilitate running clang-format only on the lines that have changed.

Here's what you can expect to see clang-format do:

Indent with two spaces.
Don't indent public:, protected:, and private: accessibility labels.
Never use more than 80 characters per source line.
Don't use tabs.
Don't indent the bodies of namespaces, even when nested.
Function result types go on the same line as the function and argument names.

Don't try to make columns of variable names or comments align vertically -- they are maintenance problems.

Always wrap the bodies of if(), else, while(), for(), do, &c. with braces, even when the body is a single statement or empty. Note that this diverges from the LLVM coding style. In parts of the codebase that make heavy use of LLVM or MLIR APIs (e.g. the Lower and Optimizer libraries), use the LLVM style instead. The opening { goes on the end of the line, not on the next line. Functions also put the opening { after the formal arguments or new-style result type, not on the next line. Use {} for empty inline constructors and destructors in classes.

If any branch of an if/else if/else cascade ends with a return statement, they all should, with the understanding that the cases are all unexceptional. When testing for an error case that should cause an early return, do so with an if that doesn't have a following else.

Don't waste space on the screen with needless blank lines or elaborate block commentary (lines of dashes, boxes of asterisks, &c.). Write code so as to be easily read and understood with a minimum of scrolling.

Avoid using assignments in controlling expressions of if() &c., even with the idiom of wrapping them with extra parentheses.

In multi-element initializer lists (especially common::visitors{...}), including a comma after the last element often causes clang-format to do a better jobs of formatting.

C++ language

Use C++17, unless some compiler to which we must be portable lacks a feature you are considering. However:

Never throw or catch exceptions.
Never use run-time type information or dynamic_cast<>.
Never declare static data that executes a constructor. (This is why #include <iostream> is contraindicated.)
Use {braced initializers} in all circumstances where they work, including default data member initialization. They inhibit implicit truncation. Don't use = expr initialization just to effect implicit truncation; prefer an explicit static_cast<>. With C++17, braced initializers work fine with auto too. Sometimes, however, there are better alternatives to empty braces; e.g., prefer return std::nullopt; to return {}; to make it more clear that the function's result type is a std::optional<>.
Avoid unsigned types apart from size_t, which must be used with care. When int just obviously works, just use int. When you need something bigger than int, use std::int64_t rather than long or long long.
Use namespaces to avoid conflicts with client code. Use one top-level Fortran project namespace. Don't introduce needless nested namespaces within the project when names don't conflict or better solutions exist. Never use using namespace ...; outside test code; never use using namespace std; anywhere. Access STL entities with names like std::unique_ptr<>, without a leading ::.
Prefer static functions over functions in anonymous namespaces in source files.
Use auto judiciously. When the type of a local variable is known, monomorphic, and easy to type, be explicit rather than using auto. Don't use auto functions unless the type of the result of an outlined member function definition can be more clear due to its use of types declared in the class.
Use move semantics and smart pointers to make dynamic memory ownership clear. Consider reworking any code that uses malloc() or a (non-placement) operator new. See the section on Pointers below for some suggested options.
When defining argument types, use values when object semantics are not required and the value is small and copyable without allocation (e.g., int); use const or rvalue references for larger values (e.g., std::string); use const references to rather than pointers to immutable objects; and use non-const references for mutable objects, including "output" arguments when they can't be function results. Put such output arguments last (pace the standard C library conventions for memcpy() & al.).
Prefer typename to class in template argument declarations.
Prefer enum class to plain enum wherever enum class will work. We have an ENUM_CLASS macro that helps capture the names of constants.
Use constexpr and const generously.
When a switch() statement's labels do not cover all possible case values explicitly, it should contain either a default:; at its end or a default: label that obviously crashes; we have a CRASH_NO_CASE macro for such situations.
On the other hand, when a switch() statement really does cover all of the values of an enum class, please insert a call to the SWITCH_COVERS_ALL_CASES macro at the top of the block. This macro does the right thing for G++ and clang to ensure that no warning is emitted when the cases are indeed all covered.
When using std::optional values, avoid unprotected access to their content. This is usually by means of x.has_value() guarding execution of *x. This is implicit when they are function results assigned to local variables in if/while predicates. When no presence test is obviously protecting a *x reference to the contents, and it is assumed that the contents are present, validate that assumption by using x.value() instead.
We use c_str() rather than data() when converting a std::string to a const char * when the result is expected to be NUL-terminated.
Avoid explicit comparisions of pointers to nullptr and tests of presence of optional<> values with .has_value() in the predicate expressions of control flow statements, but prefer them to implicit conversions to bool when initializing bool variables and arguments, and to the use of the idiom !!.

Classes

Define POD structures with struct.
Don't use this-> in (non-static) member functions, unless forced to do so in a template member function.
Define accessor and mutator member functions (implicitly) inline in the class, after constructors and assignments. Don't needlessly define (implicit) inline member functions in classes unless they really solve a performance problem.
Try to make class definitions in headers concise specifications of interfaces, at least to the extent that C++ allows.
When copy constructors and copy assignment are not necessary, and move constructors/assignment is present, don't declare them and they will be implicitly deleted. When neither copy nor move constructors or assignments should exist for a class, explicitly =delete all of them.
Make single-argument constructors (other than copy and move constructors) 'explicit' unless you really want to define an implicit conversion.

Pointers

There are many -- perhaps too many -- means of indirect addressing data in this project. Some of these are standard C++ language and library features, while others are local inventions in lib/Common:

Bare pointers (Foo *p): these are obviously nullable, non-owning, undefined when uninitialized, shallowly copyable, reassignable, and often not the right abstraction to use in this project. But they can be the right choice to represent an optional non-owning reference, as in a function result. Use the DEREF() macro to convert a pointer to a reference that isn't already protected by an explicit test for null.
References (Foo &r, const Foo &r): non-nullable, not owning, shallowly copyable, and not reassignable. References are great for invisible indirection to objects whose lifetimes are broader than that of the reference. Take care when initializing a reference with another reference to ensure that a copy is not made because only one of the references is const; this is a pernicious C++ language pitfall!
Rvalue references (Foo &&r): These are non-nullable references with ownership, and they are ubiquitously used for formal arguments wherever appropriate.
std::reference_wrapper<>: non-nullable, not owning, shallowly copyable, and (unlike bare references) reassignable, so suitable for use in STL containers and for data members in classes that need to be copyable or assignable.
common::Reference<>: like std::reference_wrapper<>, but also supports move semantics, member access, and comparison for equality; suitable for use in std::variant<>.
std::unique_ptr<>: A nullable pointer with ownership, null by default, not copyable, reassignable. F18 has a helpful Deleter<> class template that makes unique_ptr<> easier to use with forward-referenced data types.
std::shared_ptr<>: A nullable pointer with shared ownership via reference counting, null by default, shallowly copyable, reassignable, and slow.
Indirection<>: A non-nullable pointer with ownership and optional deep copy semantics; reassignable. Often better than a reference (due to ownership) or std::unique_ptr<> (due to non-nullability and copyability). Can be wrapped in std::optional<> when nullability is required. Usable with forward-referenced data types with some use of extern template in headers and explicit template instantiation in source files.
CountedReference<>: A nullable pointer with shared ownership via reference counting, null by default, shallowly copyable, reassignable. Safe to use only when the data are private to just one thread of execution. Used sparingly in place of std::shared_ptr<> only when the overhead of that standard feature is prohibitive.

A feature matrix:

indirection	nullable	default null	owning	reassignable	copyable	undefined type ok?
`*p`	yes	no	no	yes	shallowly	yes
`&r`	no	n/a	no	no	shallowly	yes
`&&r`	no	n/a	yes	no	shallowly	yes
`reference_wrapper<>`	no	n/a	no	yes	shallowly	yes
`Reference<>`	no	n/a	no	yes	shallowly	yes
`unique_ptr<>`	yes	yes	yes	yes	no	yes, with work
`shared_ptr<>`	yes	yes	yes	yes	shallowly	no
`Indirection<>`	no	n/a	yes	yes	optionally deeply	yes, with work
`CountedReference<>`	yes	yes	yes	yes	shallowly	no

Overall design preferences

Don't use dynamic solutions to solve problems that can be solved at build time; don't solve build time problems by writing programs that produce source code when macros and templates suffice; don't write macros when templates suffice. Templates are statically typed, checked by the compiler, and are (or should be) visible to debuggers.

Exceptions to these guidelines

Reasonable exceptions will be allowed; these guidelines cannot anticipate all situations. For example, names that come from other sources might be more clear if their original spellings are preserved rather than mangled to conform needlessly to the conventions here, as Google's C++ style guide does in a way that leads to weirdly capitalized abbreviations in names like Http. Consistency is one of many aspects in the pursuit of clarity, but not an end in itself.

C++ compiler bug workarounds

Below is a list of workarounds for C++ compiler bugs met with f18 that, even if the bugs are fixed in latest C++ compiler versions, need to be applied so that all desired tool-chains can compile f18.

Explicitly move noncopyable local variable into optional results

The following code is legal C++ but fails to compile with the default Ubuntu 18.04 g++ compiler (7.4.0-1ubuntu1~18.0.4.1):

class CantBeCopied {
 public:
 CantBeCopied(const CantBeCopied&) = delete;
 CantBeCopied(CantBeCopied&&) = default;
 CantBeCopied() {}
};
std::optional<CantBeCopied> fooNOK() {
  CantBeCopied result;
  return result; // Legal C++, but does not compile with Ubuntu 18.04 default g++
}
std::optional<CantBeCopied> fooOK() {
  CantBeCopied result;
  return {std::move(result)}; // Compiles OK everywhere
}

The underlying bug is actually not specific to std::optional but this is the most common case in f18 where the issue may occur. The actual bug can be reproduced with any class B that has a perfect forwarding constructor taking CantBeCopied as argument: template<typename CantBeCopied> B(CantBeCopied&& x) x_{std::forward<CantBeCopied>(x)} {}. In such scenarios, Ubuntu 18.04 g++ fails to instantiate the move constructor and to construct the returned value as it should, instead it complains about a missing copy constructor.

Local result variables do not need to and should not be explicitly moved into optionals if they have a copy constructor.

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