Coding Standards

Commenting

Comments are one critical part of readability and maintainability. Everyone knows they should comment their code, and so should you. When writing comments, write them as English prose, which means they should use proper capitalization, punctuation, etc. Aim to describe what the code is trying to do and why, not how it does it at a micro level. Here are a few critical things to document:

//===-- package/ClassName.h - Instruction class definition -------*- C++ -*-===//
//
//                     The Project Name
//
// This file is distributed under the LGPL
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
///
/// \file
/// \brief This file contains the declaration of the ClassName class, which is
/// doing this and that.
///
//===----------------------------------------------------------------------===//

Comment Formatting

In general, prefer C++ style (//) comments. They take less space, require less typing, don’t have nesting problems, etc. There are a few cases when it is useful to use C style (/* */) comments however:

1. When writing C code: Obviously if you are writing C code, use C style comments.
2. When writing a header file that may be #included by a C source file.
3. When writing a source file that is used by a tool that only accepts C style comments.

To comment out a large block of code, use #if 0 and #endif. These nest properly and are better behaved in general than C style comments.

Doxygen Use in Documentation Comments

Use the \file command to turn the standard file header into a file-level comment.

Include descriptive \brief paragraphs for all public interfaces (public classes, member and non-member functions). Explain API use and purpose in \brief paragraphs, don’t just restate the information that can be inferred from the API name. Put detailed discussion into separate paragraphs.

To refer to parameter names inside a paragraph, use the \p name command. Don’t use the \arg name command since it starts a new paragraph that contains documentation for the parameter.

Wrap non-inline code examples in \code ... \endcode.

To document a function parameter, start a new paragraph with the \param name command. If the parameter is used as an out or an in/out parameter, use the \param [out] name or \param [in,out] name command, respectively.

To describe function return value, start a new paragraph with the \returns command.

A minimal documentation comment:

/// \brief Does foo and bar.
void FooBar(bool Baz);

A documentation comment that uses all Doxygen features in a preferred way:

/// \brief Does foo and bar.
///
/// Does not do foo the usual way if \p Baz is true.
///
/// Typical usage:
/// \code
///   FooBar(false, "quux", Res);
/// \endcode
///
/// \param Quux kind of foo to do.
/// \param [out] Result filled with bar sequence on foo success.
///
/// \returns true on success.
bool FooBar(bool Baz, StringRef Quux, std::vector<int> &Result);

Don’t duplicate the documentation comment in the header file and in the implementation file. Put the documentation comments for public APIs into the header file. Documentation comments for private APIs can go to the implementation file. In any case, implementation files can include additional comments (not necessarily in Doxygen markup) to explain implementation details as needed.

Don’t duplicate function or class name at the beginning of the comment. For humans it is obvious which function or class is being documented; automatic documentation processing tools are smart enough to bind the comment to the correct declaration.

Wrong:

// In Something.h:

/// Something - An abstraction for some complicated thing.
class Something {
public:
  /// FooBar - Does foo and bar.
  void FooBar();
};

// In Something.cpp:

/// FooBar - Does foo and bar.
void Something::FooBar() { ... }

Correct:

// In Something.h:

/// \brief An abstraction for some complicated thing.
class Something {
public:
  /// \brief Does foo and bar.
  void FooBar();
};

// In Something.cpp:

// Builds a B-tree in order to do foo.  See paper by...
void Something::FooBar() { ... }

It is not required to use additional Doxygen features, but sometimes it might be a good idea to do so.

Consider:

• adding comments to any narrow namespace containing a collection of related functions or types;
• using top-level groups to organize a collection of related functions at namespace scope where the grouping is smaller than the namespace;
• using member groups and additional comments attached to member groups to organize within a class.

For example:

class Something {
  /// \name Functions that do Foo.
  /// @{
  void FooBar();
  void FooBaz();
  /// @}
  ...
};

#include Style

Immediately after the header file comment (and include guards if working on a header file), the minimal list of #includes required by the file should be listed. We prefer these #includes to be listed in this order:

1. Main Module Header
2. Local/Private Headers
3. VecGeom/...
4. System #includes

and each category should be sorted lexicographically by the full path.

The Main Module Header file applies to .cpp files which implement an interface defined by a .h file. This #include should always be included first regardless of where it lives on the file system. By including a header file first in the .cpp files that implement the interfaces, we ensure that the header does not have any hidden dependencies which are not explicitly #included in the header, but should be. It is also a form of documentation in the .cpp file to indicate where the interfaces it implements are defined.

Use Spaces Instead of Tabs

In all cases, prefer spaces to tabs in source files. People have different preferred indentation levels, and different styles of indentation that they like; this is fine. What isn’t fine is that different editors/viewers expand tabs out to different tab stops. This can cause your code to look completely unreadable, and it is not worth dealing with.

As always, follow the Golden Rule above: follow the style of existing code if you are modifying and extending it. If you like four spaces of indentation, DO NOT do that in the middle of a chunk of code with two spaces of indentation. Also, do not reindent a whole source file: it makes for incredible diffs that are absolutely worthless.

When creating a new file, prefer using two space indentation, this reduces the total length of the lines. When reindentation of file is necessary, for example for consistency or to reduce the line length, commit the reindentation changes by themselves with no functional changes; this avoid having to hunts for them in the middle of many irrelevant changes.

Indent Code Consistently

Okay, in your first year of programming you were told that indentation is important. If you didn’t believe and internalize this then, now is the time. Just do it. With the introduction of C++11, there are some new formatting challenges that merit some suggestions to help have consistent, maintainable, and tool-friendly formatting and indentation.

std::sort(foo.begin(), foo.end(), [&](Foo a, Foo b) -> bool {
  if (a.blah < b.blah)
    return true;
  if (a.baz < b.baz)
    return true;
  return a.bam < b.bam;
});

dyn_switch(V->stripPointerCasts(),
           [] (PHINode *PN) {
             // process phis...
           },
           [] (SelectInst *SI) {
             // process selects...
           },
           [] (LoadInst *LI) {
             // process loads...
           },
           [] (AllocaInst *AI) {
             // process allocas...
           });

foo({a, b, c}, {1, 2, 3});

NS::Constant *Mask[] = {
    NS::ConstantInt::Get(NS::Type::GetInt32Ty(GetContext()), 0),
    NS::ConstantInt::Get(NS::Type::GetInt32Ty(GetContext()), 1),
    NS::ConstantInt::Get(NS::Type::GetInt32Ty(GetContext()), 2)};

Treat Compiler Warnings Like Errors

If your code has compiler warnings in it, something is wrong — you aren’t casting values correctly, you have “questionable” constructs in your code, or you are doing something legitimately wrong. Compiler warnings can cover up legitimate errors in output and make dealing with a translation unit difficult.

It is not possible to prevent all warnings from all compilers, nor is it desirable. Instead, pick a standard compiler (like gcc) that provides a good thorough set of warnings, and stick to it. At least in the case of gcc, it is possible to work around any spurious errors by changing the syntax of the code slightly. For example, a warning that annoys me occurs when I write code like this:

if (V = GetValue()) {
  ...
}

gcc will warn me that I probably want to use the == operator, and that I probably mistyped it. In most cases, I haven’t, and I really don’t want the spurious errors. To fix this particular problem, I rewrite the code like this:

if ((V = GetValue())) {
  ...
}

which shuts gcc up. Any gcc warning that annoys you can be fixed by massaging the code appropriately.

Write Portable Code

In almost all cases, it is possible and within reason to write completely portable code. If there are cases where it isn’t possible to write portable code, isolate it behind a well defined (and well documented) interface.

In practice, this means that you shouldn’t assume much about the host compiler (and Visual Studio tends to be the lowest common denominator). If advanced features are used, they should only be an implementation detail of a library which has a simple exposed API, and preferably be buried in libSystem.

Do not use Static Constructors

Static constructors and destructors (e.g. global variables whose types have a constructor or destructor) should not be added to the code base, and should be removed wherever possible. Besides well known problems where the order of initialization is undefined between globals in different source files, the entire concept of static constructors is at odds with the common use case as a library linked into a larger application.

• The time to run the static constructors impacts startup time of applications — a critical time for GUI apps, among others.
• The static constructors cause the app to pull many extra pages of memory off the disk: both the code for the constructor in each .o file and the small amount of data that gets touched. In addition, touched/dirty pages put more pressure on the VM system on low-memory machines.

Use of class and struct Keywords

In C++, the class and struct keywords can be used almost interchangeably. The only difference is when they are used to declare a class: class makes all members private by default while struct makes all members public by default.

Unfortunately, not all compilers follow the rules and some will generate different symbols based on whether class or struct was used to declare the symbol (e.g., MSVC). This can lead to problems at link time.

• All declarations and definitions of a given class or struct must use the same keyword. For example:

class Foo;

// Breaks mangling in MSVC.
struct Foo { int Data; };

• As a rule of thumb, struct should be kept to structures where all members are declared public.

// Foo feels like a class... this is strange.
struct Foo {
private:
  int Data;
public:
  Foo() : Data(0) { }
  int GetData() const { return Data; }
  void SetData(int D) { Data = D; }
};

// Bar isn't POD, but it does look like a struct.
struct Bar {
  int Data;
  Foo() : Data(0) { }
};

Do not use Braced Initializer Lists to Call a Constructor

In C++11 there is a “generalized initialization syntax” which allows calling constructors using braced initializer lists. Do not use these to call constructors with any interesting logic or if you care that you’re calling some particular constructor. Those should look like function calls using parentheses rather than like aggregate initialization. Similarly, if you need to explicitly name the type and call its constructor to create a temporary, don’t use a braced initializer list. Instead, use a braced initializer list (without any type for temporaries) when doing aggregate initialization or something notionally equivalent. Examples:

class Foo {
public:
  // Construct a Foo by reading data from the disk in the whizbang format, ...
  Foo(std::string filename);

  // Construct a Foo by looking up the Nth element of some global data ...
  Foo(int N);

  // ...
};

// The Foo constructor call is very deliberate, no braces.
std::fill(foo.begin(), foo.end(), Foo("name"));

// The pair is just being constructed like an aggregate, use braces.
bar_map.insert({my_key, my_value});

If you use a braced initializer list when initializing a variable, use an equals before the open curly brace:

int data[] = {0, 1, 2, 3};

Use auto Type Deduction to Make Code More Readable

Some are advocating a policy of “almost always auto” in C++11, however we uses a more moderate stance. Use auto if and only if it makes the code more readable or easier to maintain. Don’t “almost always” use auto, but do use auto with initializers like cast<Foo>(...) or other places where the type is already obvious from the context. Another time when auto works well for these purposes is when the type would have been abstracted away anyways, often behind a container’s typedef such as std::vector<T>::iterator.

Beware unnecessary copies with auto

The convenience of auto makes it easy to forget that its default behavior is a copy. Particularly in range-based for loops, careless copies are expensive.

As a rule of thumb, use auto & unless you need to copy the result, and use auto * when copying pointers.

// Typically there's no reason to copy.
for (const auto &val : container) { Observe(val); }
for (auto &val : container) { val.Change(); }

// Remove the reference if you really want a new copy.
for (auto val : container) { val.Change(); SaveSomewhere(val); }

// Copy pointers, but make it clear that they're pointers.
for (const auto *ptr : container) { Observe(*ptr); }
for (auto *ptr : container) { ptr->Change(); }

#include as Little as Possible

File names must match the main concept or class they define or implement. Thus they will usually be nouns and start with an upper-case letter (see note on type names).

#include as Little as Possible

#include hurts compile time performance. Don’t do it unless you have to, especially in header files.

But wait! Sometimes you need to have the definition of a class to use it, or to inherit from it. In these cases go ahead and #include that header file. Be aware however that there are many cases where you don’t need to have the full definition of a class. If you are using a pointer or reference to a class, you don’t need the header file. If you are simply returning a class instance from a prototyped function or method, you don’t need it. In fact, for most cases, you simply don’t need the definition of a class. And not #includeing speeds up compilation.

It is easy to try to go too overboard on this recommendation, however. You must include all of the header files that you are using — you can include them either directly or indirectly through another header file. To make sure that you don’t accidentally forget to include a header file in your module header, make sure to include your module header first in the implementation file (as mentioned above). This way there won’t be any hidden dependencies that you’ll find out about later.

Keep “Internal” Headers Private

Many modules have a complex implementation that causes them to use more than one implementation (.cpp) file. It is often tempting to put the internal communication interface (helper classes, extra functions, etc) in the public module header file. Don’t do this!

If you really need to do something like this, put a private header file in the same directory as the source files, and include it locally. This ensures that your private interface remains private and undisturbed by outsiders.

Use Early Exits and continue to Simplify Code

When reading code, keep in mind how much state and how many previous decisions have to be remembered by the reader to understand a block of code. Aim to reduce indentation where possible when it doesn’t make it more difficult to understand the code. One great way to do this is by making use of early exits and the continue keyword in long loops. As an example of using an early exit from a function, consider this “bad” code:

Value *DoSomething(Instruction *i) {
  if (!IsA<TerminatorInst>(i) &&
      i->HasOneUse() && DoOtherThing(i)) {
    ... some long code ....
  }

  return 0;
}

This code has several problems if the body of the 'if' is large. When you’re looking at the top of the function, it isn’t immediately clear that this only does interesting things with non-terminator instructions, and only applies to things with the other predicates. Second, it is relatively difficult to describe (in comments) why these predicates are important because the if statement makes it difficult to lay out the comments. Third, when you’re deep within the body of the code, it is indented an extra level. Finally, when reading the top of the function, it isn’t clear what the result is if the predicate isn’t true; you have to read to the end of the function to know that it returns null.

It is much preferred to format the code like this:

Value *DoSomething(Instruction *i) {
  // Terminators never need 'something' done to them because ...
  if (IsA<TerminatorInst>(i))
    return 0;

  // We conservatively avoid transforming instructions with multiple uses
  // because goats like cheese.
  if (!i->HasOneUse())
    return 0;

  // This is really just here for example.
  if (!DoOtherThing(i))
    return 0;

  ... some long code ....
}

This fixes these problems. A similar problem frequently happens in for loops. A silly example is something like this:

for (BasicBlock::iterator ii = bb->begin(), e = bb->end(); ii != e; ++ii) {
  if (BinaryOperator *bo = dyn_cast<BinaryOperator>(ii)) {
    Value *lhs = bo->GetOperand(0);
    Value *rhs = bo->GetOperand(1);
    if (lhs != rhs) {
      ...
    }
  }
}

When you have very, very small loops, this sort of structure is fine. But if it exceeds more than 10-15 lines, it becomes difficult for people to read and understand at a glance. The problem with this sort of code is that it gets very nested very quickly. Meaning that the reader of the code has to keep a lot of context in their brain to remember what is going immediately on in the loop, because they don’t know if/when the if conditions will have elses etc. It is strongly preferred to structure the loop like this:

for (BasicBlock::iterator ii = bb->begin(), e = bb->end(); ii != e; ++ii) {
  BinaryOperator *bo = dynamic_cast<BinaryOperator*>(ii);
  if (!bo) continue;

  Value *lhs = bo->GetOperand(0);
  Value *rhs = bo->GetOperand(1);
  if (lhs == rhs) continue;

  ...
}

This has all the benefits of using early exits for functions: it reduces nesting of the loop, it makes it easier to describe why the conditions are true, and it makes it obvious to the reader that there is no else coming up that they have to push context into their brain for. If a loop is large, this can be a big understandability win.

Don’t use else after a return

For similar reasons above (reduction of indentation and easier reading), please do not use 'else' or 'else if' after something that interrupts control flow — like return, break, continue, goto, etc. For example, this is bad:

case 'J': {
  if (Signed) {
    Type = context.GetSigJmpBufType();
    if (Type.isNull()) {
      Error = ASTContext::GE_Missing_sigjmp_buf;
      return QualType();
    } else {
      break;
    }
  } else {
    Type = context.GetSigJmpBufType();
    if (Type.isNull()) {
      Error = ASTContext::GE_Missing_jmp_buf;
      return QualType();
    } else {
      break;
    }
  }
}

It is better to write it like this:

case 'J':
  if (Signed) {
    Type = context.GetSigJmpBufType();
    if (Type.isNull()) {
      Error = ASTContext::GE_Missing_sigjmp_buf;
      return QualType();
    }
  } else {
    Type = context.GetJmpBufType();
    if (Type.isNull()) {
      Error = ASTContext::GE_Missing_jmp_buf;
      return QualType();
    }
  }
  break;

Or better yet (in this case) as:

case 'J':
  if (Signed)
    Type = context.GetSigJmpBufType();
  else
    Type = context.GetJmpBufType();

  if (Type.isNull()) {
    Error = Signed ? ASTContext::GE_Missing_sigjmp_buf :
                     ASTContext::GE_Missing_jmp_buf;
    return QualType();
  }
  break;

The idea is to reduce indentation and the amount of code you have to keep track of when reading the code.

Turn Predicate Loops into Predicate Functions

It is very common to write small loops that just compute a boolean value. There are a number of ways that people commonly write these, but an example of this sort of thing is:

bool foundFoo = false;
for (unsigned i = 0, e = barList.size(); i != e; ++i)
  if (barList[i]->IsFoo()) {
    foundFoo = true;
    break;
  }

if (foundFoo) {
  ...
}

This sort of code is awkward to write, and is almost always a bad sign. Instead of this sort of loop, we strongly prefer to use a predicate function (which may be static) that uses early exits to compute the predicate. We prefer the code to be structured like this:

/// \returns true if the specified list has an element that is a foo.
static bool ContainsFoo(const std::vector<Bar*> &list) {
  for (unsigned i = 0, e = list.size(); i != e; ++i)
    if (list[i]->IsFoo())
      return true;
  return false;
}
...

if (ContainsFoo(barList)) {
  ...
}

There are many reasons for doing this: it reduces indentation and factors out code which can often be shared by other code that checks for the same predicate. More importantly, it forces you to pick a name for the function, and forces you to write a comment for it. In this silly example, this doesn’t add much value. However, if the condition is complex, this can make it a lot easier for the reader to understand the code that queries for this predicate. Instead of being faced with the in-line details of how we check to see if the barList contains a foo, we can trust the function name and continue reading with better locality.

Name Types, Functions, Variables, and Enumerators Properly

Poorly-chosen names can mislead the reader and cause bugs. We cannot stress enough how important it is to use descriptive names. Pick names that match the semantics and role of the underlying entities, within reason. Avoid abbreviations unless they are well known. After picking a good name, make sure to use consistent capitalization for the name, as inconsistency requires clients to either memorize the APIs or to look it up to find the exact spelling.

In general, names should be in camel case (e.g. TextFileReader and isLValue()). Different kinds of declarations have different rules:

• Namespace names should be nouns and start with lower-case letter (e.g. vecGeom).

• Type names (including classes, structs, enums, etc) should be nouns and start with an upper-case letter (e.g. TextFileReader).

• Typedefs should follow the type names rules and end in a trailing _t or if they indicates types that are 'intrisinc' vectors (on some platforms) the typedef should end in a trailing _v.

• Variable names should be nouns (as they represent state). The name should be camel case, and start with a lower case letter if it is a local variable and with a prefix (see next) followed by an upper case letter (e.g. greatLeader or fBoats).

  • Data members begin with f (for field).
  • Static variables begin with g; this applies to static variables in functions and global variables (excluding static data members of a class).
  • Static data members begin with fg (i.e. class globals).
  • Constants begin with k; including names of enumeration constants and constant statics.

• Template parameter names should follow the convention for type or variable and be suffixed by a capital letter T. e.g. template <typename ValueT, int kSizeT>.. Note that just T is an acceptable name for a type template name, as it is a standard well known choice.

• Function names should be verb phrases (as they represent actions), and command-like function should be imperative. The name should be camel case, and start with an upper case letter (e.g. OpenFile() or IsFoo()).

• Enum declarations (e.g. enum Foo {...}) are types, so they should follow the naming conventions for types and start with an e. A common use for enums is as a discriminator for a union, or an indicator of a subclass. When an enum is used for something like this, it should have a Kind suffix (e.g. EValueKind).

• Enumerators (e.g. enum { Foo, Bar }) and public member variables should start with a k. Unless the enumerators are defined in their own small namespace or inside a class, enumerators should have a prefix corresponding to the enum declaration name. For example, enum EValueKind { ... }; may contain enumerators like VK_Argument, VK_kBasicBlock, etc. Enumerators that are just convenience constants are exempt from the requirement for an additional prefix. For instance:

  enum {
    kMaxSize = 42,
    kDensity = 12
  };
  

As an exception, classes that mimic STL classes can have member names in STL’s style of lower-case words separated by underscores (e.g. begin(), push_back(), and empty()). Classes that provide multiple iterators should add a singular prefix to begin() and end() (e.g. global_begin() and use_begin()).

Here are some examples of good and bad names:

class VehicleMaker {
  ...
  Factory<Tire> fF;            // Bad -- abbreviation and non-descriptive.
  Factory<Tire> fFactory;      // Better.
  Factory<Tire> fTireFactory;  // Even better -- if VehicleMaker has more than one
                               // kind of factories.
};

Vehicle MakeVehicle(VehicleType Type) {
  VehicleMaker m;                         // Might be OK if having a short life-span.
  Tire tmp1 = m.MakeTire();               // Bad -- 'Tmp1' provides no information.
  Light headlight = m.MakeLight("head");  // Good -- descriptive.
  headlight.TurnOn();
  ...
}

Assert Liberally

Use the “assert” macro to its fullest. Check all of your preconditions and assumptions, you never know when a bug (not necessarily even yours) might be caught early by an assertion, which reduces debugging time dramatically. The “<cassert>” header file is probably already included by the header files you are using, so it doesn’t cost anything to use it.

To further assist with debugging, make sure to put some kind of error message in the assertion statement, which is printed if the assertion is tripped. This helps the poor debugger make sense of why an assertion is being made and enforced, and hopefully what to do about it. Here is one complete example:

inline Value *GetOperand(unsigned i) {
  assert(i < gOperands.size() && "GetOperand() out of range!");
  return gOperands[i];
}

Here are more examples:

assert(ty->IsPointerType() && "Can't allocate a non-pointer type!");

assert((opcode == kShl || Opcode == kShr) && "ShiftInst Opcode invalid!");

assert(idx < GetNumSuccessors() && "Successor # out of range!");

assert(v1.GetType() == v2.GetType() && "Constant types must be identical!");

assert(IsA<PhiNode>(succ->front()) && "Only works on PHId BBs!");

You get the idea.

In the past, asserts were used to indicate a piece of code that should not be reached. These were typically of the form:

assert(0 && "Invalid radix for integer literal");

This has a few issues, the main one being that some compilers might not understand the assertion, or warn about a missing return in builds where assertions are compiled out.

If needed we could import something better coming from llvm: llvm_unreachable:

llvm_unreachable("Invalid radix for integer literal");

When assertions are enabled, this will print the message if it’s ever reached and then exit the program. When assertions are disabled (i.e. in release builds), llvm_unreachable becomes a hint to compilers to skip generating code for this branch. If the compiler does not support this, it will fall back to the “abort” implementation.

Another issue is that values used only by assertions will produce an “unused value” warning when assertions are disabled. For example, this code will warn:

unsigned size = v.size();
assert(size > 42 && "Vector smaller than it should be");

bool newToSet = myset.insert(value);
assert(newToSet && "The value shouldn't be in the set yet");

These are two interesting different cases. In the first case, the call to V.size() is only useful for the assert, and we don’t want it executed when assertions are disabled. Code like this should move the call into the assert itself. In the second case, the side effects of the call must happen whether the assert is enabled or not. In this case, the value should be cast to void to disable the warning. To be specific, it is preferred to write the code like this:

assert(v.size() > 42 && "Vector smaller than it should be");

bool newToSet = myset.insert(Value); (void)newToSet;
assert(newToSet && "The value shouldn't be in the set yet");

Do Not Use using namespace std

We prefer to explicitly prefix all identifiers from the standard namespace with an “std::” prefix, rather than rely on “using namespace std;”.

In header files, adding a 'using namespace XXX' directive pollutes the namespace of any source file that #includes the header. This is clearly a bad thing.

In implementation files (e.g. .cpp files), the rule is more of a stylistic rule, but is still important. Basically, using explicit namespace prefixes makes the code clearer, because it is immediately obvious what facilities are being used and where they are coming from. And more portable, because namespace clashes cannot occur between our code and other namespaces. The portability rule is important because different standard library implementations expose different symbols (potentially ones they shouldn’t), and future revisions to the C++ standard will add more symbols to the std namespace. As such, we never use 'using namespace std;'.

The exception to the general rule (i.e. it’s not an exception for the std namespace) is for implementation files. For example, all of the code in our project implements code that lives in a specific namespace. As such, it is ok, and actually clearer, for the .cpp files to have a 'using namespace vecGeom;' directive at the top, after the #includes. This reduces indentation in the body of the file for source editors that indent based on braces, and keeps the conceptual context cleaner. The general form of this rule is that any .cpp file that implements code in any namespace may use that namespace (and its parents’), but should not use any others.

Provide a Virtual Method Anchor for Classes in Headers

If a class is defined in a header file and has a vtable (either it has virtual methods or it derives from classes with virtual methods), it must always have at least one out-of-line virtual method in the class. Without this, the compiler will copy the vtable and RTTI into every .o file that #includes the header, bloating .o file sizes and increasing link times.

Don’t evaluate end() every time through a loop

Because C++ doesn’t have a standard “foreach” loop (though it can be emulated with macros and may be coming in C++‘0x) we end up writing a lot of loops that manually iterate from begin to end on a variety of containers or through other data structures. One common mistake is to write a loop in this style:

BasicBlock *bb = ...
for (BasicBlock::iterator i = bb->begin(); i != bb->end(); ++i)
  ... use i ...

The problem with this construct is that it evaluates “bb->end()” every time through the loop. Instead of writing the loop like this, we strongly prefer loops to be written so that they evaluate it once before the loop starts. A convenient way to do this is like so:

BasicBlock *bb = ...
for (BasicBlock::iterator i = bb->begin(), e = bb->end(); i != e; ++i)
  ... use i ...

The observant may quickly point out that these two loops may have different semantics: if the container (a basic block in this case) is being mutated, then “bb->end()” may change its value every time through the loop and the second loop may not in fact be correct. If you actually do depend on this behavior, please write the loop in the first form and add a comment indicating that you did it intentionally.

Why do we prefer the second form (when correct)? Writing the loop in the first form has two problems. First it may be less efficient than evaluating it at the start of the loop. In this case, the cost is probably minor — a few extra loads every time through the loop. However, if the base expression is more complex, then the cost can rise quickly. I’ve seen loops where the end expression was actually something like: “SomeMap[X]->end()” and map lookups really aren’t cheap. By writing it in the second form consistently, you eliminate the issue entirely and don’t even have to think about it.

The second (even bigger) issue is that writing the loop in the first form hints to the reader that the loop is mutating the container (a fact that a comment would handily confirm!). If you write the loop in the second form, it is immediately obvious without even looking at the body of the loop that the container isn’t being modified, which makes it easier to read the code and understand what it does.

While the second form of the loop is a few extra keystrokes, we do strongly prefer it.

#include <iostream> is Forbidden

The use of #include <iostream> in library files is hereby forbidden, because many common implementations transparently inject a static constructor into every translation unit that includes it.

Note that using the other stream headers (<sstream> for example) is not problematic in this regard — just <iostream>.

Avoid std::endl

The std::endl modifier, when used with iostreams outputs a newline to the output stream specified. In addition to doing this, however, it also flushes the output stream. In other words, these are equivalent:

std::cout << std::endl;
std::cout << '\n' << std::flush;

Most of the time, you probably have no reason to flush the output stream, so it’s better to use a literal '\n'.

Don’t use inline when defining a function in a class definition

A member function defined in a class definition is implicitly inline, so don’t put the inline keyword in this case.

Don’t:

class Foo {
public:
  inline void Bar() {
    // ...
  }
};

Do:

class Foo {
public:
  void Bar() {
    // ...
  }
};

Spaces Before Parentheses

We prefer to put a space before an open parenthesis only in control flow statements, but not in normal function call expressions and function-like macros. For example, this is good:

if (x) ...
for (i = 0; i != 100; ++i) ...
while (gGeantRocks) ...

Somefunc(42);
assert(3 != 4 && "laws of math are failing me");

a = Foo(42, 92) + Bar(X);

and this is bad:

if(x) ...
for(i = 0; i != 100; ++i) ...
while(gGeantRocks) ...

Somefunc (42);
assert (3 != 4 && "laws of math are failing me");

a = Foo (42, 92) + Bar (x);

The reason for doing this is not completely arbitrary. This style makes control flow operators stand out more, and makes expressions flow better. The function call operator binds very tightly as a postfix operator. Putting a space after a function name (as in the last example) makes it appear that the code might bind the arguments of the left-hand-side of a binary operator with the argument list of a function and the name of the right side. More specifically, it is easy to misread the “A” example as:

a = Foo ((42, 92) + Bar) (x);

when skimming through the code. By avoiding a space in a function, we avoid this misinterpretation.

We prefer to not put white space after the opening parathensis and before the closing parathensis and to put white space surrounding operators:

void Foo(int a, float b);
for (int i = 0; i < 10; ++i) // No padding whitespace

namespace vecGeom {
namespace knowledge {

/// This class represents things that Smith can have an intimate
/// understanding of and contains the data associated with it.
class Grokable {
...
public:
  explicit Grokable() { ... }
  virtual ~Grokable() = 0;

  ...

};

} // end namespace knowledge
} // end namespace vecGeom

namespace {
class StringSort {
...
public:
  StringSort(...)
  bool operator<(const char *rhs) const;
};
} // end anonymous namespace

static void RunHelper() {
  ...
}

bool StringSort::operator<(const char *rhs) const {
  ...
}

namespace {

class StringSort {
...
public:
  StringSort(...)
  bool operator<(const char *rhs) const;
};

void RunHelper() {
  ...
}

bool StringSort::operator<(const char *rhs) const {
  ...
}

} // end anonymous namespace

Const qualifiers

In C++, the outer most const qualifier can be placed before or after the type. Whether to use one option or the other depends on the context. When there is only one const qualifier and it is the outer most, place the const on the left of the type as it follows the natural english language reading order (left to right). When there is more than one const qualifier or the qualifier is not the outer most, place it after the type as it is much easier in that case to read from right to left to properly parse/understand the type.

const int var1;        // const value
const int *var2;       // modifiable pointer to a const int
int const *const var3; // const pointer to a const int
int *const var4;       // const pointer to a modifiable int

int * const var1; // This is BAD
int *const a;
int *a;
const std::vector<int> &GetContent() const;

if (x is true) {
   we do y
}

int Function(int x)
{
   body of function
}

void Foo(int a0, int a1, int a2, int a3,
         int a4, int a5) const;