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C++ Ethereum Coding Style

Code Formatting

Use clang-format tool to format your changes, see CONTRIBUTING for details.


  1. No using namespace declarations in header files.
  2. All symbols should be declared in a namespace except for final applications.
  3. Preprocessor symbols should be prefixed with the namespace in all-caps and an underscore.
       // WRONG:
       #include <cassert>
       using namespace std;
       tuple<float, float> meanAndSigma(vector<float> const& _v);

       // CORRECT:
       #include <cassert>
       std::tuple<float, float> meanAndSigma(std::vector<float> const& _v);


  1. File comment is always at top, and includes:

    • Copyright.
    • License.
  2. Never use #ifdef/#define/#endif file guards. Prefer #pragma once as first line below file comment.

  3. Prefer static const variable to value macros.

  4. Prefer inline constexpr functions to function macros.


GOLDEN RULE: Preprocessor: ALL_CAPS; C++: camelCase.

  1. Use camelCase for splitting words in names, except where obviously extending STL/boost functionality in which case follow those naming conventions.
  2. The following entities' first alpha is upper case:
    • Type names.
    • Template parameters.
    • Enum members.
    • static const variables that form an external API.
  3. All preprocessor symbols (macros, macro arguments) in full uppercase with underscore word separation.

All other entities' first alpha is lower case.

Variable prefixes

  1. Leading underscore _ to parameter names.
    • Exception: o_parameterName when it is used exclusively for output. See also Declarations.5.
    • Exception: io_parameterName when it is used for both input and output. See also Declarations.5.
  2. Leading c_ to const variables (unless part of an external API).
  3. Leading g_ to global (non-const) variables.
  4. Leading s_ to static (non-const, non-global) variables.

Error reporting

Prefer exception to bool/int return type.


  1. {Typename} + {qualifiers} + {name}. (TODO: Against NL.26)
  2. Only one per line.
  3. Favour declarations close to use; don't habitually declare at top of scope ala C.
  4. Always pass non-trivial parameters with a const& suffix.
  5. To return multiple "out" values, prefer returning a tuple or struct. See F.21.
  6. Never use a macro where adequate non-preprocessor C++ can be written.
  7. Make use of auto whenever type is clear or unimportant:
    • Always avoid doubly-stating the type.
    • Use to avoid vast and unimportant type declarations.
    • However, avoid using auto where type is not immediately obvious from the context, and especially not for arithmetic expressions.
  8. Don't pass bools: prefer enumerations instead.
  9. Prefer enum class to straight enum.
       // WRONG:
       const double d = 0;
       int i, j;
       char *s;
       float meanAndSigma(std::vector<float> _v, float* _sigma, bool _approximate);
       Derived* x(dynamic_cast<Derived*>(base));
       for (map<ComplexTypeOne, ComplexTypeTwo>::iterator i = l.begin(); i != l.end(); ++l) {}

       // CORRECT:
       enum class Accuracy
       double const d = 0;
       int i;
       int j;
       char* s;
       std::tuple<float, float> meanAndSigma(std::vector<float> const& _v, Accuracy _a);
       auto x = dynamic_cast<Derived*>(base);
       for (auto i = x.begin(); i != x.end(); ++i) {}

Structs & classes

  1. Structs to be used when all members public and no virtual functions.
    • In this case, members should be named naturally and not prefixed with m_
  2. Classes to be used in all other circumstances.


  1. One member per line only.
  2. Private, non-static, non-const fields prefixed with m_.
  3. Avoid public fields, except in structs.
  4. Use override, final and const as much as possible.
  5. No implementations with the class declaration, except:
    • template or force-inline method (though prefer implementation at bottom of header file).
    • one-line implementation (in which case include it in same line as declaration).
  6. For a property foo
    • Member: m_foo;
    • Getter: foo(); also: for booleans, isFoo()
    • Setter: setFoo();


  1. Collection conventions:
    • ...s means std::vector e.g. using MyTypes = std::vector<MyType>
    • ...Set means std::set e.g. using MyTypeSet = std::set<MyType>
    • ...Hash means std::unordered_set e.g. using MyTypeHash = std::unordered_set<MyType>
  2. Class conventions:
    • ...Face means the interface of some shared concept. (e.g. FooFace might be a pure virtual class.)
  3. Avoid unpronounceable names:
    • If you need to shorten a name favour a pronouncable slice of the original to a scattered set of consonants.
    • e.g. Manager shortens to Man rather than Mgr.
  4. Avoid prefixes of initials (e.g. DON'T use IMyInterface, CMyImplementation)
  5. Find short, memorable & (at least semi-) descriptive names for commonly used classes or name-fragments.
    • A dictionary and thesaurus are your friends.
    • Spell correctly.
    • Think carefully about the class's purpose.
    • Imagine it as an isolated component to try to decontextualise it when considering its name.
    • Don't be trapped into naming it (purely) in terms of its implementation.


  1. Prefer using to typedef. E.g. using ints = std::vector<int> rather than typedef std::vector<int> ints.
  2. Generally avoid shortening a standard form that already includes all important information:
    • e.g. stick to shared_ptr<X> rather than shortening to ptr<X>.
  3. Where there are exceptions to this (due to excessive use and clear meaning), note the change prominently and use it consistently.
    • e.g.
    using Guard = std::lock_guard<std::mutex>; ///< Guard is used throughout the codebase since it's clear in meaning and used commonly.
  4. In general expressions should be roughly as important/semantically meaningful as the space they occupy.


  1. Comments should be doxygen-compilable, using @notation rather than \notation.
  2. Document the interface, not the implementation.
    • Documentation should be able to remain completely unchanged, even if the method is reimplemented.
    • Comment in terms of the method properties and intended alteration to class state (or what aspects of the state it reports).
    • Be careful to scrutinise documentation that extends only to intended purpose and usage.
    • Reject documentation that is simply an English transaction of the implementation.


Logging should be performed at appropriate verbosities depending on the logging message. The more likely a message is to repeat (and thus cause noise) the higher in verbosity it should be.

We use boost log, some rules to keep in mind:

Level Description
Trace Information that can be useful to back-trace certain events - mostly useful than debug logs.
Debug Informational events most useful for developers to debug application. Only applicable if NDEBUG is not defined (for non-VC++) or _DEBUG is defined (for VC++).
Fatal Very severe error event that will presumably lead the application to abort.
Error Error information but will continue application to keep running.
Warning Information representing errors in application but application will keep running.
Info Mainly useful to represent current progress of application.

Recommended reading

Herb Sutter and Bjarne Stroustrup

Herb Sutter and Andrei Alexandrescu

  • "C++ Coding Standards: 101 Rules, Guidelines, and Best Practices"

Scott Meyers

  • "Effective C++: 55 Specific Ways to Improve Your Programs and Designs (3rd Edition)"
  • "More Effective C++: 35 New Ways to Improve Your Programs and Designs"
  • "Effective Modern C++: 42 Specific Ways to Improve Your Use of C++11 and C++14"


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