Keywords and naming identifiers

By Md Shoriful Islam

 

Keywords and naming identifiers

Keywords

C++ reserves a set of 92 words (as of C++20) for its own use. These words are called keywords (or reserved words), and each of these keywords has a special meaning within the C++ language.

Here is a list of all the C++ keywords (through C++20):

  • alignas
  • alignof
  • and
  • and_eq
  • asm
  • auto
  • bitand
  • bitor
  • bool
  • break
  • case
  • catch
  • char
  • char8_t (since C++20)
  • char16_t
  • char32_t
  • class
  • compl
  • concept (since C++20)
  • const
  • consteval (since C++20)
  • constexpr
  • constinit (since C++20)
  • const_cast
  • continue
  • co_await (since C++20)
  • co_return (since C++20)
  • co_yield (since C++20)
  • decltype
  • default
  • delete
  • do
  • double
  • dynamic_cast
  • else
  • enum
  • explicit
  • export
  • extern
  • false
  • float
  • for
  • friend
  • goto
  • if
  • inline
  • int
  • long
  • mutable
  • namespace
  • new
  • noexcept
  • not
  • not_eq
  • nullptr
  • operator
  • or
  • or_eq
  • private
  • protected
  • public
  • register
  • reinterpret_cast
  • requires (since C++20)
  • return
  • short
  • signed
  • sizeof
  • static
  • static_assert
  • static_cast
  • struct
  • switch
  • template
  • this
  • thread_local
  • throw
  • true
  • try
  • typedef
  • typeid
  • typename
  • union
  • unsigned
  • using
  • virtual
  • void
  • volatile
  • wchar_t
  • while
  • xor
  • xor_eq

The keywords marked (C++20) were added in C++20. If your compiler is not C++20 compliant (or does have C++20 functionality, but it’s turned off by default), these keywords may not be functional.

C++ also defines special identifiers: override, final, import, and module. These have a specific meaning when used in certain contexts but are not reserved.

You have already run across some of these keywords, including int and return. Along with a set of operators, these keywords and special identifiers define the entire language of C++ (preprocessor commands excluded). Because keywords and special identifiers have special meaning, your IDEs will likely change the text color of these words to make them stand out from other identifiers.

By the time you are done with this tutorial series, you will understand what almost all of these words do!

Identifier naming rules 

As a reminder, the name of a variable (or function, type, or other kind of item) is called an identifier. C++ gives you a lot of flexibility to name identifiers as you wish. However, there are a few rules that must be followed when naming identifiers:

  • The identifier can not be a keyword. Keywords are reserved.
  • The identifier can only be composed of letters (lower or upper case), numbers, and the underscore character. That means the name can not contain symbols (except the underscore) nor whitespace (spaces or tabs).
  • The identifier must begin with a letter (lower or upper case) or an underscore. It can not start with a number.
  • C++ is case sensitive, and thus distinguishes between lower and upper case letters. nvalue is different than nValue is different than NVALUE.

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