C++ Comments


A comment is a way to put arbitrary text inside source code without having the C++ compiler interpret it with any functional meaning. Comments are used to give insight into the design or method of a program.

There are two types of comments in C++:

Single-Line Comments

The double forward-slash sequence // will mark all text until a newline as a comment:

int main()
   // This is a single-line comment.
   int a;  // this also is a single-line comment
   int i;  // this is another single-line comment

C-Style/Block Comments

The sequence /* is used to declare the start of the comment block and the sequence */ is used to declare the end of comment. All text between the start and end sequences is interpreted as a comment, even if the text is otherwise valid C++ syntax. These are sometimes called "C-style" comments, as this comment syntax is inherited from C++'s predecessor language, C:

int main()
    *  This is a block comment.
   int a;

In any block comment, you can write anything you want. When the compiler encounters the symbol */, it terminates the block comment:

int main()
   /* A block comment with the symbol /*
      Note that the compiler is not affected by the second /*
      however, once the end-block-comment symbol is reached,
      the comment ends.
   int a;

The above example is valid C++ (and C) code. However, having additional /* inside a block comment might result in a warning on some compilers.

Block comments can also start and end within a single line. For example:

void SomeFunction(/* argument 1 */ int a, /* argument 2 */ int b);

Importance of Comments

As with all programming languages, comments provide several benefits:

  • Explicit documentation of code to make it easier to read/maintain
  • Explanation of the purpose and functionality of code
  • Details on the history or reasoning behind the code
  • Placement of copyright/licenses, project notes, special thanks, contributor credits, etc. directly in the source code.

However, comments also have their downsides:

  • They must be maintained to reflect any changes in the code
  • Excessive comments tend to make the code less readable

The need for comments can be reduced by writing clear, self-documenting code. A simple example is the use of explanatory names for variables, functions, and types. Factoring out logically related tasks into discrete functions goes hand-in-hand with this.

Comment markers used to disable code

During development, comments can also be used to quickly disable portions of code without deleting it. This is often useful for testing or debugging purposes, but is not good style for anything other than temporary edits. This is often referred to as “commenting out”.

Similarly, keeping old versions of a piece of code in a comment for reference purposes is frowned upon, as it clutters files while offering little value compared to exploring the code's history via a versioning system.