Comments serve as a sort of in-code documentation. When inserted into a program, they are effectively ignored by the compiler; they are solely intended to be used as notes by the humans that read source code. Although specific documentation is not part of the C++ standard, several utilities exist that parse comments with different documentation formats.

[edit]Syntax

All comments are removed from the program attranslation phase 3 by replacing each comment with a single whitespace character.

[edit]C-style

C-style comments are usually used to comment large blocks of text, however, they can be used to comment single lines. To insert a C-style comment, simply surround text with/* and*/; this will cause the contents of the comment to be ignored by the compiler. Although it is not part of the C++ standard,/** and*/ are often used to indicate documentation blocks; this is legal because the second asterisk is simply treated as part of the comment. C-style comments cannot be nested.

[edit]C++-style

C++-style comments are usually used to comment single lines, however, multiple C++-style comments can be placed together to form multi-line comments. C++-style comments tell the compiler to ignore all content between// and a new line.

[edit]Notes

Because commentsare removed before the preprocessor stage, a macro cannot be used to form a comment and an unterminated C-style comment doesn't spill over from an #include'd file.

Besides commenting out, other mechanisms used for source code exclusion are

#if 0std::cout<<"this will not be executed or even compiled\n";#endif

and

if(false){std::cout<<"this will not be executed\n";}

[edit]Example

#include <iostream> /* C-style comments can containmultiple lines *//* or just one */ /************** *  you can insert any *, but *  you can't make comments nested */ // C++-style comments can comment one line // or, they can// be strung together int main(){// comments are removed before preprocessing,// so ABC is "1", not "1//2134", and "1 hello world"// will be printed#define ABC 1//2134std::cout<< ABC<<" hello world\n"; // The below code won't be run// return 1; // The below code will be runreturn0;}

1 hello world

[edit]See also

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