compiler-warning-level-4-c4001.md

description	title	ms.date	f1_keywords	helpviewer_keywords
Learn more about: Compiler Warning (level 4, no longer emitted) C4001	Compiler Warning (level 4, no longer emitted) C4001	11/04/2016	C4001	C4001

Compiler Warning (level 4, no longer emitted) C4001

nonstandard extension 'single line comment' was used

Note

This warning is removed in Visual Studio 2017 version 15.5 because single-line comments are standard in C99.

Single-line comments are standard in C++ and standard in C starting with C99. Under strict ANSI compatibility (/Za), C files that contain single-line comments, generate C4001 due to the usage of a nonstandard extension. Since single-line comments are standard in C++, C files containing single-line comments don't produce C4001 when compiling with Microsoft extensions (/Ze).

Example

To disable warning, uncomment #pragma warning(disable:4001).

// C4001.cpp
// compile with: /W4 /Za /TC
// #pragma warning(disable:4001)
int main()
{
   // single-line comment in main
   // C4001
}