Compiler Warning (level 4) C4001
nonstandard extension 'single line comment' was used
Single-line comments are standard in C++ and nonstandard in C. 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 do not 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
}