Filename and line information - cppreference.com

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Changes the source code's line number and, optionally, the current file name, in the preprocessor.

Syntax


`#line` lineno new-line	(1)

`#line` lineno `"`filename`"` new-line	(2)

`#line` pp-tokens new-line	(3)

1) Changes the current preprocessor line number to lineno.

2) Also changes the current preprocessor file name to filename.

3) If neither (1) nor (2) is matched, pp-tokens will undergo macro replacement. The directive after replacement will be tried to match with (1) or (2) again.

new-line	-	The new-line character
lineno	-	A sequence of one or more decimal digits (`0` to `9`), optional single quotes (`'`) may be inserted between the digits as a separator(since C++14)
filename	-	A sequence of one or more s-char s (see string literal)
pp-tokens	-	A sequence of one or more preprocessing tokens

Explanation

Expansions of the macro __LINE__ beyond a #line directive will expand to lineno plus the number of actual source code lines encountered since. If filename is provided, expansions of the macro __FILE__ from this point will produce filename .

If the decimal number specified by lineno is 0 or greater than 32767(until C++11)2147483647(since C++11), the behavior is undefined(until C++26)the program is ill-formed(since C++26).

Notes

This directive is used by some automatic code generation tools which produce C++ source files from a file written in another language. In that case, #line directives may be inserted in the generated C++ file referencing line numbers and the file name of the original (human-editable) source file.

Example

#include <cassert>
#define FNAME "test.cc"
int main()
{
#line 777 FNAME
        assert(2+2 == 5);
}

Possible output:

test: test.cc:777: int main(): Assertion `2+2 == 5' failed.

References

C++23 standard (ISO/IEC 14882:2024):

15.7 Line control [cpp.line]

C++20 standard (ISO/IEC 14882:2020):

15.7 Line control [cpp.line]

C++17 standard (ISO/IEC 14882:2017):

19.4 Line control [cpp.line]

C++14 standard (ISO/IEC 14882:2014):

16.4 Line control [cpp.line]

C++11 standard (ISO/IEC 14882:2011):

16.4 Line control [cpp.line]

C++98 standard (ISO/IEC 14882:1998):

16.4 Line control [cpp.line]