# Filename and line information

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< cpp‎ | preprocessor

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

### Syntax

`#line` lineno | (1) |
`#line` lineno `"`filename`"` | (2) |

### Explanation

1) Changes the current preprocessor line number to lineno. Expansions of the macro __LINE__ beyond this point will expand to lineno plus the number of actual source code lines encountered since.

2) Also changes the current preprocessor file name to filename. Expansions of the macro __FILE__ from this point will produce filename.

Any preprocessing tokens (macro constants or expressions) are permitted as arguments to #line as long as they expand to a valid decimal integer optionally following a valid character string.

lineno must be a sequence of at least one decimal digit (the program is ill-formed, otherwise) and is always interpreted as decimal (even if it starts with `0`).

If lineno is `0` or greater than `32767`(until C++11)`2147483647`(since C++11), the behavior is undefined.

### 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

Run this code

    #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]

### See also

source_location (C++20) | a class representing information about the source code, such as file names, line numbers, and function names (class)
C documentation for Filename and line information

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