Filename and line information

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Diagnostics library

Memory management library

Metaprogramming library (C++11)

General utilities library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library (C++17)

Concurrency support library (C++11)

Execution control library (C++26)

Technical specifications

Symbols index

External libraries

 

C++ language

General topics
Preprocessor Comments	Keywords Escape sequences
Flow control
Conditional execution statements
if	switch
Iteration statements (loops)
for range-for (C++11)	while do-while
Jump statements
continue - break	goto - return
Functions
Function declaration
Lambda function expression
inline specifier
Dynamic exception specifications (until C++17*)
noexcept specifier (C++11)
Exceptions
throw-expression try block	catch handler
Namespaces
Namespace declaration	Namespace aliases
Types
Fundamental types Enumeration types Function types	Class/struct types Union types
Specifiers
const/volatile decltype (C++11) auto (C++11)	constexpr (C++11) consteval (C++20) constinit (C++20)
Storage duration specifiers
Initialization
Default-initialization Value-initialization Zero-initialization Copy-initialization Direct-initialization	Aggregate initialization List-initialization (C++11)   Constant initialization Reference initialization

Expressions
Value categories Order of evaluation	Operators Operator precedence
Alternative representations
Literals
Boolean - Integer - Floating-point
Character - String - nullptr (C++11)
User-defined (C++11)
Utilities
Attributes (C++11)
Types
typedef declaration
Type alias declaration (C++11)
Casts
Implicit conversions static_cast const_cast	Explicit conversions dynamic_cast reinterpret_cast
Memory allocation
new expression	delete expression
Classes
Class declaration Constructors this pointer	Access specifiers friend specifier
Class-specific function properties
Virtual function override specifier (C++11)   final specifier (C++11)	explicit (C++11) static
Special member functions
Default constructor Copy constructor Move constructor (C++11)	Copy assignment Move assignment (C++11) Destructor
Templates
Class template Function template	Template specialization Parameter packs (C++11)
Miscellaneous
Inline assembly	History of C++

 

Preprocessor

#if#ifdef#ifndef#else#elif#elifdef#elifndef#endif (C++23)(C++23)

#define#undef#,## operators

#include__has_include (C++17)

#error#warning (C++23)

#pragma_Pragma (C++11)

#line

#embed (C++26)

 

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

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