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)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support 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	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

 

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