FILE without full path

Question

The C++ macro __FILE__ typically includes the full path (with many compilers).

For my tracing, I want just the file name, not the full path.

Is there a built-in macro for this in any version of C++ or do I have to write my own?

This is not quite a duplicate of __FILE__ macro shows full path. That question is targeted at the 'C' language, and answers to that question which are C++ are actually incorrect for that question. This question is targeted specifically at C++.

Is there any particular reason this should be a macro? The C++ language itself is very capable at solving this problem. — Drew Dormann
– Drew Dormann, Commented Jun 3 at 0:55
Note there also is std::source_location. If you can, avoid using macros — Pepijn Kramer
– Pepijn Kramer, Commented Jun 3 at 3:23
std::source_location still gives the full path, so it doesn't actually solve the problem. Yes, you could create a function that does the same thing, but a macro doesn't require () and is also easier to read for new team members who aren't used to C++. They see __LINE__ and __FUNCTION__ which they're used to and then [new macro] which looks the same. For the curious, they may not think that macro existed, but they can then find out that it's implemented in the code. — Samuel Wenker
– Samuel Wenker, Commented Jun 3 at 15:17
Indeed, C++ offers different possibilities than C (for example en.cppreference.com/w/cpp/utility/source_location/… ) and the currently identified duplicate seems not suitable. I vote to reopen this question. — Christophe
– Christophe, Commented Jun 3 at 18:10
For me, c++ foo.cpp the __FILE__ is foo.cpp. But c++ /home/eljay/proj/foo.cpp the __FILE__ is /home/eljay/proj/foo.cpp. — Eljay
– Eljay, Commented Jun 4 at 13:46

Samuel Wenker · Accepted Answer · 2025-06-04 01:11:36Z

2

In C++20 or higher, the following macro implements the desired behavior.

It's a small snippit of code and works the same way that __FILE__ does.

#define FILENAME ([]{ \
  constexpr auto ret {std::string_view(__FILE__).substr(std::string_view(__FILE__).find_last_of("/\\") + 1).data()};         \
  return ret;           \
}())

edited Jun 4 at 1:11

answered Jun 2 at 23:22

Samuel Wenker

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4 Comments

Pete Becker Jun 2 at 23:56

The name __FILENAME__ contains two consecutive underscores, so it is reserved for use by the implementation. Don’t use it in your code.

Remy Lebeau Jun 3 at 1:29

@DrewDormann No it should not throw anything. If there is a slash then find_last_of() will return its index, and strstr(index+1) will return everything following the slash, as expected. But, if there is no slash then find_last_of() will return string_view::npos, which is an unsigned type of maximum value, so npos + 1 will simply wrap back to 0, which is well-defined behavior, and substr(0) will just return the original string as-is. So, there is no out of bounds access in either case, and the result will be the correct file name whether a slash is present or not.

Jesper Juhl Jun 3 at 15:13

Identifiers beginning with underscore followed by a capital letter (as in _FILENAME_) are reserved for the implementation, as are many others. See en.cppreference.com/w/cpp/preprocessor/replace.html , en.cppreference.com/w/cpp/language/identifiers.html & stackoverflow.com/questions/228783/…

Samuel Wenker Jun 3 at 15:21

Fixed Jesphers remark. Also updated the macro to ensure that it forces compile-time evaluation regardless of optimization settings for the compiler.

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