Does C++20 mandate source code being stored in files?osseg r
A slightly strange question, however, if I remember correctly , C++ source code doesn't require a file system to store its files.
Having a compiler that scans handwritten papers via a camera would be a conforming implementation. Although practically not making that much sense.
However C++20 now adds source location with file_name. Does this now imply that source code should always be stored in a file?
2 Answers
No, source code doesn't have to come from a file.
You can compile C++ completely within a pipe, putting your compiler in the middle, e.g.
generate_source | g++ -o- -xc++ - | do_something_with_the_binary
and it's been like that for decades. See also:
- Is it possible to get gcc to read from a pipe?
- How to make GCC output to stdout?
The introduction of std::source_location in C++20 doesn't change that. It's just that some code will not have a well-defined source location (or it may be well-defined, but not very meaningful). Actually, I'd say that the insistence on defining source_location using files is a bit myopic... although in fairness, it's just a macro-less equivalent of __FILE__ and __LINE__ which already exist in C++ (and C), and have for decades.
@HBv6 if you print the value of __FILE__ when compiling using GCC from the standard input stream:
echo -e '#include <iostream>\\n int main(){std::cout << __FILE__ ;}' | g++ -xc++ -
running the resulting executable prints <stdin>.
Are you a language lawyer? Ok, so let's consult the standard..
The question of whether C++ program sources need to come from files is not answered clearly in the language standard. Looking at a draft of the C++17 standard (n4713), section 5.1 [lex.separate] reads:
- The text of the program is kept in units called source files in this document. A source file together with all the headers (20.5.1.2) and source files included (19.2) via the preprocessing directive #include, less any source lines skipped by any of the conditional inclusion (19.1) preprocessing directives, is called a translation unit.
So, the source code is not necessarily kept in a file per se, but in a "unit called a file". But then, where do the includes come from? One would assume they come from files... but that too is not mandated.
At any rate, std::source_location does not seem to change that in C++20 (AFAICT).
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3That pipe is a "source file" for the purposes of the standard. – melpomene 22 hours ago
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4I'm looking at the C standard, which defines: "The text of the program is kept in units called source files, (or preprocessing files) in this International Standard." So wherever the code is stored, that's a "source file" in Standardese. (Addendum: Similar language is found in the C++ standard under [lex].) – melpomene 22 hours ago
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7@melpomene: The units are just called source files, it doesn't say that they actually have to be source files. But I'll edit the answer to include this. – einpoklum 22 hours ago
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2@Barmar: The answer seems to be "Yes, we are." But hey - don't shoot the messenger, I didn't write the standard :-) – einpoklum 13 hours ago
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8Just tried this with GCC: "echo '#include <stdio.h>\\nint main(){printf("%s\\\\n", __FILE__); return 1;}' | gcc -o test -xc -" (without quotes). When executed, it prints out <stdin>. – HBv6 10 hours ago
Even before c++20, the standard has had:
__FILE__The presumed name of the current source file (a character string literal).
The definition is same for source_location::file_name.
As such, there has not been a change in regard to support for filesystemless implementations in c++20.
The standard doesn't exactly define what "source file" means, so whether it refers to a file system may be up to interpretation. Presumably, it could be conforming for an implementation to produce "the handwritten note that you gave to me just then" if that indeed identifies the "source file" in that implementation of the language.
In conclusion: Yeah, sources are referred to as "files" by the standard, but what a "file" is and whether a file system is involved is unspecified.
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What does "presumed" mean in this context? Can there be an ambiguity? – Yksisarvinen 23 hours ago
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4
#linecan update the name – JVApen 23 hours ago -
I'm wondering, does my initial claim (not stored in file) contain a mistake, or is there a behavior for that case? – JVApen 23 hours ago
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2@Yksisarvinen I don't know exactly the intention of the "presumption" qualification of the rule, but I presume :) that it is an clarification that the file name doest need to be absolute or canonical, but rather a relative name from perspective of the compiler is sufficient. I could be wrong. – eerorika 22 hours ago
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I can just see
scanner-c++returning "Left-cabinet, third-drawer, fourth red-tabbed folder, page 17". – dmckee 2 hours ago
__FILE__. Classsource_locationjust allows you to get it at function call site. – StaceyGirl 23 hours ago<iostream>that may not be files (if you see what I mean), not the files written by developers? – Neil Butterworth 23 hours ago