there. As title saying.
Use #include"test.h" is legal.
Use #include"test/test.h" is also legal.
But is it legal to use #include"../test.h"?
For example, #pragma once is legal in almost every compilers.
But this is not standard in C++.
I cannot find any documents which saying .. "guarantee" to be meaning of parent directory.
Can anyone help me?
The C++ standard says in §2.8/1:
Header name preprocessing tokens shall only appear within a
#includepreprocessing directive (16.2). The sequences in both forms of header-names are mapped in an implementation-defined manner to headers or to external source file names as specified in 16.2.
So here you have it: It's all implementation-defined. §16.2 then says, among some other things not directly related to your question:
A preprocessing directive of the form
#include"q-char-sequence"new-linecauses the replacement of that directive by the entire contents of the source file identified by the specified sequence between the
"delimiters. The named source file is searched for in an implementation-defined manner.
And this makes a lot of sense. Why shouldn't you be able to create a C++ compiler for file systems that do not use the ".." convention?
In summary, to answer your question: Yes, it is legal according to the C++ standard, but that's not surprising because almost everything is technically legal for an #include directive.
The more interesting question is what the documentation of your C++ implementation has to say about this. But even then you may be left with a rather generic explanation on the topic. For example, the MSVC 2015 documentation for #include says:
The path-spec is a file name that may optionally be preceded by a directory specification. The file name must name an existing file. The syntax of the path-spec depends on the operating system on which the program is compiled.