C++ object-like macro containing URL interpreted as comment - c++

Question

Recently I run into strange behavior of my code:
#define STR(X) #X
#define XSTR(X) STR(X)
#define TEST_SERVER_HTTP http://host.cz/import.php
qDebug() << "Test server: " XSTR(TEST_SERVER_HTTP);
outputting just "http:" text.
I have figured out, that the "//" part of the URL is here interpreted as C++ comment, cutting the rest of URL text away.
Obvious workaround would be
#define TEST_SERVER_HTTP http:\/\/host.cz\/import.php
however I am getting (G++) warning
invalid string literal, ignoring final '\' [enabled by default]
wherever macro is used, even though the compiled code runs as expected.
Questions are:
Is there some "correct" way in C++ how to quote slash to suppress double slash meaning of start of comment ?
Is there some "canonical" workaround for putting URL constants as replace value of object-like macro ?
And no
#define TEST_SERVER_HTTP "http://host.cz/import.php"
is not acceptable for reasons given in the rest of my code.

Answer 1

It can be done easily, but you need to hack into it (as always):
#define STRINGIFY_(x) #x
#define STRINGIFY(x) STRINGIFY_(x)
#define SLASH() /
#define DOUBLE_SLASH() SLASH()SLASH()
#define STH_BEFORE() sth/before
#define STH_AFTER() sth/after
#define STH_DOUBLE_SLASH_STH STH_BEFORE()DOUBLE_SLASH()STH_AFTER()
int main()
{
//test
std::cout << STRINGIFY(STH_DOUBLE_SLASH_STH) << std::endl;
}
Output:
sth/before//sth/after
Usable new-macro-non-definiable version would be:
#define PLACEBO(x) x
#define STH_DOUBLE_SLASH_STH(x,y) PLACEBO(x)DOUBLE_SLASH()PLACEBO(y)
int main()
{
std::cout << STRINGIFY(STH_DOUBLE_SLASH_STH(before, after)) << std::endl;
}
Tested with MSVC 2015 and GCC 5.3.0.
Also note, that MSVC allows such comment (this is not sane!)
DOUBLE_SLASH() fun comment

Answer 2

You can't escape tokens in C/C++; the only place that backslash has an effect is inside string literals. Outside of a string literal (and when not part of a universal character name), a backslash is just a one-character token which is only valid during the preprocessing phases.
It is, however, easy enough to prevent multi-character tokens from being formed during the initial division of code into tokens, and you can do so without introducing whitespace. All you need is the identity macro:
#define I(X) X
Now you can use that to avoid the double slash becoming comments:
#define STR(X) #X
#define XSTR(X) STR(X)
#define TEST_SERVER_HTTP http:I(/)I(/)host.cz/import.php
qDebug() << "Test server: " XSTR(TEST_SERVER_HTTP);
There is no point using that on the single slash, or indeed any other character.
That solution will not, however, work to escape quotes and apostrophes (or whitespace, for that matter). As far as I know, there is no way to introduce an unmatched quote or apostrophe as a token.
Disclaimer: I didn't try that with MSVC, which is known to have a non-compliant preprocessor implementation. But it should work without issues on any standards-compliant implementation.