How to add a hash character (#) to a qmake variable - qmake

Question

I want to write a configuration file with qmake that #defines a few values. But I cannot simply create variables that contain the hash or pound character (#). Nonworking example:
lines = "/* Autogenerated: do not edit */"
if(foo): lines += "#define MYLIB_WITH_FOO 1"
else: lines += "#define MYLIB_WITH_FOO 0"
write_file(config.h, lines)
The hash starts a comment (inside the string!), so this won't work. How to generate the proper #defines for write_file under qmake?

Answer 1

There's a predefined variable called LITERAL_HASH specially created to deal with this problem.
If this name seems too long you can create one of your own:
H = $$LITERAL_HASH
lines = "/* Autogenerated: do not edit */"
if(foo): lines += "$${H}define MYLIB_WITH_FOO 1"
else: lines += "$${H}define MYLIB_WITH_FOO 0"
write_file(config.h, lines)

Answer 2

The trick is to use $$system() to create the hash character. This example works for me under Windows and Linux:
pound = $$system(printf $$system_quote(\43))
if(foo): lines += "$${pound}define MYLIB_WITH_FOO 1"
else: lines += "$${pound}define MYLIB_WITH_FOO 0"

Answer 3

It is usual on a C or C++ application sources to include a "config.h" header, that is generated by the buildsystem from a template (for instance "config.h.in"). This is available using autotools, and also CMake - see: configure_file(). But what about Qmake?
Here is an alternative using QMAKE_SUBSTITUTES. Another reference.
test.pro
TEMPLATE = app
QT = core
CONFIG += cmdline c++11
VERSION = 1.2.3
FOO = 1
QMAKE_SUBSTITUTES = config.h.in
SOURCES += main.cpp
DISTFILES += config.h.in
config.h.in
/* Autogenerated: do not edit */
#ifndef CONFIG_H
#define CONFIG_H
#define MYLIB_VERSION '"$$VERSION"'
#define MYLIB_BANNER '"Project version $$VERSION created with Qt $$QT_VERSION"'
#define MYLIB_WITH_FOO $$FOO
#endif // CONFIG_H
main.cpp
#include <QCoreApplication>
#include <QDebug>
#include "config.h"
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
qDebug() << MYLIB_BANNER;
if (MYLIB_WITH_FOO) {
qDebug() << "Foo included!";
}
return 0;
}
Output:
Project version 1.2.3 created with Qt 5.12.5
Foo included!