Question: Sometimes when I change code in my Qt project (Qt Creator 2.1.0 with mingw32), the changes don't get reflected in the produced machine code after building it. This happens mostly when I change things like default values in constructors or the order of parameters in methods/constructors. Usually, a full rebuild fixes that (but takes a few minutes).

I'm helping myself by deleting the generated executables or librarys before building, which seems to help most of the time. Does that mean that theres something going wrong when linking the object files?

I'm coming from java/.net and I'm used to a different behaviour. I'd be happy if anyone could explain me what I'm doing wrong and/or point me to some related articles.

Thank you!

Answer: 
Usually, after a change in a header, all source files including that header should be rebuilt.
However, qmake is a bit peculiar in this regard, you need to set DEPENDPATH for include folders other than the current directory. E.g., if you have

```
INCLUDEPATH += somepath_in_my_project

```

also add

```
DEPENDPATH += some_path_in_my_project

```

Only with DEPENDPATH, files built by the .pro files are rebuilt if some header in some\_path\_in\_my\_project changes (if they include that header)!

I suggest to add for each INCLUDEPATH line an identical DEPENDPATH line, unless you include some system directory you don't expect to change.

**Edit:**

A similar problem exists when linking statically with qmake: If the static lib foo.a changes, binaries linking against it are not relinked. That's a bug in QMake, not generating the correct dependencies.

A workaround I found in a former project:

```
static:unix:TARGETDEPS += path_to_my/somestaticlib.a
static:win32:TARGETDEPS += path_to_my/somestaticlib.lib

```

**Edit edit:**

Since some time (Qt 5?), above code should use POST\_TARGETDEPS instead of TARGETDEPS.
If your `Makefile` (or your moral equivalent to a `Makefile`) is missing dependency information, then you can get out-of-sync builds. For every file with `#include "header.h"`, you need to make sure the `Makefile` puts `header.h` as a dependency on that file.

Here is a small piece of a `Makefile` I've got:

```
parser_yacc.c parser_yacc.h: parser_yacc.y parser.h
        $(YACC) $(YFLAGS) -o parser_yacc.c parser_yacc.y

parser_lex.c: parser_lex.l parser_yacc.h parser.h
        $(LEX) ${LEXFLAGS} -o$@ $<

parser_lex.o: parser_lex.c parser.h parser_yacc.h
        $(CC) $(EXTRA_CFLAGS) -c -o $@ $<

parser_misc.o: parser_misc.c parser.h parser_yacc.h af_names.h cap_names.h
        $(CC) $(EXTRA_CFLAGS) -c -o $@ $<

```

Each object file clearly depends upon the corresponding source and header files. If we forgot `cap_names.h`, then `parser_misc.o` wouldn't be rebuilt when `cap_names.h` is modified. Bugs can ensue.

I'm afraid the fix is long and tedious: inspect every file, make a list of its dependencies, and add missing dependencies to the `Makefile`. On a Linux platform, you could use the `strace(1)` tool to discover which files the compiler needs to open when compiling each source file, and improve the dependencies with that list. I do not know if any similar tool exists on Windows, but it would be worth spending a few minutes looking for one before diving in.