用.depends指定SUBDIRS项目之间的依赖项不起作用

Question

I'm having problems setting up dependencies between subprojects in the context of a qmake SUBDIRS project.

I'm using the .depends option to establish the dependency of one project on another, expecting the former to be recompiled when the latter is modified, but this doesn't happen - the latter is recompiled after the modification, but the dependent project is not.

Am I misunderstanding the meaning of the .depends option?

Here's a minimal example that illustrates the problem:

+mysubdirs/
| mysubdirs.pro
| +mylib/
| | mylib.pro
| | MyClass.h
| | MyClass.cpp
| +myapp/
| | myapp.pro
| | main.cpp

mysubdirs.pro :

TEMPLATE = subdirs
SUBDIRS += mylib
SUBDIRS += myapp
CONFIG += ordered
myapp.depends = mylib

mylib.pro :

TEMPLATE = lib
CONFIG += staticlib
HEADERS = MyClass.h
SOURCES = MyClass.cpp

myapp.pro :

TEMPLATE = app
SOURCES = main.cpp
LIBS += "../../build-mysubdirs/mylib/libmylib.a"
INCLUDEPATH += ../mylib

MyClass.h :

#ifndef MYCLASS_H
#define MYCLASS_H
class MyClass
{
public:
    MyClass();
};
#endif // MYCLASS_H

MyClass.cpp :

#include "MyClass.h"
#include <iostream>
MyClass::MyClass()
{
    std::cout << "in constructor" << std::endl;
}

main.cpp :

#include "MyClass.h"
int main()
{
    MyClass x;
    return 0;
}

Compile the subdirs project. Then change the string "in constructor" to something else and recompile: the output of myapp is unchanged.

Answer 1

Am I misunderstanding the meaning of the .depends option?

Yes, you are.

Basically, qmake is a "Makefile"-generator. And ".depends" here means "a dependency in a generated Makefile". So myapp.depends = mylib becomes myapp: mylib in a root Makefile.

However, both mylib and myapp (being SUBDIRS ) are just .PHONY targets which perform recursive make invocations. So, at the end of the day, make will run two sub-makes, which absolutely do not depend on each other (as it's usual with a recursive make pattern), except one sub-make always runs before the other (because of a target-prerequisite relation in a root Makefile).

By the way, it means that CONFIG += ordered makes no sense here and should be omitted (in fact, it's deprecated in favour of .depends ).

Now, considering recompilation of myapp : it turns out that your executable DOES NOT depend on your library (in a "make-sense"), except that library is mentioned in $$LIBS (ie linker flags). To fix this issue you have to add your library to the application target dependency list manually:

myapp.pro

PRE_TARGETDEPS += path/to/mylib.a