foo will be printed first, as the objects are initialized in the order of their declarations. Run and see:
By the way, __attribute__((constructor)) is not Standard C++. It is GCC’s extension. So the behavior of your program depends on how GCC has defined it. In short, it is implementation-defined, according to it foo is printed first.
The doc says,
The constructor attribute causes the function to be called automatically before execution enters main (). Similarly, the destructor attribute causes the function to be called automatically after main () has completed or exit () has been called. Functions with these attributes are useful for initializing data that will be used implicitly during the execution of the program.
You may provide an optional integer priority to control the order in which constructor and destructor functions are run. A constructor with a smaller priority number runs before a constructor with a larger priority number; the opposite relationship holds for destructors. So, if you have a constructor that allocates a resource and a destructor that deallocates the same resource, both functions typically have the same priority. The priorities for constructor and destructor functions are the same as those specified for namespace-scope C++ objects (see C++ Attributes).
I think the text in bold implies, the objects are initialized in the order of their declarations, as I said before, which is pretty much confirmed by online demo also.
I guess you would also like to read this:
If you want to control/alter the initialization order, you can use init_priority attribute, providing priority. Taken from the page:
Some_Class A __attribute__ ((init_priority (2000)));
Some_Class B __attribute__ ((init_priority (543)));
Here, B is initialized before A.