Global vector emptying itself between calls?

Question

I have a vector in a header, like so:

extern std::vector g_vector;

In the associated cpp file I have this:

std::         


        

Answer 1

Not sure what the issue really is, but I guess the following pattern will help solve it: define an accessor to the global variable and allocate it as a static function variable as shown below.

In the header file:

std::vector<Foo> &getGlobalVector();

In the cpp file:

std::vector<Foo> &getGlobalVector()
{
  static std::vector<Foo> s_vector;
  return s_vector;
}

This pattern is inspired from Andrei Alexandrescu's "generic singleton" implementation in Modern C++ design.

I've taken the habit of systematically using this pattern whenever I fell upon an existing global variable while maintaining existing applications (or in the rare occasions I actually chose to use one myself), and it may have helped in eliminating a couple of hard-to-reproduce bugs in said applications.

At any rate, this should really help avoiding any multiple-initialization or order-of-initialization related issue.

Answer 2

Order of initialization of global values is not defined.

Read here about the static initialization fiasco.

When you declare Bar in a function - the g_vector will be initialized before, because its promised to be initialized before the program runs. If Bar is a global variable - then you have a problem.