In C++, Is it good form to write code that executes before main()?

Question

The constructors of globally declared classes are invoked before main is entered. While this can be confusing to a new reader of the code because it is done so infrequently, is it necessarily a bad idea?

Answer 1

It's not necessarily a bad idea, but usually, yes.

First, it's global data, and globals are usually a bad thing. The more global state you have, the harder it becomes to reason about your program.

Second, C++ doesn't guarantee intialization order of static objects defined in different translation units (.cpp files) -- so if they depend on one another, you might be in trouble.

Answer 2

Yes, this is bad. Since you will have no way to catch exceptions and deal with them, the default handler will be used. In C++ that means calling terminate...

Example: contents a.cpp

#include <stdexcept>

int f() { throw std::runtime_error("boom"); return 42; }
int i = f();

int main(int argc, char* argv[])
{
  return 0;
}

Output of : g++ a.cpp && ./a.out

terminate called after throwing an instance of 'std::runtime_error'
  what():  boom
Aborted (core dumped)

You can try add a try ... catch in your main, that won't help.

Edit: Jalf's points are valid too. Listen to his advice.

Answer 3

In addition to being of questionable form - It will not be portable to some platforms.