Is it possible for a missing #include to break the program at runtime?

Question

Is there any case, where missing a #include would break the software at runtime, while the build still goes through?

In other words, is it possible that

#include "some/code.h" complexLogic(); cleverAlgorithms(); 

and

complexLogic(); cleverAlgorithms(); 

would both build successfully, but behave differently?

Answer 1

Yes, it's perfectly possible. I'm sure there are lots of ways, but suppose the include file contained a global variable definition which called a constructor. In the first case the constructor would execute, and in the second it wouldn't.

Putting a global variable definition in a header file is poor style, but it's possible.

Answer 2

Yes, that's possible.

Everything concerning #includes happens at compile time. But compile time things can change behavior at runtime, of course:

some/code.h:

#define FOO int foo(int a) { return 1; } 

then

#include <iostream> int foo(float a) { return 2; }  #include "some/code.h"  // Remove that line  int main() {   std::cout << foo(1) << std::endl;   #ifdef FOO     std::cout << "FOO" std::endl;   #endif } 

With the #include, overload resolution finds the more appropriate foo(int) and hence prints 1 instead of 2. Also, since FOO is defined, it additionally prints FOO.

That's just two (unrelated) examples that came to my mind immediately, and I'm sure there are plenty more.