Question

As I have understood, a class can be defined in multiple translation units aslong they’re identical. With that in mind, consider the following examples:

 //1.cpp

class Foo{
 public:
  int i;
};


void FooBar();

void BarFoo(){
  Foo f;
}


int main(){
 FooBar();
 BarFoo();
}

//2.cpp

class Foo{
 public:
  std::string s;
};

void FooBar(){
  Foo f;
}

This compiles and I don’t get a crash.

If I do the following changes:

//1.cpp
 Foo FooBar();
//2.cpp
 Foo FooBar(){
   Foo f;
   return f;
 }

I get a crash. Why does one result in a crash and the other doesn’t. Also, am I not violating ODR in the first example? If I am, why does it compile ok?

Answer 1

The program is ill-formed for the reason you stated. The compiler is not required a diagnostics, but I don’t see a point in discussing reasons for a crash in an ill-formed program.

Still, let’s do it:

The first example probably doesn’t crash because FooBar‘s behavior doesn’t affect the run of main. The method is called, it does something, and that’s it.

In the second example, you attempt to return a Foo. FooBar returns the version of Foo defined in 2.cpp. main appears in 1.cpp so it expects the version of Foo defined in 1.cpp, which is a completely different version – different members, sizes. You most likely get a corruption on the destructor. (just a guess)

EDIT: this does break the one definition rule:

3.2 One definition rule [basic.def.odr]

6) There can be more than one definition of a class type […] in a program provided that each definition
appears in a different translation unit, and provided the definitions satisfy the following requirements. […]

• each definition of D shall consist of the same sequence of tokens;

[…]