What are the dangers of forward declarations?

Question

I just had an interview. I was asked what is a "forward declaration". I was then asked if they were dangers associated with forward declarations.

I could not answer to the second question. A search on the net didn't show up any interesting result.

So, do someone know any dangers associated with the use of forward declarations ?

Answer 1

Well, apart from the issues about duplication...

... there is at least one sore spot in the Standard.

If you call delete on a pointer to an incomplete type, you get undefined behavior. In practice, the destructor may not get called.

We can see that on LiveWorkSpace using the following command and sample:

// -std=c++11 -Wall -W -pedantic -O2  #include <iostream>  struct ForwardDeclared;  void throw_away(ForwardDeclared* fd) {    delete fd; }  struct ForwardDeclared {    ~ForwardDeclared() {       std::cout << "Hello, World!\n";    } };  int main() {    ForwardDeclared* fd = new ForwardDeclared();    throw_away(fd); } 

Diagnosis:

Compilation finished with warnings:  source.cpp: In function 'void throw_away(ForwardDeclared*)':  source.cpp:6:11: warning: possible problem detected in invocation of delete operator: [enabled by default]  source.cpp:5:6: warning: 'fd' has incomplete type [enabled by default]   source.cpp:3:8: warning: forward declaration of 'struct ForwardDeclared' [enabled by default]  source.cpp:6:11: note: neither the destructor nor the class-specific operator delete will be called, even if they are declared when the class is defined 

Don't you want to thank your compiler for warning you ;) ?