Question

If I create a class like so:

// B.h #ifndef _B_H_ #define _B_H_  class B { private:     int x;     int y; };  #endif // _B_H_ 

and use it like this:

// main.cpp #include <iostream> #include <vector>  class B; // Forward declaration.  class A { public:     A() {         std::cout << v.size() << std::endl;     }  private:     std::vector<B> v; };  int main() {     A a; } 

The compiler fails when compiling main.cpp. Now the solution I know is to #include 'B.h', but I’m curious as to why it fails. Neither g++ or cl‘s error messages were very enlightening in this matter.

Answer 1

The compiler needs to know how big ‘B’ is before it can generate the appropriate layout information. If instead, you said std::vector<B*>, then the compiler wouldn’t need to know how big B is because it knows how big a pointer is.