C++ FAQ / Section 39 / FAQ 39.14

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Revised Jul 4, 2012

[39.14] Why can't I put a forward-declared class in a std::vector<>?

Because the std::vector<> template needs to know the sizeof() its contained elements, plus the std::vector<> probably accesses members of the contained elements (such as the copy constructor, the destructor, etc.). For example,

class Fred;  // Okay: forward declaration

class Barney {
  std::vector<Fred> x;  // Error: the declaration of Fred is incomplete
};

class Fred {
  Barney* y;
};

One solution to this problem is to change Barney so it uses a std::vector<> of Fred pointers rather than a std::vector<> of Fred objects:

class Fred;  // Okay: forward declaration

class Barney {
  std::vector<Fred*> x;  // Okay: Barney can use Fred pointers
};

class Fred {
  Barney* y;
};

Another solution to this problem is to reverse the order of the classes so Fred is defined before Barney:

class Barney;  // Okay: forward declaration

class Fred {
  Barney* y;  // Okay: the first can point to an object of the second
};

class Barney {
  std::vector<Fred> x;  // Okay: Fred is fully defined at this point
};

Just remember this: Whenever you use a class as a template parameter, the declaration of that class must be complete and not simply forward declared.