Can't allocate class with forward declared value in std::map member variable

c++, compiler-errors, forward-declaration, stdmap

Solution

Note: as of C++11, the explanation below is outdated: standard library containers now work with incomplete types.

Why is this?

Because the C++ standard library containers are not defined to work with incomplete member types. This is by design1 – but it’s arguably an error (and might be changed in future versions of C++). The Boost.Containers library fixes this.

Your code with pointers works because a pointer to an incomplete type is itself a complete type. However, this obviously changes the semantics of your type quite drastically (in particular, who manages the memory?) and it’s generally not a good idea to use this as a replacement.

1 It’s worth pointing out that the article claims that you technically cannot implement `std::map` to work with incomplete types. However, this claim is wrong.

Problem

In `test.h`: ``` #ifndef TEST_H #define TEST_H #include <map> struct Incomplete; class Test { std::map<int, Incomplete> member; public: Test(); int foo() { return 0; } }; #endif ``` In `test.cpp`: ``` #include "test.h" struct Incomplete {}; Test::Test() {} ``` In `main.cpp` ``` #include "test.h" int main() { Test test; return test.foo(); } ``` g++ 4.7 gives me an error that `struct Incomplete` is forward declared when I write `g++ main.cpp test.h -o main.o`. However, if I change `std::map<int, Incomplete> member` to `std::map<int, Incomplete*> member`, `main.o` compiles. Why is this?

Original source