Uses undefined class - Forward Declaration C++
1473
When can I use a forward declaration?
127172
Why is template argument deduction disabled with std::forward?
6292
What is an undefined reference/unresolved external symbol error and how do I fix it?
129766
decltype in class method declaration: error when used before "referenced" member is declared
66040
C++ unresolved external symbol
113128
C++ Default Argument Error
72605
How to solve "Unresolved inclusion: " in a C++ file in Eclipse CDT?
70199
When do I use a dot, arrow, or double colon to refer to members of a class in C++?
73291
How to use a member variable as a default argument in C++?
101951
Undefined reference to class::function in c++
54646
How undefined is undefined behavior?
29948
How to forward declare a template class in namespace std?
148678
What's the difference between std::move and std::forward
140952
warning in extern declaration
56989
Why is "using namespace std;" considered bad practice?
68463
How to resolve the linker error in C++ compiler
153232
Forward declaration of a typedef in C++