Template build error: Undefined symbols for architecture x86_64?

Sounds like you need to place the template definitions of stack back in the header file. Templates form a plan for code generation, so if the compiler can't see the entire template definition and only sees the declaration, code for that specific instantion of the template will not be generated. It will simply trust the declaration and expect that at link time there exists an objects file with the instantions of those templates.

The solution to this is 1) keep the template definitions in the header file or 2) pregenerate the required definitions so the linker can find them at link time.

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