MSVC build errors > LNK2001

LNK2001: unresolved external symbol

error LNK2001: unresolved external symbol "public: static int Counter::count" (?count@Counter@@2HA)

Like LNK2019, the linker found no definition of a symbol, but here no function references it directly: the reference comes from data, such as a virtual function table or a variable.

Causes and fixes

1. A static data member is declared but not defined. Define it once in a .cpp file (int Counter::count = 0;), or declare it inline static in C++17.
2. A virtual function has no definition. Define every virtual function that is not pure, including the destructor. A class whose first virtual function is never defined has no virtual table.
3. A global variable is only declared extern. Define it, without extern, in exactly one .cpp file.
4. A library or a source file is missing. The causes of LNK2019 apply too: check that the defining file is in the project and that its library is linked for this configuration and platform.

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Related errors: LNK2019, LNK1120, LNK2038. See all guides.