MSVC build errors > LNK2005

LNK2005: symbol already defined

error LNK2005: "int counter" (?counter@@3HA) already defined in main.obj

The linker found two definitions of the same function or variable. C++ allows one definition per program; LNK1169 usually follows with the number of symbols defined more than once.

Causes and fixes

1. A variable is defined in a header. Every .cpp file that includes the header defines it again. Declare it extern int counter; in the header and define it in one .cpp file, or make it inline int counter = 0; in C++17.
2. A function is defined in a header without inline. Mark it inline, or keep only its declaration in the header and move its body to a .cpp file. Member functions defined inside the class are already inline.
3. A .cpp file is included. #include "util.cpp" compiles that file twice. Include its header instead.
4. Runtime library symbols are defined twice. Symbols already defined in LIBCMT.lib or MSVCRT.lib mean that libraries built with /MT and /MD are mixed: use the same runtime everywhere, as for LNK2038.
5. Two libraries define the same symbol. Link only one of them, or rename the symbol in the code you control.

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Related errors: LNK2038, LNK2019. See all guides.