Xcode C++ :: Duplicate Symbols for Architecture x86_64

For me, changing 'No Common Blocks' from Yes to No ( under Targets->Build Settings->Apple LLVM - Code Generation ) fixed the problem.

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Problem is that main.cpp has included B.cpp and A.cpp. In your build process, you are also compiling B.cpp and A.cpp and trying to link B.o and A.o alongwith main.o.

Linking B.o and A.o causes symbols display and square to be defined multiple times. display is defined 3 times and square defined 2 times.

You just compile and build main.cpp. Do not build A.cpp and B.cpp.

Second way is that make A.cpp and B.cpp to A.h and B.h and functions inline. So, they will be compiled only once.

Third way, do not include B.cpp in main.cpp. Just put function declaration instead of inclusion.

//main.cpp

void square(int);

int main() {
  square(5);
  return 0;
}

Generally, function declarations are put in header files. If that is required in multiple cases, make a header file.