C++: Undefined symbols for architecture x86_64: namespace not recognized [duplicate]

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• What is an undefined reference/unresolved external symbol error and how do I fix it? 27 answers

I am trying to compile a program in c++ using xCode but I am getting the following error:

   Undefined symbols for architecture x86_64:
   "BPN::NetworkTrainer::Train(BPN::TrainingData const&)", referenced from:
  _main in main.o
  "BPN::NetworkTrainer::NetworkTrainer(BPN::NetworkTrainer::Settings const&, BPN::Network*)", referenced from:
  _main in main.o
  "BPN::TrainingDataReader::ReadData()", referenced from:
  _main in main.o
  "BPN::TrainingDataReader::TrainingDataReader(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, int, int)", referenced from:
  _main in main.o
  "BPN::Network::Network(BPN::Network::Settings const&)", referenced from:
  _main in main.o
  ld: symbol(s) not found for architecture x86_64
  clang: error: linker command failed with exit code 1 (use -v to see invocation)

Do you know what am I missing?

Thank you!!!