问题：
Undefined symbols for architecture x86_64:
"VZNetwork::VZSocket::socket_instance", referenced from:
VZNetwork::VZSocket::initSocket() in VZSocket.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
解决方案：
socket_instance 变量是静态的，类型是int, 去掉static就可以了
