Mac OS下编译C++代码，出现如下的错误，如何解决？
$ g++ -o test main.cpp -I /Users/txgcwm/Documents/sourcecode/cplusplus/design_pattern/observer -v
Apple LLVM version 7.3.0 (clang-703.0.31)
Target: x86_64-apple-darwin15.5.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
"/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang" -cc1 -triple x86_64-apple-macosx10.11.0 -Wdeprecated-objc-isa-usage -Werror=deprecated-objc-isa-usage -emit-obj -mrelax-all -disable-free -disable-llvm-verifier -main-file-name main.cpp -mrelocation-model pic -pic-level 2 -mthread-model posix -mdisable-fp-elim -masm-verbose -munwind-tables -target-cpu core2 -target-linker-version 264.3.102 -v -dwarf-column-info -resource-dir /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/clang/7.3.0 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk -I /Users/txgcwm/Documents/sourcecode/cplusplus/design_pattern/observer -stdlib=libc++ -fdeprecated-macro -fdebug-compilation-dir /Users/txgcwm/Documents/sourcecode/cplusplus/design_pattern/observer -ferror-limit 19 -fmessage-length 181 -stack-protector 1 -fblocks -fobjc-runtime=macosx-10.11.0 -fencode-extended-block-signature -fcxx-exceptions -fexceptions -fmax-type-align=16 -fdiagnostics-show-option -fcolor-diagnostics -o /var/folders/3t/hr6c83h539s3kgfqx8fgjf7c0000gn/T/main-6f9bfd.o -x c++ main.cpp
clang -cc1 version 7.3.0 (clang-703.0.31) default target x86_64-apple-darwin15.5.0
ignoring nonexistent directory "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/usr/include/c++/v1"
ignoring nonexistent directory "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/usr/local/include"
ignoring nonexistent directory "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/Library/Frameworks"
#include "..." search starts here:
#include <...> search starts here:
/Users/txgcwm/Documents/sourcecode/cplusplus/design_pattern/observer
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/clang/7.3.0/include
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/usr/include
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/System/Library/Frameworks (framework directory)
End of search list.
"/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ld" -demangle -dynamic -arch x86_64 -macosx_version_min 10.11.0 -syslibroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk -o test /var/folders/3t/hr6c83h539s3kgfqx8fgjf7c0000gn/T/main-6f9bfd.o -lc++ -lSystem /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/clang/7.3.0/lib/darwin/libclang_rt.osx.a
Undefined symbols for architecture x86_64:
"ConcreteSubject::ConcreteSubject()", referenced from:
_main in main-6f9bfd.o
"ConcreteSubject::~ConcreteSubject()", referenced from:
_main in main-6f9bfd.o
"ConcreteObserver::ConcreteObserver()", referenced from:
_main in main-6f9bfd.o
"ConcreteObserver::~ConcreteObserver()", referenced from:
_main in main-6f9bfd.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)问题找到了，是头文件中的构造和析构函数存在问题。