C++中基类如果是纯虚类，并且析构函数做了声明但未定义的话，编译可以过，链接时会导致报错说该类未定义。类有static变量成员时，如果未定义，链接会报错这个变量未定义。这些语法细节不记得的话，遇到错误时只看表面的错误信息，一下想不起来为什么报错有点浪费时间，做下记录。
例如：
E0103 22:47:55.505376 42954 class_loader_utility.cc:218] [mainboard]LibraryLoadException: /apollo/bazel-bin/modules/perception/image_stitch/../../../_solib_local/_U_S_Smodules_Sperception_Simage_Ustitch_Cimagestitch_Udceplus___Umodules_Sperception_Simage_Ustitch_Sbuild_Ux86/libdcepluscuda.so: undefined symbol: _ZTIN6apollo10perception6camera11imagestitch3pre13NormalizationE library_path:/apollo/bazel-bin/modules/perception/image_stitch/libimage_stitch_component.so
E0103 22:47:55.505409 42954 class_loader_utility.cc:234] [mainboard]shared library failed: /apollo/bazel-bin/modules/perception/image_stitch/libimage_stitch_component.so
E0103 22:47:55.505439 42954 class_loader_manager.h:78] [mainboard]Invalid class name: ImageStitchComponent
E0103 22:47:55.505457 42954 module_controller.cc:69] [mainboard]Failed to load module: modules/perception/image_stitch/dag/image_stitch.dag
E0103 22:47:55.505462 42954 class_loader_utility.cc:256] [mainboard]Attempt to UnloadLibrary lib, but can't find lib: /apollo/bazel-bin/modules/perception/image_stitch/libimage_stitch_component.so
E0103 22:47:55.505466 42954 mainboard.cc:39] [mainboard]module start error.
执行命令
nm -D /apollo/bazel-bin/modules/perception/image_stitch/../../../_solib_local/_U_S_Smodules_Sperception_Simage_Ustitch_Cimagestitch_Udceplus___Umodules_Sperception_Simage_Ustitch_Sbuild_Ux86/libdceplusnorm.so |grep "_ZTIN6apollo10perception6camera11imagestitch3pre13NormalizationE"
得到结果
U _ZTIN6apollo10perception6camera11imagestitch3pre13NormalizationE
输出显示 U（Undefined），说明这个符号在 libdceplusnorm.so 中是被引用的，而不是被定义的。执行下面的命令查看typeinfo:
c++filt _ZTIN6apollo10perception6camera11imagestitch3pre13NormalizationE
输出 typeinfo for apollo::perception::camera::imagestitch::pre::Normalization
那可能的原因是：包含Normalization类定义的源文件没有被编译进任何库或类是纯虚类，但缺少至少一个虚函数的实现，或者链接时缺少包含该类实现的库。这里属于第一种情况，Normalization类是纯虚基类，要确保至少析构函数在 .cpp 文件中有实现定义，哪怕是空的：
normalization.hpp
class Normalization {
public:
virtual ~Normalization();
// ...
};
需要在normalization.cpp里增加下面这句才不会报链接错误，因为子类析构时需要调用到基类的析构函数，基类的析构函数需要有实现：
Normalization::~Normalization() = default;
或者把normalization.hpp里的virtual ~Normalization(); 删掉也不会报链接报错，但这个不规范，相当于让编译器去隐式实现，编译器的实现做得不好可能会在运行时出现莫名其妙的崩溃问题。
加了定义后再执行命令
nm -D /apollo/bazel-bin/modules/perception/image_stitch/../../../_solib_local/_U_S_Smodules_Sperception_Simage_Ustitch_Cimagestitch_Udceplus___Umodules_Sperception_Simage_Ustitch_Sbuild_Ux86/libdceplusnorm.so |grep "_ZTIN6apollo10perception6camera11imagestitch3pre13NormalizationE"
可以看到库文件里的符号有对应地址了：
00000000002a3fb8 V _ZTIN6apollo10perception6camera11imagestitch3pre13NormalizationE
E0124 18:43:53.675529 6208 class_loader_utility.cc:218] [mainboard]LibraryLoadException: /apollo/bazel-bin/modules/perception/multi_sensor_fusion/../../../_solib_local/_U_S_Smodules_Sperception_Smulti_Usensor_Ufusion_Cmulti_Usensor_Ufusion_Ulib___Umodules_Sperception_Smulti_Usensor_Ufusion/libmulti_sensor_fusion_lib.so: undefined symbol: _ZN6apollo10perception6fusion26MultiSensorFusionComponent16s_mutex_pkslots_E library_path:/apollo/bazel-bin/modules/perception/multi_sensor_fusion/libmulti_sensor_fusion.so
E0124 18:43:53.675557 6208 class_loader_utility.cc:234] [mainboard]shared library failed: /apollo/bazel-bin/modules/perception/multi_sensor_fusion/libmulti_sensor_fusion.so
用c++filt _ZN6apollo10perception6fusion26MultiSensorFusionComponent16s_mutex_pkslots_E
可以看到MultiSensorFusionComponent类的静态成员变量s_mutex_pkslots_未正确定义，在头文件中声明了:
static std::mutex s_mutex_pkslots_;
但在.cc文件中没有定义，增加std::mutex MultiSensorFusionComponent::s_mutex_pkslots_;即可消除链接错误。