“Undefined reference to” 的处理

Undefined reference to 错误:这类错误是在连接过程中出现的，可能有两种原因∶一是使用者自己定义的函数或者全局变量所在源代码文件，没有被编译、连接，或者干脆还没有定义，这需要使用者根据实际情况修改源程序，给出全局变量或者函数的定义体；二是未定义的符号是一个标准的库函数，在源程序中使用了该库函数，而连接过程中还没有给定相应的函数库的名称，或者是该档案库的目录名称有问题.

“undefined reference to ”一般是在找不到相应的库文件的时候产生的，也跟具体的makefile文件有关系。

转别人的：

尝试了许多关键词后终于在搜索“undefined reference to `typeinfo”时在http://www.wellho.net/上看到：
undefined reference to typeinfo - C++ error message

There are some compiler and loader error messages that shout obviously as to their cause, but there are others that simply don't give the new user much of an indication as to what's really wrong. And most of those I get to know pretty quickly, so that I can whip around a room during a course, making suggestions to delegate to check for missing ; characters or double quotes, to check that they have used the right type of brackets for a list subscript and haven't unintentionally written a function call, etc.

Here's one of the more obscure messages from the Gnu C++ compiler - or rather from the loader:

g++ -o polygon shape.o circle.o square.o polygon.o
circle.o(.gnu.linkonce.r._ZTI6Circle+0x8): undefined reference to `typeinfo for Shape'
square.o(.gnu.linkonce.r._ZTI6Square+0x8): undefined reference to `typeinfo for Shape'
polygon.o(.gnu.linkonce.t._ZN5ShapeC2Ev+0x8): In function `Shape::Shape()':
: undefined reference to `vtable for Shape'
collect2: ld returned 1 exit status


And you can be scratching you head for hour over that one!

The error? shape.o contains a base class from which classes are derived in circle.o and square.o .. but virtual function(s) in shape's definition are missing null bodies.

The fix? You've got line(s) like
virtual float getarea() ;
that should read
virtual float getarea() {} ;

The complete (working) source code files for this example are available here
(written 2006-07-15 06:22:59)
        按照文中所说稍微修改了一下，在析构函数后面添加了{}，再make，成功了，高兴啊！问题终于解决了。我的所有虚函数都是有定义的，没想到就因为写基类的这个虚析构函数大意，没写函数体就出现了一个困扰我几天的莫名其妙的错误。就virtual ~CSgAnalyseStatBase();和virtual ~CSgAnalyseStatBase() {};的区别，编译可以通过却搞出个莫名其妙的链接错误。链接器linker需要将虚函数表vtable 放入某个object file，但是linker无法找到正确的object文件。这个错误常见于刚刚创建一系列有继承关系的class的时候，这个时候很容易忘了给base class的virtual function加上函数实现。解决办法：给基类的virtual函数加上本来就应该有的function body。当含有虚函数的类未将析构函数声明为virtual时也会出现这个链接错误。不管如何，问题解决了，再辛苦也是值得的，以后在写代码时一定要严谨。