已定义方法 （在btw类中），但是编译器（gcc 6.3）仍然说不是：

template<OperatorId op>
class ChangeTypeExpression : public UnaryExpression {
 public:
  /* Ctor here... */

  // The compiler complains about the destructor
  virtual ~ChangeTypeExpression() {}

  // ... and it doesn't like this method too
  virtual string ToString(bool verbose) const {
    return ...;
  }

  // ...

  // Maybe relevant - I already don't know...
  DISALLOW_COPY_AND_ASSIGN(ChangeTypeExpression);
};

编译器输出如下：

[ 24%] Building CXX object src/engine/executor/CMakeFiles/supersonic.dir/expression/core/regexp_expressions.cc.o
/home/asg/work/raijin/src/engine/executor/expression/core/elementary_expressions.cc:61:11: warning: ‘supersonic::{anonymous}::ChangeTypeExpression<op>::~ChangeTypeExpression() noexcept [with supersonic::OperatorId op = (supersonic::OperatorId)265u]’ declared ‘static’ but never defined [-Wunused-function]
   virtual ~ChangeTypeExpression() {}
           ^
/home/asg/work/raijin/src/engine/executor/expression/core/elementary_expressions.cc:67:18: warning: ‘std::__cxx11::string supersonic::{anonymous}::ChangeTypeExpression<op>::ToString(bool) const [with supersonic::OperatorId op = (supersonic::OperatorId)265u]’ declared ‘static’ but never defined [-Wunused-function]
   virtual string ToString(bool verbose) const {
                  ^~~~~~~~

可以找到原始代码here，请注意，这不是我的代码-Google的人已经编写了它。我没想到为什么会发生这种情况，有人知道答案吗？