c – 将类更改为其子类

class Target
{
public:
    virtual void do_something()
    { log_message("NO!"); }
};

class TargetChild : public Target
{
public:
    virtual void do_something()
    { log_message("YES!"); }
};

class Caller
{
private:
    Target target;

public:
    void call_target()
    { target.do_something(); }
    void set_target(Target set_target)
    { target = set_target; }
};

int main( int argc,const char* argv[] )
{
    TargetChild targetChild;

    Caller caller;
    caller.call_target();
    caller.set_target(targetChild);
    caller.call_target();
}

日志文件中的结果是“NO！YES！”但它写了NO！两次.我真的不明白它有什么问题.