cannot use cvimageptr -> expression

edit

I am trying to write a converter from rgb to gray image

and here comes the code :

cv_bridge::CvImagePtr cv_ptr;

cv_ptr = cv_bridge::toCvCopy(img_ , enc::MONO8);

sensor_msgs::Image img2_ = cv_ptr->toImageMsg();

image_pub_.publish(img2_, *ci);

But the compiler says

/root/catkin_ws/src/usb_cam/nodes/usb_cam_node.cpp:178:51: error: conversion from ‘sensor_msgs::ImagePtr {aka boost::shared_ptr<sensor_msgs::image_<std::allocator<void> > >}’ to non-scalar type ‘sensor_msgs::Image’ requested

which maybe means it cannot access the function toImageMsg() from cv_ptr

Hope anyone could help me out of this !