have to create a pointer object to change device.

ofVideoGrabber change device

#include "ofApp.h"
vector<ofVideoDevice> devices;
int deviceCount = 0;
ofVideoGrabber * vidGrabber = NULL;
int camWidth;
int camHeight;
//--------------------------------------------------------------
void ofApp::setup() {
	
    camWidth = 320;
    camHeight = 240;

	vidGrabber = new ofVideoGrabber();
	devices = vidGrabber->listDevices();
    for(int i = 0; i < devices.size(); i++){
        if(devices[i].bAvailable){
            ofLogNotice() << devices[i].id << ": " << devices[i].deviceName;
        }else{
            ofLogNotice() << devices[i].id << ": " << devices[i].deviceName << " - unavailable ";
        }
    }
	
	setDevice(devices[0]);
    ofSetVerticalSync(true);
}


//--------------------------------------------------------------
void ofApp::update(){
    if (vidGrabber) vidGrabber->update();
}

//--------------------------------------------------------------
void ofApp::draw(){
    ofSetHexColor(0xffffff);
    if (vidGrabber) vidGrabber->draw(20, 20);
}

//--------------------------------------------------------------
void ofApp::setDevice(ofVideoDevice &device) {
	
	if(vidGrabber && vidGrabber->isInitialized()) {
		vidGrabber->close();
		delete vidGrabber;
	}
	vidGrabber = new ofVideoGrabber();
	cout << "Device " << device.deviceName << " ID " << device.id << endl;
	vidGrabber->setDeviceID(device.id);
	vidGrabber->initGrabber(camWidth, camHeight);
	camWidth = vidGrabber->getWidth(); camHeight = vidGrabber->getHeight();
}

//--------------------------------------------------------------
void ofApp::keyPressed(int key){
	if (key == OF_KEY_UP) {
		deviceCount ++;
		deviceCount %= devices.size();
		setDevice(devices[deviceCount]);
	}
	if (key == OF_KEY_DOWN) {
		deviceCount ++;
		if(deviceCount < 0) {
			deviceCount = devices.size()-1;
		}
		setDevice(devices[deviceCount]);
	}
}

//--------------------------------------------------------------
void ofApp::keyReleased(int key){
}

//--------------------------------------------------------------
void ofApp::mouseMoved(int x, int y){
}

//--------------------------------------------------------------
void ofApp::mouseDragged(int x, int y, int button){
}

//--------------------------------------------------------------
void ofApp::mousePressed(int x, int y, int button){
}

//--------------------------------------------------------------
void ofApp::mouseReleased(int x, int y, int button){
}

//--------------------------------------------------------------
void ofApp::mouseEntered(int x, int y){
}

//--------------------------------------------------------------
void ofApp::mouseExited(int x, int y){
}

//--------------------------------------------------------------
void ofApp::windowResized(int w, int h){
}

//--------------------------------------------------------------
void ofApp::gotMessage(ofMessage msg){
}

//--------------------------------------------------------------
void ofApp::dragEvent(ofDragInfo dragInfo){
}