在头文件中声明变量

我正试图为几个opencv相机编写一个包装器，但我得到了错误：‘VideoCapture’ in namespace ‘cv’ does not name a type。我想这是因为我没有在头文件中正确声明cv::VideoCapture left和cv::VideoCapture right？

立体声.h:

#ifndef _GUARD_STEREO_GUARD_
#define _GUARD_STEREO_GUARD_
#include "cv.h"
class Stereo {
public:
Stereo(int, int);
cv::Mat getLeft();
cv::Mat getRight();
private:
cv::VideoCapture left;
cv::VideoCapture right;
};
#endif

Stereo.cpp:

#include "cv.h"
#include <iostream>
#include "stereo.h"
using namespace cv;
Stereo::Stereo(int leftId, int rightId) {
    left = VideoCapture(leftId);
    right = VideoCapture(rightId);
    if (!left.opened() || !right.opened()) {
            std::cerr << "Could not open camera!" << std::endl;
    }
}
Mat Stereo::getLeft() {
    Mat frame;
    left >> frame;
    return frame;
}
Mat Stereo::getRight() {
    Mat frame;
    right >> frame;
    return frame;
}

但是，我可以成功地使用VideoCapture like this:

cv::VideoCapture capLeft; // open the Left camera
cv::VideoCapture capRight; // open the Right camera
capLeft  = cv::VideoCapture(0);
capRight = cv::VideoCapture(1);