'＆'：对绑定成员函数表达式的非法操作。从KeyPoint向量OpenCV获取容量

我的项目有问题，就像我在主题中写的一样。我想从的表中获取容量KeyPoint vector。请告诉我，我的问题在哪里？

这是我的代码

class SidesOfFigure
{
public:
    SidesOfFigure() {
    };


    ~SidesOfFigure() {
    };

    //sideNumber == 0 => top
    //sideNumber == 1 => down
    //sideNumber == 2 => left
    //sideNumber == 3 => right
    void setSideOfFigure(vector <KeyPoint> keyPoints, int sideNumber) {
        sideOfFigure[sideNumber] = keyPoints;
    }
    uint64 getCapacityOfSide(int numberOfSide) {
        try
        {
            return sideOfFigure[numberOfSide].capacity;
        }
        catch (const std::exception&)
        {
            throw exception("Illegal number!");
        }
    }

private:
    vector <KeyPoint> sideOfFigure[4];
};