无法将double转换为字符串

我一直在开发一个计算盒子体积的程序，使用类来帮助我了解如何使用类。作为程序的一部分，我想将对象的长度、宽度和高度转换为字符串，以显示框的尺寸。当我从主文件运行代码时，它会崩溃。当我从类文件中运行它时，我得到"无法将Box:：length从double转换为std:：string。如何修复转换错误？

#include <iostream>
#include <stdexcept>
#include <sstream>
#include <iomanip>
#include <string>
using namespace std;
class Box
{
    public:
        double length;//length of the box
        double height;//height of the box
        double width;//with of the box
        Box(): length(1), height(1), width(1){}
        //Parameterized Constructor
        Box(double length, double width, double height);

        double getVolume(void);
        //Mutators
        void setLength(double leng);
        void setWidth(double wid);
        void setHeight(double hei);
        //Acessors
        string toString() const;
        string getLength(); 
        string getWidth();
        string getHeight();
};//end class   
//member function definitions
    double Box::getVolume(void)//get volume will cal and output the volume when called
    {
        return length * width * height;
    }
    void Box::setLength(double leng)
    {
        const double MIN_LENGTH = 0.1;//constants for min/max for range check and out_of_range exception
        const double MAX_LENGTH = 99;
        if (length > MAX_LENGTH || length < MIN_LENGTH)
        {
            stringstream strOut;//declare string stream
            strOut << "Length is out of range. Length must be between" << MIN_LENGTH << " and " << MAX_LENGTH << ".";//error msg
            throw out_of_range(strOut.str());
        }
        else
        {
            length = leng;// if length is within range, store it 
        }
    }
    string Box::getLength()
    {
        return length;
    }
    void Box::setWidth(double wid)
    {
        const double MIN_WIDTH = 0.1;//constants for min/max for range check and out_of_range exception
        const double MAX_WIDTH = 99;
        if (length > MAX_WIDTH || length < MIN_WIDTH)
        {
            stringstream strOut;//declare string stream
            strOut << "Width is out of range. Width must be between" << MIN_WIDTH << " and " << MAX_WIDTH << ".";//error msg
            throw out_of_range(strOut.str());
        }
        else
        {
            width = wid;// width is in range, store it
        }
    }   
    string Box::getWidth()
    {
        return width;
    }
    void Box::setHeight(double hei)
    {
        const double MIN_HEIGHT = 0.1;//constants for min/max for range check and out_of_range exception
        const double MAX_HEIGHT = 99;
        if (length > MAX_HEIGHT || length < MIN_HEIGHT)
        {
            stringstream strOut;//declare string stream
            strOut << "Height is out of range. Height must be between" << MIN_HEIGHT << " and " << MAX_HEIGHT << ".";//error msg
            throw out_of_range(strOut.str());
        }
        else
        {
            height = hei;// height is in range, store it
        }
    }   
    string Box::getHeight()
    {
        return height;
    }
    string Box::toString() const
    {
        stringstream strOut;
        strOut << "Length: " << getLength() << endl
        << "Width: " << getWidth() << endl <<
        "Height: " << getHeight() << endl;
        return strOut;
    }