有一个双重模板化的错误实现

在GameBoard.h文件里：

//
// Created by Administrator on 2019/2/6.
//

#ifndef GAMEBOARDTEMPLATE_GAMEBOARD_H
#define GAMEBOARDTEMPLATE_GAMEBOARD_H
#include<iostream>
#include <vector>

template<typename T>
class GameBoard
{
public:
    GameBoard();
    GameBoard(size_t inWidth,size_t inHeight);

    template<typename E>
    GameBoard(const GameBoard<E>& src);
    template<typename E>
    GameBoard<T>& operator = (const GameBoard<E>& rhs);

    void setElem(size_t x,size_t y,const GameBoard& inElem);
    T& getElem(size_t x,size_t y);
    virtual ~GameBoard();

public:
    static const size_t mDefalutWidth = 10;
    static const size_t mDefalutHeight = 10;
    size_t getWidth(){return mWidth;}
    size_t getHeight(){return mHeight;}

private:
    size_t mWidth,mHeight;
    std::vector<std::vector<T>> mCells;
    template<typename E>
    void copyFrom(const GameBoard<E>& src);
    void initialContainer();
    bool inRange(size_t x,size_t inX)
    {
        return x > inX ? false : true;
    }
};



#endif //GAMEBOARDTEMPLATE_GAMEBOARD_H

在GameBoard.cpp文件里：

#include "GameBoard.h"

template<typename T>
GameBoard<T>::GameBoard()
:mWidth(0)
,mHeight(0)
{
    initialContainer();
}

template<typename T>
GameBoard<T>::GameBoard(size_t inWidth,size_t inHeight)
:mWidth(inWidth > mDefalutWidth ? mDefalutHeight : inWidth)
,mHeight(inHeight > mDefalutHeight ? mDefalutHeight : inHeight)
{

}

template<typename T>
template<typename E>
GameBoard<T>::GameBoard(const GameBoard<E>& src)
{
    copyFrom(src);
}

template<typename T>
template<typename E>
GameBoard<T>& GameBoard<T>::operator = (const GameBoard<E>& rhs)
{
    copyFrom(rhs);
    return *this;
}

template<typename T>
void GameBoard<T>::setElem(size_t x,size_t y,const GameBoard& inElem)
{
    if(!inRange(x,mWidth) || !inRange(y,mHeight))
    {
        throw std::invalid_argument("invalid argument
");
    }
    this->mCells[x][y] = inElem;
}

template<typename T>
T& GameBoard<T>::getElem(size_t x,size_t y)
{
    if(!inRange(x,mWidth) || !inRange(y,mHeight))
    {
        throw std::invalid_argument("invalid argument
");
    }
    return this->mCells[x][y];
}

template<typename T>
GameBoard<T>::~GameBoard()
{

}

template<typename T>
template<typename E>
void GameBoard<T>::copyFrom(const GameBoard<E> &src)
{
    mWidth = src.getWidth();
    mHeight = src.getHeight();
    initialContainer();

    for(int i = 0;i < mWidth;++i)
    {
        for(int j = 0;j <mHeight;++j)
        {
            mCells[i][j] = src.mCells[i][j];
        }
    }
}

template<typename T>
void GameBoard<T>::initialContainer()
{
    mCells.resize(mWidth);
    for(auto &item : mCells)
    {
        item.resize(mHeight);
    }
}

在main()函数体内：

#include <iostream>
#include "GameBoard.h"
#include "GameBoard.cpp"
int main()
{
    GameBoard<int> myIntBoard;
    GameBoard<double> myDoubleBoard;

    myDoubleBoard = myIntBoard;
    return 0;
}

会报错这是为什么！

错误是：

required from 'GameBoard<T>& GameBoard<T>::operator=(const GameBoard<E>&) [with E = int; T = double]'

passing 'const GameBoard<int>' as 'this' argument discards qualifiers [-fpermissive]mWidth = src.getWidth();