有一点看不懂关于四则运算

//添加内容    较复杂的四则运算
int oopcalc::mixed()
{
    int rtn=mul();
    while(shizi[pos]=='+'||shizi[pos]=='-')
    {
        int op=shizi[pos++];
        int opr2=mul();
        if(op=='+')
            rtn+=opr2;
        else
            rtn-=opr2;
    }
    return rtn;
}
int oopcalc::mul()
{
    int rtn=number();
    while(shizi[pos]=='*'||shizi[pos]=='/')
    {
        int op=shizi[pos++];
        int opr2=number();
        if(op=='*')
            rtn*=opr2;
        else
            rtn/=opr2;
    }
    return rtn;
}
int oopcalc::number()
{
    int rtn;                                                    //这里开始看不懂
    if(shizi[pos]=='(')
    {
        pos++;
        rtn=mixed();
        pos++;
    return rtn;                                              //到这里   到底什么意思
    }
    rtn=atoi(shizi+pos);
    while(isdigit(shizi[pos]))
        pos++;
    return rtn;
}


void main()
{
    oopcalc a ;
    a.menu();
}