我目前正在研究一个非常基本的汇编程序。汇编程序需要接收汇编指令并输出16位二进制指令，以便与我们正在使用的计算机一起使用。
我的设计策略是创建一个Command类，它有3个子类。每种类型的命令都有一个：A命令，C命令和L命令。为了识别我正在使用的命令类型，我添加了一个字符串command_type，它是＆＃34; A＆＃34;，＆＃34; C＆＃34;或＆＃34; L＆＃34;分别
修改
我在确定如何正确推导这些类时仍然遇到很多麻烦。基本上，A和L命令应该有一个＆＃34;符号＆＃34; string，表示需要转换的整数值，而C命令有＆＃34; dest＆＃34;，＆＃34; comp＆＃34;和＆＃34; jump＆＃34;也必须访问的值，但是它们没有＆＃34;符号＆＃34;值。
Command.h
#include <fstream>
#include <string>
class Command {
std::string command_type = "";
protected:
void set_commandType(std::string x){command_type = x;}
public:
Command();
virtual ~Command();
std::string commandType() const {return command_type;}
};
class A_COMMAND : public Command
{
std::string symbol;
public:
A_COMMAND(std::string s);
std::string get_symbol(){return symbol;}; //Returns the symbol or decimal Xxx of the current command @Xxx or (Xxx) . Should be called only when commandType() is A_COMMAND or L_COMMAND.
};
class C_COMMAND : public Command
{
std::string comp;
std::string dest;
std::string jump;
public:
C_COMMAND(std::string s, std::string d, std::string j);
std::string get_comp(){return comp;}; //Returns the comp mnemonic in the current C-command (28 possibilities). Should be called only when commandType() is C_COMMAND.
std::string get_dest(){return dest;}; //Returns the dest mnemonic in the current C-command (8 possibilities). Should be called only when commandType() is C_COMMAND.
std::string get_jump(){return jump;}; //Returns the jump mnemonic in the current C-command (8 possibilities). Should be called only when commandType() is C_COMMAND.
};
class L_COMMAND : public Command
{
std::string symbol;
public:
L_COMMAND(std::string s);
std::string get_symbol(){return symbol;}; //Returns the symbol or decimal Xxx of the current command @Xxx or (Xxx) . Should be called only when commandType() is A_COMMAND or L_COMMAND.
};
Command.cpp
#include "Command.h"
//---------------------------------------------
//A-Command functions
Command::Command(){}
A_COMMAND::A_COMMAND(std::string s) : symbol(s)
{
set_commandType("A");
}
//---------------------------------------------
//C-Command functions
C_COMMAND::C_COMMAND(std::string c, std::string d, std::string j) : comp(c), dest(d), jump(j)
{
set_commandType("C");
}
//---------------------------------------------
//L-Command functions
L_COMMAND::L_COMMAND(std::string s) : symbol(s)
{
set_commandType("L");
}
我有一个处理输入的Parser.cpp和Parser.h，负责创建一个命令副本：
Parser.h
#include "Command.h"
#include <vector>
#include <deque>
class Parser {
private:
std::deque<Command> commands;
public:
Parser(std::vector<std::string>);
bool hasMoreCommands() //are there more commands in the input?
{
if(commands.size() != 0)
return true;
else
return false;
}
void advance(){commands.pop_front();} //move to next command, should only work if hasMoreCommands returns false}
Command currentCommand(){return commands.front();}
std::vector<std::string> translateCommands(); //convert commands into binary strings
};
Parser.cpp
#include "Parser.h"
#include "Command.h"
#include <vector>
#include <iostream>
#include <string>
#include <unordered_map>
bool inList(std::string& str, std::vector<std::string> list) //check if a given string contains one of the elements in the comp, dest, jump vectors. if so extract string for use in constructor
{
for(auto i = list.begin(); i!=list.end(); ++i)
{
std::size_t found = str.find(*i);
if(found!=std::string::npos)
{
return true;
}
}
return false;
}
Parser::Parser(std::vector<std::string> input) {
std::vector<std::string> dest_list = {"","M","D","MD","A","AM","AD","AMD"}; //all possible dests
std::vector<std::string> comp_list = {"0","1","D","A","!D","!A","-D","-A","D+1","A+1","D-1","A-1","D+A","D-A","A-D","D&A","D|A","M","!M","-M","M+1","M-1","D+M","D-M","M-D","D&M","D|M"}; //all possible comps
std::vector<std::string> jump_list = {"","JGT","JEQ","JGE","JLT","JNE","JLE","JMP"}; //all possible jumps
std::string dest, comp, jump;
std::deque<Command> commands;
for(std::vector<std::string>::const_iterator i = input.begin(); i != input.end(); ++i)
{
std::string line = *i;
if(*line.begin()=='@') //A-command
{
A_COMMAND command(line.substr(1));
std::cout << "Command type: " << command.commandType() << "\n";
std::cout << "symbol: " << command.get_symbol() << "\n";
commands.push_back(command);
}
else if(*line.begin()=='(' && *line.rbegin() == ')' && line.size() > 2) //L-command
{
L_COMMAND command(line.substr(1, line.size() - 2));
std::cout << "Command type: " << command.commandType() << "\n";
std::cout << "symbol: " << command.get_symbol() << "\n";
commands.push_back(command); }
else
{
std::string rhs = line;
std::string dest_string = "";
std::string comp_string = "";
std::string jump_string = "";
size_t equals_pos = line.find('='); //position of = in string, if present
size_t semi_pos = line.find(';'); //position of ; in string, if present
if(equals_pos != line.npos) //if there is an = then we have a dest
{
dest_string = line.substr(0,equals_pos);
rhs = line.substr(equals_pos+1);
}
if(semi_pos != line.npos) //jump
{
comp_string = rhs.substr(0,semi_pos);
jump_string = rhs.substr(semi_pos+1);
}
else //no jump
{
comp_string = rhs;
}
//now confirm if inputs are valid
if(inList(dest_string, dest_list))
dest = dest_string;
else
std::cout << "invalid dest \n";
if(inList(comp_string, comp_list))
comp = comp_string;
else
std::cout << "invalid comp \n";
if(inList(jump_string, jump_list))
jump = jump_string;
else
std::cout << "invalid jump \n";
C_COMMAND command(comp, dest, jump);
std::cout << "Command type: " << command.commandType() << "\n";
std::cout << "dest: " << command.get_dest() << "\n";
std::cout << "comp: " << command.get_comp() << "\n";
std::cout << "jump: " << command.get_jump() << "\n";
commands.push_back(command);
}
}
}
我的main.cpp加载输入，并将其传递给解析器。我遇到的问题是我无法对输入做任何事情。
我试过写一个这样的函数：
string translateLine(Command command, Code code) //Code is a table for translating the command
{
string output;
if(command.commandType() == "A")
{
string symbol = parser.currentCommand().get_symbol();
cout << symbol << endl;
//perform binary conversion
}
/*else if(command.commandType() == "C")
{
string dest = command.get_dest();
}*/
//shouldn't be any L commands in symbol-less version
else
{
std::cout << "unexpected command value \n";
}
return output;
}
但是一旦我调用get_symbol（），编译器就不会识别该函数。我知道这是因为基本命令没有get_symbol（）函数，但我无法弄清楚如何正确地将函数添加到基类并将它们导出到较低的3.我可以＆ ＃39; t只是制作纯虚拟，因为并非每个类都使用所有函数。我怎样才能正确地完成这个？
答案 0 :(得分：2)
首先，如果translateLine()应该能够接受A_COMMAND，C_COMMAND或L_COMMAND个对象，那么它需要采用Command*参数，而不是一个Command参数。指向基类的指针可以包含指向从该基类派生的类的指针，但基类的对象不能保存派生对象。
其次，即使使用A_COMMAND指针实际指向Command对象而不执行A_COMMAND，也无法调用属于dynamic_cast的函数。 dynamic_cast可以在运行时将指针从Command*转换为A_COMMAND，如果指向的对象实际上不是NULL对象，则会返回A_COMMAND。
答案 1 :(得分：0)
基本问题是您正在重新创建运行时类型信息。您无需向class A_COMMAND添加“command_type = A”。如果至少有一个虚方法，C ++已经知道你的对象有什么类型。看起来你的类需要一个虚拟的析构函数。