使用map& lt; const字符串，function& lt ;?(?)& gt;命令; [英] Parsing commands using map<const string, function<? (?)>> commands;

问题描述

问题陈述:

给出交互式输入，在适当的对象(银行，BankRegister，BasicAccount等)上调用适当的方法.

Given the interactive input, call the appropriate methods on appropriate objects (Bank, BankRegister, BasicAccount, etc...).

输入(每个命令在单独的行上):

input (each command on a separate line):

create_bank <bank_name>\n
create_customer <bank_number> <customer_name>\n
create_account <bank_number> <customer_number>\n
deposit <amount> <account_number>/<bank_number>\n
etc...

建议的解决方案:

#include <functional>

bool create_bank(const string& arguments){
    const string& bankName = arguments; //no need to parse here
    bankRegister.registerBank(new Bank(bankName,
                                       &bankRegister)); //can't access bankRegister!
    return true;
}

int main(){
    map<string, function<bool (string)>> commands;
    commands.emplace("create_bank",create_bank);
    ...
    BankRegister bankRegister;

    string command, arguments;
    cin>>command;
    getline(cin, arguments);
    commands[command](arguments);
    ...
}

为什么它不起作用:

1. 不能通过函数访问bankRegister对象.我可能会通过对函数的常量引用来传递它，但只有某些函数需要它.我当时正在考虑使其成员(m_banks，m_next_bankNumber)成为静态成员，但随后我必须将其公开，这可能是一个错误的设计决定.
2. 参数解析应该在哪里进行?这样可以吗?

3. 返回值应代表什么?

1. The bankRegister object can't be accessed from the functions. I might pass it by constant reference to the functions, but only some functions need it. I was thinking of making its members (m_banks, m_next_bankNumber) static, but then I would have to make them public, which is probably a bad design decision.
2. Where should the argument parsing happen? Is it OK this way?

3. What should the return value represent?

a)参数解析成功

b)参数成功解析并且命令成功执行

b) arguments parsed successfully and command executed successfully

对于某些错误，我使用了异常，但是除了cerr之外，有些是无声的.

I use the exceptions for some of the errors, but some are silent except for cerr.

我可以在这里使用可变参数函数来解析参数吗?

Could I use variadic functions here to parse the arguments?

推荐答案

您可以更改 create_bank 函数的签名，并使用 std :: bind 绑定 BankRegister 参数，使用调用签名 bool(const string&)

You could change the signature of the create_bank function and use std::bind to bind the BankRegister argument, creating a function object with the call signature bool(const string&)

bool create_bank(const string& arguments, BankRegister& bankRegister){
    const string& bankName = arguments; //no need to parse here
    bankRegister.registerBank(new Bank(bankName,
                                       &bankRegister));
    return true;
}

int main(){
    BankRegister bankRegister;
    map<string, function<bool (string)>> commands;
    using std::placeholders::_1;
    commands.emplace("create_bank",std::bind(create_bank, _1, std::ref(bankRegister)));

您可以使用lambda函数执行相同的操作:

You can do the same thing with a lambda function:

    commands.emplace("create_bank", [&bankRegister](const string& args) { return create_bank(args, bankRegister); });

bind 返回的函数对象和lambda表达式创建的闭包对象都具有必需的调用签名 bool(const string&)，但它们还包含对 bankRegister 对象的引用，并将其传递给 create_bank

The function object returned by bind and the closure object created by the lambda expression both have the required call signature, bool(const string&), but they also contain a reference to the bankRegister object and can pass it to create_bank

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