在Ruby中使用eval函数调用其他函数

Question

我使用下面的类与processQuestion函数调用其他方法。

通过调用其他类的CONSTANTS来调用此函数。

# Is responsible for executing a particular question. Question types are in the Question object. A question will 
# always have a responding method in this class. That method will take the parameters defined by the question and 
# should provide the answer in the format expected. 
class QuestionProcessor 
    NO_ROUTE = "NO SUCH ROUTE" 

    def initialize(routeList) 
    @routeList = routeList 
    end 


    # Finds the method and runs it. This should provide the answer object 
    def processQuestion(question) 
    return eval("get"+question.command+"(question)") 
    end 


    # Finds the total distance using the exact stations specified, or returns NO_ROUTE if no route was stored in the route list 
    # this method ignores the constraints and actions 
    def getDistance(question) 
    distance = 0 
    currentStation = nil 

    question.parameters.each do |nextStation| 
     if (! currentStation.nil?) 
     route = @routeList.getDirectRoute(currentStation, nextStation) 
     if (route.nil?) 
      return NO_ROUTE 
     end 
     distance += route.distance 
     end 
     currentStation = nextStation; 
    end 

    return distance; 
    end 


    # Finds the shortest route possible for the given constraint. This method requires a constraint and action to be provided 
    def getShortestRoute(question) 
    startStation = question.parameters[0] 
    endStation = question.parameters[1] 

    routeProcessor = ShortestRouteProcessor.new(@routeList, question.constraint, question.action) 
    routeProcessor.getRoute(startStation, endStation) 

    return routeProcessor.shortestRoute == Constants::INTEGER_MAX ? NO_ROUTE : routeProcessor.shortestRoute 
    end 


    # Counts the number of routes based on the condition provided. Intended to count the number of routes, but could potentially provide a total distance 
    # or anything else produced by the action. 
    def getCountRoutes(question) 
    startStation = question.parameters[0] 
    endStation = question.parameters[1] 

    routeProcessor = RouteProcessor.new(@routeList, question.constraint, question.action) 
    routeProcessor.getRoute(startStation, endStation) 

    return routeProcessor.totalSuccessfulRoutes 
    end 
end 

我认为这是一个很好的方法来保持干燥，但我听到eval是邪恶的。

这是一个好方法，还是应该以更面向对象的方式寻找其他方法？

Answer 1

在这种情况下，你可以放心地在这个例子中使用send代替eval，如：

def processQuestion(question) 
    return send("get#{question.command}", question) 
end 

要知道，如果你不净化你的输入send可能为eval危险（question.command在这种情况下）。

如果可能的话，请致电send（或eval）之前白名单过滤，否则有人可以通过它做你不想做的事情的命令。

Answer 2

正是这个原因，发送函数在ruby中有一个函数。它是Object类的一部分，所以一切都有它。

在这里阅读更多： http://ruby-doc.org/core-2.1.1/Object.html#method-i-send

元编程，我建议你读这整个教程： https://rubymonk.com/learning/books/2-metaprogramming-ruby/