简单的C ++ getter / setters [英] Simple C++ getter/setters

问题描述

最近我写的getter和setter为（注意：真正的类在getter / setter中做更多的事情）：

Lately I'm writing my getter and setters as (note: real classes do more things in getter/setter):

struct A {
  const int& value() const { return value_; } // getter
        int& value()       { return value_; } // getter/setter
 private:
  int value_;
};

它允许我执行以下操作：

which allows me to do the following:

auto a = A{2}; // non-const object a

// create copies by "default" (value always returns a ref!):
int b = a.value();  // b = 2, is a copy of value :)
auto c = a.value(); // c = 2, is a copy of value :)

// create references explicitly:
auto& d = a.value();  // d is a ref to a.value_ :)
decltype(a.value()) e = a.value(); // e is a ref to a.value_ :)
a.value() = 3; // sets a.value_ = 3 :)

cout << b << " " << c << " " << d << " " << e << endl; // 2 2 3 3

const auto ca = A{1};
const auto& f = ca.value();  // f is a const ref to ca.value_ :)
auto& g = ca.value(); // no compiler error! :(
// g = 4; // compiler error :)
decltype(ca.value()) h = ca.value(); // h is a const ref to ca.value_ :)
//ca.value() = 2; // compiler error! :)

cout << f << " " << g << " " << h << endl; // 1 1 1

这种方法不允许我：

• 验证setter的输入（这是一个很大的BUT），


• 在const成员函数中返回值我想让编译器捕捉到const对象的赋值： ca.value（）= 2 ）。更新：请参阅下面的cluracan答案。

• validate the input for the setter (which is a big BUT),
• return by value in the const member function (because I want the compiler to catch assignment to const objects: ca.value() = 2). Update: see cluracan answer below.

但是，我仍然使用这个很多，因为

However, I'm still using this a lot because

• 大多数时候我不需要它，


• 这使我能够从我们的接口中分离出我的类的实现细节，这就是我想要的。

示例：

struct A {
  const int& value(const std::size_t i) const { return values_[i]; }
        int& value(const std::size_t i)       { return values_[i]; }
  private:
   std::vector<int> values_; 
   // Storing the values in a vector/list/etc is an implementation detail.
   // - I can validate the index, but not the value :(
   // - I can change the type of values, without affecting clients :)
 }; 

现在回答问题：

• 这种方法还有其他缺点吗？


• 为什么人们喜欢：
  
  
  
  • getter / setters方法具有不同的名称？
  
  
  • 将值作为参数传递？
    只是用于验证输入或有其他主要原因吗？
  
  

  推荐答案

  
  
  • 通常使用访问器/ mutator是一个设计的气味，你的类public接口是不完整的。通常来说，你想要一个有用的公共接口，提供有意义的功能，而不是简单的get / set（这只比一个或两个步骤比我们在结构和函数的C）。每次你想写一个mutator，并且很多时候你想写一个访问器，只是退后一步，问你自己我真的*需要这个吗？
  
  
  • 只有习惯用语的人可能不准备期望这样的功能，所以它会增加维护者的时间来梳理你的代码。
  
  
  • 同名方法几乎与公共成员相同：在这种情况下只使用公共成员。当方法做两个不同的事情，命名他们两个不同的东西。
  
  
  • 非const引用返回的mutator将允许各种各样的别名问题，给成员，依靠它以后存在。通过使用单独的设置器功能，您可以防止用户对您的私人数据进行别名。
  
  
  • Generally using accessors/mutators at all is a design smell that your class public interface is incomplete. Typically speaking you want a useful public interface that provides meaningful functionality rather than simply get/set (which is just one or two steps better than we were in C with structs and functions). Every time you want to write a mutator, and many times you want to write an accessor first just take a step back and ask yourself "do I *really* need this?".
  • Just idiom-wise people may not be prepared to expect such a function so it will increase a maintainer's time to grok your code.
  • The same-named methods are almost the same as the public member: just use a public member in that case. When the methods do two different things, name them two different things.
  • The "mutator" returning by non-const reference would allow for a wide variety of aliasing problems where someone stashes off an alias to the member, relying on it to exist later. By using a separate setter function you prevent people from aliasing to your private data.

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