在函数中使用非静态值作为默认参数 [英] Using a non static value as default argument in a function

问题描述

是否有一种很好的方法将非静态值作为函数的默认参数?我已经看到了对同一问题的一些较旧的回答，这些回答总是最终明确地写出重载.在C ++ 17中仍然有必要吗?

Is there a nice way to have a non static value as default argument in a function? I've seen some older responses to the same question which always end up in explicitly writing out the overload. Is this still necessary in C++17?

我想做的事情类似于

class C {
  const int N; //Initialized in constructor

  void foo(int x = this->N){
    //do something
  }
}

不必写

class C {
  const int N; //Initialized in constructor

  void foo(){
    foo(N);
  }

  void foo(int x){
    //do something
  }
}

这使得过载的目的不那么明显.

which makes the purpose of the overload less obvious.

推荐答案

一种比较优雅的方式(我认为)是使用std::optional接受参数，如果未提供参数，则使用默认值对象:

One relatively elegant way (in my opinion) would be to use std::optional to accept the argument, and if no argument was provided, use the default from the object:

class C {
  const int N_; // Initialized in constructor
    public:
    C(int x) :N_(x) {}

  void foo(std::optional<int> x = std::nullopt) {
        std::cout << x.value_or(N_) << std::endl;
  }
};

int main() {
  C c(7);
  c.foo();
  c.foo(0);
}

您可以在该标准的第11.3.6节中找到有关什么有效/无效的完整解释.第9小节介绍了成员访问权限(节选):

You can find the full explanation of what works/doesn't work in section 11.3.6 of the standard. Subsection 9 describes member access (excerpt):

非静态成员不得出现在默认参数中，除非它 显示为类成员访问表达式的id表达式 (8.5.1.5)或除非用于形成指向成员的指针 (8.5.2.1).[示例:在以下示例中X :: mem1()的声明 格式错误，因为没有为非静态对象提供任何对象 memberX :: a用作初始化程序.

A non-static member shall not appear in a default argument unless it appears as the id-expressionof a class member access expression (8.5.1.5) or unless it is used to form a pointer to member (8.5.2.1).[Example:The declaration of X::mem1()in the following example is ill-formed because no object is supplied for the non-static memberX::a used as an initializer.

int b;
class X {
   int a;
   int mem1(int i = a);// error: non-static memberaused as default argument
   int mem2(int i = b);// OK; useX::b
   static int b;
};

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