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可以将非静态值作为函数中的默认参数?我看到过对同一问题的一些较旧的回答，这些回答总是以明确写出重载而告终.这在 C++17 中还有必要吗?

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?

我想做的是做一些类似的事情

What I'd like to do is do something akin to

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

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

而不必写

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

  void foo(){
    foo(N);
  }

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

这使得重载的目的不那么明显.

which makes the purpose of the overload less obvious.

推荐答案

一种相对优雅的方式(在我看来)是使用 std::optional 来接受参数，如果没有参数提供，使用对象的默认值:

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);
}

您可以在标准的第 11.3.6 节中找到有关哪些有效/无效的完整说明.第 9 小节描述了成员访问(摘录):

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):

非静态成员不应出现在默认参数中，除非它显示为类成员访问表达式的 id 表达式(8.5.1.5) 或者除非它用于形成指向成员的指针(8.5.2.1).[示例:下例中X::mem1()的声明格式错误，因为没有为非静态提供对象memberX::a 用作初始化器.

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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