了解 Lambda 闭包类型如何删除默认构造函数

Question

来自 5.1.2

[19] The closure type associated with a lambda-expression has a deleted (8.4.3) default constructor and a deleted copy assignment operator. It has an implicitly-declared copy constructor (12.8) and may have an implicitlydeclared move constructor (12.8). [ Note: The copy/move constructor is implicitly defined in the same way as any other implicitly declared copy/move constructor would be implicitly defined. —end note ]

我正在通读 C++ Primer 14.8.1，其中解释了编译器将 lambda 表达式翻译成未命名 class 的未命名对象。如果删除默认构造函数，我如何定义不包含 lambda 捕获的 lambda 函数对象？

 auto g = [](){};

这在概念上与...不同吗

 class lambdaClass{
 public:
      lambdaClass() = delete;
      lambdaClass& operator=(const lambdaClass&) = delete;
      void operator()(){ }

      //copy/move constructor and destructor implicitly defined
};

auto g = lambdaClass(); //would be an error since default is deleted.

如果有一个捕获，那么将定义一个构造函数 other 而不是默认构造函数，并且可以初始化这样的对象（只要传递了一个参数） .但是如果没有capture，删除了默认的构造函数，那么lambda class object 是可以创建的，在概念上似乎并不一致。

编辑：嗯，也许 lambda class 根据其 lambda 捕获创建构造函数的概念是没有根据的，尽管这是 C++ Primer 中描述的方式（我在标准中找不到它的引用），因为即使我希望它在概念上有效，以下代码也不起作用：

int sz = 2;
auto a = [sz](){ return sz;};
decltype(a) b(10); //compiler error
decltype(a) b = a; //all good though

Answer 1

closure到lambda的关系类似于object到class.

C++11 标准 说 closure! 类型没有默认构造函数，这是正确的，因为它没有不要说它没有构造函数。

lambda 用于创建闭包。但是您引用的段落将更改 C++14.

ClosureType() = delete;                     // (until C++14)
ClosureType(const ClosureType& ) = default; // (since C++14)
ClosureType(ClosureType&& ) = default;      // (since C++14)

Closure types are not DefaultConstructible. Closure types have a deleted (until C++14) no (since C++14) default constructor. The copy constructor and the move constructor are implicitly-declared (until C++14) declared as defaulted (since C++14) and may be implicitly-defined according to the usual rules for copy constructors and move constructors.

http://en.cppreference.com/w/cpp/language/lambda

Answer 2

因为赋值右边是一个临时对象(右值)，所以'g'被移动赋值

了解 Lambda 闭包类型如何删除默认构造函数

Understanding how Lambda closure type has deleted default constructor

c++

lambda

constructor

default