c++ - 为什么非可变 lambda 表达式中的字段在捕获常量值或常量引用时使用 "const"？

Question

如问题 lambda capture by value mutable doesn't work with const &? 所示, 当捕获类型为 const T& 的值时使用其名称或 [=]在可变 lambda 中，隐藏类中的字段的类型为 const T .可以争论的是，这对于可变 lambda 来说是正确的做法。
但是为什么对非可变 lambda 也这样做呢？在非可变 lambda 表达式中，operator()(...)已声明 const ，因此无论如何它都无法修改捕获的值。
当我们移动 lambda 时，就会发生这种不良后果，例如将它包装在 std::function 中时。 .
请参阅以下两个示例:

#include <cstdio>
#include <functional>

std::function<void()> f1, f2;

struct Test {
    Test() {puts("Construct");}
    Test(const Test& o) {puts("Copy");}
    Test(Test&& o) {puts("Move");}
    ~Test() {puts("Destruct");}
};

void set_f1(const Test& v) {
    f1 = [v] () {}; // field type in lambda object will be "const Test"
}

void set_f2(const Test& v) {
    f2 = [v = v] () {}; // field type in lambda object will be "Test"
}

int main() {
    Test t;
    puts("set_f1:");
    set_f1(t);
    puts("set_f2:");
    set_f2(t);
    puts("done");
}

我们得到以下编译器生成的 lambda 类:

class set_f1_lambda {
    const Test v;
public:
    void operator()() const {}
};

class set_f2_lambda {
    Test v;
public:
    void operator()() const {}
};

该程序打印以下内容(使用 gcc 或 clang):

Construct
set_f1:
Copy
Copy
Copy
Destruct
Destruct
set_f2:
Copy
Move
Move
Destruct
Destruct
done
Destruct
Destruct
Destruct

v在第一个示例中，值被复制不少于 3 次 set_f1 .
在第二个例子中 set_f2 ，唯一的拷贝是在捕获值时(如预期)。使用两次移动这一事实是 libstdc++ 中的一个实现细节。第一步发生在内部 operator=在 std::function当按值将仿函数传递给内部函数时(为什么这个函数签名不使用传递引用？)。第二个移动发生在移动构造最终堆分配的仿函数时。
但是，如果字段是 const，则 lambda 仿函数对象的移动构造函数不能为字段使用移动构造函数。 (因为这样的构造函数不能在窃取其内容后“清除”const 变量)。这就是为什么必须将复制构造函数用于此类字段的原因。
所以对我来说，将值捕获为 const 似乎只有负面影响。在非可变 lambda 表达式中。我是否遗漏了一些重要的东西，或者它只是通过这种方式标准化以某种方式使标准更简单？

Answer 1

Did I miss something important, or has it simply been standardised this way to make the standard more simpler somehow?

最初的 lambda 提议，

N2550: Lambda Expressions and Closures:Wording for Monomorphic Lambdas (Revision 4)

区分捕获对象的类型和 lambda 闭包类型的相应数据成员的类型:

/6 The type of the closure object is a class with a unique name, call it F, considered to be defined at the point where thelambda expression occurs.

Each name N in the effective capture set is looked up in the contextwhere the lambda expression appears to determine its object type;in the case of a reference, the object type is the type to which the reference refers. For each element in the effective capture set, Fhas a private non-static data member as follows:

• if the element is this, the data member has some unique name, call it t, and is of the type of this ([class.this],9.3.2);
• if the element is of the form & N, the data member has the name N and type “reference to object type of N”;5.19. CONSTANT EXPRESSIONS 3
• otherwise, the element is of the form N, the data member has the name N and type “cv-unqualified object type of N”.

在这个原始措辞中，OP 的示例不会导致 const - 合格数据成员 v .我们可能还注意到，我们承认措辞

in the case of a reference, the object type is the type to which the reference refers

存在于 [expr.prim.lambda.capture]/10 中(但直接说明数据成员的类型而不是对象类型)关于 lambdas 的最终措辞的(最新草案):

The type of such a data member is the referenced type if the entity is a reference to an object, an lvalue reference to the referenced function type if the entity is a reference to a function, or the type of the corresponding captured entity otherwise.

发生的事情是

N2927: New wording for C++0x Lambdas (rev. 2)

其中重写了 N2550 的大部分措辞:

During the meeting of March 2009 in Summit, a large number of issues relating to C++0xLambdas were raised and reviewed by the core working group (CWG). After deciding on a cleardirection for most of these issues, CWG concluded that it was preferable to rewrite the sectionon Lambdas to implement that direction. This paper presents this rewrite.

特别是，对于这个问题，解决 CWG 问题

CWG 756. Dropping cv-qualification on members of closure objects

[...] Consider the following example:

void f() {
  int const N = 10;
  [=]() mutable { N = 30; }  // Okay: this->N has type int, not int const.
  N = 20;  // Error.
}

That is, the N that is a member of the closure object is not const,even though the captured variable is const. This seems strange, ascapturing is basically a means of capturing the local environment in away that avoids lifetime issues. More seriously, the change of typemeans that the results of decltype, overload resolution, and templateargument deduction applied to a captured variable inside a lambdaexpression can be different from those in the scope containing thelambda expression, which could be a subtle source of bugs.

之后的措辞(从 N2927 开始)变成了我们最终看到的 C++11

The type of such a data memberis the type of the corresponding captured entity if the entity is not a reference to anobject, or the referenced type otherwise.

如果我敢推测，CWG 756 的决议还意味着保留 cv 限定符用于引用类型的实体的值捕获，这可能是一种疏忽。