lambda-capture by reference 中的 this 指针

Question

struct CL
{
    int i;
    void fnc()
    {
        [&this](){i=1;};  // (1) error
        [&](){i=1;};      // (2) ok
    }
};

第一种情况无效，但第二种情况有效。这是为什么？
我看到标准 5.1.2/1:

capture:
identifier
& identifier
this

（即 &this 不存在）

和 5.1.2/15：

An entity is captured by reference if it is implicitly or explicitly captured but not captured by copy

在第二种情况下 "this" 未根据 5.1.2/14 的副本捕获：

An entity is captured by copy if it is implicitly captured and the capture-default is = or if it is explicitly captured with a capture that does not include an &

但是我怎样才能通过值显式捕获 "this" 指针？或者只能隐含地使用 default-capture & ?

Answer 1

形式上，[&] 确实通过引用捕获了 this，根据 5.1.2/16 "An entity is captured by reference if it is implicitly or explicitly captured but not captured by copy." 但这并不重要, 因为

5.1.2/18 ... If this is captured, each odr-use of this is transformed into an access to the corresponding unnamed data member of the closure type, cast (5.4) to the type of this. [Note: The cast ensures that the transformed expression is a prvalue. —end note]

因此对于this，复制捕获和引用捕获是无法区分的。出于所有实际目的，this 始终由副本捕获。

lambda-capture by reference 中的 this 指针

this pointer in lambda-capture by reference

c++

lambda

c++11