提供给std :: unique_ptr的简单自定义删除器lambda：为什么使用by-reference capture default（[＆]）over no capture（[]）？

Question

Background

Cppreference:s section on std::unique_ptr shows the following demo for supplying a custom deleter to the unique_ptr instance:

 std::unique_ptr<D, std::function<void(D*)>> p(new D, [&](D* ptr) { std::cout << "destroying from a custom deleter...\\n"; delete ptr; }); 

Where D , for the purpose of this question, is just as simple custom type, say

struct D
{
    D() { std::cout << "D CTOR\n"; }
    ~D() { std::cout << "D DTOR\n"; }
};

Moreover, the reference above states the following type requirement on the deleter:

Type requirements

Deleter must be FunctionObject or lvalue reference to a FunctionObject or lvalue reference to function, callable with an argument of type unique_ptr<T, Deleter>::pointer

...

Member types

pointer : std::remove_reference<Deleter>::type::pointer if that type exists, otherwise T* . Must satisfy NullablePointer .

As a capture list of the deleter lambda in the example above, [&] is used. Based on Cppreference:s section on lambdas , as I see it, the only effect of this capture list in the deleter example above would be to capture the "current object by reference" [emphasis mine]:

[&] captures all automatic variables odr-used in the body of the lambda by reference and current object by reference if exists .

But as I understand it from above, the supplied lambda will be simply called with the object's unique_ptr<T, Deleter>::pointer , no matter if we choose [&] or [] as capture list to the lambda. I don't understand myself why we'd want to use by-reference capture (of the object , which is the unique_ptr instance here?) default here, but I'm pretty sure I'm missing something essential (hence the question).

Question

• Is there any particular reason to use by-reference capture default ( [&] ) in the deleter lambda in the example above, as compared to simply using no-capturing ( [] )?

Answer 1

Here's what the standard says about the deleter of unique_ptr ( [unique.ptr.single]/1 ):

A client-supplied template argument D shall be a function object type (20.9), lvalue-reference to function, or lvalue-reference to function object type for which, given a value d of type D and a value ptr of type unique_ptr::pointer , the expression d(ptr) is valid and has the effect of disposing of the pointer as appropriate for that deleter.

Judging by the above, both [] and [&] are perfectly valid.