unique_ptr没有使用默认删除器初始化 [英] unique_ptr is not getting init with default deleter

问题描述

当我使用 deleter 创建 unique_ptr 时，它会起作用：

When I create an unique_ptr with deleter, it works:

std::unique_ptr<Animal<Cat>, void(*)(Animal<Cat>*)> ptr(new Animal<Cat>, [](Animal<Cat> *ls) {
    delete ls;
});

但是，此代码抛出错误：

But, this code is throwing error:

std::unique_ptr<Animal<Cat>, void(*)(Animal<Cat>*)> ptr;
ptr = std::unique_ptr<Animal<Cat>, void(*)(Animal<Cat>*)>(new Animal<Cat>, [](Animal<Cat> *ls) {
    delete ls;
});

错误：

/usr/bin/../lib/c++/v1/memory:2561:13: error: static_assert failed "unique_ptr constructed with null function pointer deleter"
                static_assert(!is_pointer<deleter_type>::value,
                ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
note: in instantiation of member function 'std::__1::unique_ptr<Animal<Cat>, void (*)(Animal<Cat> *)>::unique_ptr' requested here
            std::unique_ptr<Animal<Cat>, void(*)(Animal<Cat>*)> ptr;
                                                                ^

我的编译器版本：

Apple clang version 4.0 (tags/Apple/clang-421.0.60) (based on LLVM 3.1svn)
Target: x86_64-apple-darwin12.4.0
Thread model: posix

Animal和Cat类很简单。这就是整个代码。

The Animal and Cat classes are trivial. This is the entire code.

推荐答案

显然不可能默认构造一个 unique_ptr ，带有删除器的指针类型。 unique_ptr 构造函数在§20.7.1.2.1[unique.ptr.single.ctor]中进行了详细说明，其中包括：

It as apparently not possible to default-construct a unique_ptr with a pointer type for a deleter. The unique_ptr constructors are detailed in §20.7.1.2.1 [unique.ptr.single.ctor] which says, among other things:

constexpr unique_ptr（）noexcept;

4 备注：如果此构造函数使用模板
参数 D [删除器类型]的指针类型或引用类型实例化，则程序为

4 Remarks: If this constructor is instantiated with a pointer type or reference type for the template argument D [the deleter type], the program is ill-formed.

类似于您想要的符合标准的东西

Something similar to what you want that is standard conforming:

unique_ptr<int, void(*)(int*)> ptr{nullptr,[](int*p){delete p;}};
// ...
ptr.reset(new int(42));

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