C ++ std :: unique_ptr：为什么lambdas没有任何大小费用？ [英] C++ std::unique_ptr : Why isn't there any size fees with lambdas?

问题描述

我正在读有效的现代C ++。在与 std :: unique_ptr 相关的项目中声明，如果自定义删除程序是无状态对象，则不会发生大小费用，但如果它是一个函数指针或 std :: function 发生大小。

I am reading "Effective Modern C++". In the item related to std::unique_ptr it's stated that if the custom deleter is a stateless object, then no size fees occur, but if it's a function pointer or std::function size fee occurs. Could you explain why?

假设我们有以下代码：

auto deleter_ = [](int *p) { doSth(p); delete p; };
std::unique_ptr<int, decltype(deleter_)> up(new int, deleter_);

根据我的理解， unique_ptr 类型为 decltype（deleter _）的对象，并将 deleter _ 分配给该内部对象。但显然这不是发生了什么。

To my understanding, the unique_ptr should have an object of type decltype(deleter_) and assign deleter_ to that internal object. But obviously that's not what's happening. Could you explain the mechanism behind this using smallest possible code example?

推荐答案

A unique_ptr 必须始终存储其删除程序。现在，如果删除器是没有状态的类类型，则 unique_ptr 可以使用空基本优化，以便删除程序不使用任何额外的空间。

A unique_ptr must always store its deleter. Now, if the deleter is a class type with no state, then the unique_ptr can make use of empty base optimization so that the deleter does not use any additional space.

这是如何完成不同的实现。例如， libc ++ 和MSVC都将托管指针和删除器存储在压缩对，如果其中一个所涉及的类型是一个空类。

How exactly this is done differs between implementations. For instance, both libc++ and MSVC store the managed pointer and the deleter in a compressed pair, which automatically gets you empty base optimization if one of the types involved is an empty class.

从上面的libc ++链接

From the libc++ link above

template <class _Tp, class _Dp = default_delete<_Tp> >
class _LIBCPP_TYPE_VIS_ONLY unique_ptr
{
public:
    typedef _Tp element_type;
    typedef _Dp deleter_type;
    typedef typename __pointer_type<_Tp, deleter_type>::type pointer;
private:
    __compressed_pair<pointer, deleter_type> __ptr_;

libstdc ++ 在 std :: tuple 中存储两个，一些Google搜索建议他们的 tuple 实现使用空基本优化，但我找不到任何明确说明的文档。

libstdc++ stores the two in an std::tuple and some Google searching suggests their tuple implementation employs empty base optimization but I can't find any documentation stating so explicitly.

无论如何，此示例演示了libc ++和libstdc ++都使用EBO来减小 unique_ptr 与一个空的删除者。

In any case, this example demonstrates that both libc++ and libstdc++ use EBO to reduce the size of a unique_ptr with an empty deleter.

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