我们可以将lambda用作带有std :: unique_ptr的删除器吗？实际上，我使用clang ++做到了，并且很高兴这样做。
我std::swap用来交换到std::unique_ptr<ObjType, decltyp(deleter)>;哪里auto deleter = [](struct addrinfo* ptr){if (ptr != nullptr) {freeaddrinfo(ptr);} };。Clang的交换似乎不需要复制赋值运算符，但是gcc的std :: swap却需要复制赋值运算符，如您在那些日志中所看到的：
In file included from /usr/include/c++/4.8.1/memory:81:0,
from /home/zenol/proj/src/PROJ/TCPClient.cpp:28:
/usr/include/c++/4.8.1/bits/unique_ptr.h: In instantiation of ‘std::unique_ptr<_Tp, _Dp>& std::unique_ptr<_Tp, _Dp>::operator=(std::unique_ptr<_Tp, _Dp>&&) [with _Tp = addrinfo; _Dp = Proj::TCPClient::connect(const Proj::SocketAddress&, int)::__lambda0]’:
/usr/include/c++/4.8.1/bits/move.h:176:11: required from ‘void std::swap(_Tp&, _Tp&) [with _Tp = std::unique_ptr<addrinfo, Proj::TCPClient::connect(const Proj::SocketAddress&, int)::__lambda0>]’
/home/zenol/proj/src/Proj/SocketHelp.hpp:109:50: required from ‘void Proj::retrieve_addresses(std::string, int, addrinfo&, addrinfo*&, T&, U) [with T = std::unique_ptr<addrinfo, Proj::TCPClient::connect(const Proj::SocketAddress&, int)::__lambda0>; U = Proj::TCPClient::connect(const Proj::SocketAddress&, int)::__lambda0; std::string = std::basic_string<char>]’
/home/zenol/proj/src/PROJ/TCPClient.cpp:65:49: required from here
/usr/include/c++/4.8.1/bits/unique_ptr.h:193:16: erreur: use of deleted function ‘Proj::TCPClient::connect(const Proj::SocketAddress&, int)::__lambda0& Proj::TCPClient::connect(const Proj::SocketAddress&, int)::__lambda0::operator=(const Proj::TCPClient::connect(const Proj::SocketAddress&, int)::__lambda0&)’
get_deleter() = std::forward<deleter_type>(__u.get_deleter());
^
/home/zenol/proj/src/Proj/TCPClient.cpp:56:21: note: a lambda closure type has a deleted copy assignment operator
auto deleter = [](struct addrinfo* ptr)
^
该说什么标准？我可以设法包装这两个std :: unique_ptr吗？他们是解决方法吗？（也许将lambda封装在std :: function中？？）
编辑：这是一个小例子，应该差不多是同一件事：
auto deleter = [](struct addrinfo* ptr)
{if (ptr != nullptr) {freeaddrinfo(ptr);} };
std::unique_ptr<struct addrinfo, decltype(deleter)>
resources_keeper(nullptr, deleter);
int main()
{
decltype(resources_keeper) plouf1(nullptr, deleter);
decltype(resources_keeper) plouf2(nullptr, deleter);
std::swap(plouf1, plouf2);
return 0;
}
错误 ：
In file included from /usr/include/c++/4.8.1/bits/stl_pair.h:59:0,
from /usr/include/c++/4.8.1/bits/stl_algobase.h:64,
from /usr/include/c++/4.8.1/memory:62,
from mini.cpp:1:
/usr/include/c++/4.8.1/bits/move.h: In instantiation of ‘void std::swap(_Tp&, _Tp&) [with _Tp = __lambda0]’:
/usr/include/c++/4.8.1/tuple:381:36: required from ‘void std::_Tuple_impl<_Idx, _Head, _Tail ...>::_M_swap(std::_Tuple_impl<_Idx, _Head, _Tail ...>&) [with long unsigned int _Idx = 1ul; _Head = __lambda0; _Tail = {}]’
/usr/include/c++/4.8.1/tuple:382:35: required from ‘void std::_Tuple_impl<_Idx, _Head, _Tail ...>::_M_swap(std::_Tuple_impl<_Idx, _Head, _Tail ...>&) [with long unsigned int _Idx = 0ul; _Head = addrinfo*; _Tail = {__lambda0}]’
/usr/include/c++/4.8.1/tuple:667:33: required from ‘void std::tuple<_T1, _T2>::swap(std::tuple<_T1, _T2>&) [with _T1 = addrinfo*; _T2 = __lambda0]’
/usr/include/c++/4.8.1/tuple:1050:7: required from ‘void std::swap(std::tuple<_Elements ...>&, std::tuple<_Elements ...>&) [with _Elements = {addrinfo*, __lambda0}]’
/usr/include/c++/4.8.1/bits/unique_ptr.h:269:21: required from ‘void std::unique_ptr<_Tp, _Dp>::swap(std::unique_ptr<_Tp, _Dp>&) [with _Tp = addrinfo; _Dp = __lambda0]’
/usr/include/c++/4.8.1/bits/unique_ptr.h:484:7: required from ‘void std::swap(std::unique_ptr<_Tp, _Dp>&, std::unique_ptr<_Tp, _Dp>&) [with _Tp = addrinfo; _Dp = __lambda0]’
mini.cpp:21:29: required from here
/usr/include/c++/4.8.1/bits/move.h:176:11: erreur: use of deleted function ‘__lambda0& __lambda0::operator=(const __lambda0&)’
__a = _GLIBCXX_MOVE(__b);
^
mini.cpp:9:17: note: a lambda closure type has a deleted copy assignment operator
auto deleter = [](struct addrinfo* ptr)
^
In file included from /usr/include/c++/4.8.1/bits/stl_pair.h:59:0,
from /usr/include/c++/4.8.1/bits/stl_algobase.h:64,
from /usr/include/c++/4.8.1/memory:62,
from mini.cpp:1:
/usr/include/c++/4.8.1/bits/move.h:177:11: erreur: use of deleted function ‘__lambda0& __lambda0::operator=(const __lambda0&)’
__b = _GLIBCXX_MOVE(__tmp);
^
交换到unique_ptrs时，还必须交换其删除器。您看到的问题可以归结为：clang可以交换两个相同类型的lambda，gcc不能（并且标准允许Jonathan引用两者）。示范：
#include <utility>
int main() {
auto f = [](){};
auto g(f);
std::swap(f, g);
}
此代码适用于clang，但无法使用gcc进行编译。（那没关系。）
这就是为什么它正在发生。
我建议以下内容：
#include <memory>
#include <utility>
struct addrinfo { };
void freeaddrinfo(addrinfo* ) { }
struct deleter {
void operator()(struct addrinfo* ptr) {
if (ptr != nullptr)
freeaddrinfo(ptr);
}
};
using resources_keeper = std::unique_ptr<struct addrinfo, deleter>;
int main() {
resources_keeper plouf1(nullptr);
resources_keeper plouf2(nullptr);
std::swap(plouf1, plouf2);
return 0;
}
请注意，代码也变得更加简洁易读。
如果你绝对要与lambda表达式解决这个问题，那么也许你可以尝试一些hackish的是这样的：只交换指针而不是删除者。
#include <iostream>
#include <memory>
#include <utility>
using namespace std;
template <class T, class D>
void swap_pointers_but_not_deleters(unique_ptr<T,D>& x, unique_ptr<T,D>& y) noexcept {
T* x_ptr = x.release();
x.reset(y.release());
y.reset(x_ptr);
}
int main() {
auto deleter = [](int* p){ delete p; };
unique_ptr<int,decltype(deleter)> a(new int(1),deleter);
unique_ptr<int,decltype(deleter)> b(new int(2),deleter);
swap_pointers_but_not_deleters(a, b);
cout << "a = " << *a << ", b = " << *b << endl;
}
尽管此代码似乎有效，但我真的不喜欢它。我建议第一个不使用lambda的解决方案。
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我来说两句