swap std::unique_ptr with lambda as deleter -- GCC

Question

Can we use a lambda as a deleter with a std::unique_ptr ? Actualy, I did it with clang++ and it was happy to do so. I'm using `std::swap` to swap to `std::unique_ptr;` where `auto deleter = [](struct addrinfo* ptr){if (ptr != nullptr) {freeaddrinfo(ptr);} };`. Clang's swap seems to do not need a copy assignment operator, but gcc's std::swap did, as you can see in those logs : ``` 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]’ /home/zenol/proj/src/Proj/Socke…

Accepted Answer

To expand on Jonathan Wakely's answer: When you swap to `unique_ptr`s, you also have to swap their deleters. The problem you are seeing boils down to this: clang can swap two lambdas of the same type, gcc cannot (and the standard allows both as Jonathan quotes it). Demonstration: ``` #include int main() { auto f = [](){}; auto g(f); std::swap(f, g); } ``` This code works with clang but fails to compile with gcc. (And that is OK.) That is why it is happening. I suggest the following: ``` #include #include struct addrinfo { }; void freeaddrinfo(addrinfo* ) { } struct deleter { void operator()(struct addrinfo* ptr) { if (ptr != nullptr) freeaddrinfo(ptr); } }; using resources_keeper = std::unique_ptr; int main() { resources_keeper plouf1(nullptr); resources_keeper plouf2(nullptr); std::swap(plouf1, plouf2); return 0; } ``` Note that the code became cleaner and more readable as well. If you absolutely have to solve this with lambdas, then perhaps you could try something hackish like this: Swap only the pointers but not the deleters. ``` #include #include #include using namespace std; template void swap_pointers_but_not_deleters(unique_ptr& x, unique_ptr& 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 a(new int(1),deleter); unique_ptr b(new int(2),deleter); swap_pointers_but_not_deleters(a, b); cout << "a = " << *a << ", b = " << *b << endl; } ``` Although this code seems to work, I really don't like it. I suggest the first solution that does not use lambdas.

swap std::unique_ptr with lambda as deleter -- GCC

Solution

Problem