Copy-assigning C++ lambdas of the same type -


c++ lambdas have deleted copy-assignment operator. don't know why. limitation can worked around. don't feel it. wrong following approach? live code.

template <class t> void assign(t& dest, t&& val)  {    dest.~t();      new (&dest) t(std::forward<t>(val));  }  auto make_lambda(int i) {      return [v=std::make_shared<int>(i)] {std::cout << *v << "\n"; };  }  int main() {     auto 1 = make_lambda(1);      assign(one, make_lambda(2));      one();  // prints 2  }

there possibly 2 reasons can think of:

  1. construction may throw , dest main remain uninitialized , destroyed again later in main. double deletion.
  2. destructor may throw if badly written library throws in destructor.

exception during construction can worked-around strong exception safety guarantee. consider assignv2:

template <class t> void assignv2(t& dest, const t& src)  {   static std::allocator<t> alloc;   static typename std::aligned_storage<sizeof(t), alignof(t)>::type storage;   std::memcpy(&storage, &dest, sizeof(t));   try {     new (&dest) t(src);   }   catch(...) {     std::memcpy(&dest, &storage, sizeof(t));     throw;   }   reinterpret_cast<t*>(&storage)->~t(); }

you don't need able (copy / move) assign lambdas.

class lambda {     std::shared_ptr<int> v; public:     explicit lambda(int i) : v(make_shared(i)) {}     void operator()() { std::cout << *v << "\n"; } }  auto make_lambda(int i) {        return lambda(i); }

or, memoization case, don't change memoise:

template <typename func> auto memoized_recursion(func func, cache c = cache::no_reclaim) {   static std::unordered_map<func, decltype(memoise(func))> functor_map;    if(cache::reclaim == c)     return functor_map.insert_or_assign(func, memoize(func)).first->second;   else     return functor_map.insert(func, memoize(func)).first->second; }

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