Cannot move std::unique_ptr with NULL deleter to std::shared_ptr?

Question

I want to move a NULL std::unique_ptr to a std::shared_ptr, like so:

std::unique_ptr test = nullptr;
std::shared_ptr          


        

Answer 1

I'll echo Brian's answer and add that in situations like this where the function shouldn't be null, you can use a function reference, which are nonnullable like all C++ references, instead of function pointers.

void delete_float(float *f) {delete f;}

using Deleter = void(float*);

// Function pointers
constexpr void(*fptr)(float*) = delete_float;
constexpr Deleter *fptr_typedef = delete_float;
constexpr auto fptr_auto = delete_float;
constexpr auto *fptr_auto2 = delete_float;

// Function references
constexpr void(&fref)(float*) = delete_float;
constexpr Deleter &fref_typedef = delete_float;
constexpr auto &fref_auto = delete_float;

The one gotcha you need to keep in mind with function references is that lambdas implicitly convert to function pointers, but not function references, so you need to use the dereference operator * to convert a lambda to a function reference.

const Deleter *fptr_lambda = [](float *f) {delete f;};
// const Deleter &fref_lambda = [](float *f) {delete f;}; // error
const Deleter &fref_lambda_fixed = *[](float *f) {delete f;};