How to implement std::any? - c++

Question

Try to implement a tiny-any:
#include <iostream>
#include <type_traits>
#include <memory>
class TinyAny
{
public:
template <typename Tp>
TinyAny(Tp&& val) : data(std::make_shared<unsigned char[]>(sizeof(val))) {
new(data.get()) Tp(std::forward<Tp>(val));
}
template <typename Tp>
TinyAny &operator =(Tp&& val)
{
data = std::make_shared<unsigned char[]>(sizeof(val));
new(data.get()) Tp(std::forward<Tp>(val));
return *this;
}
template <typename Tp>
Tp get()
{
return *reinterpret_cast<Tp *>(data.get());
}
private:
std::shared_ptr<unsigned char[]> data;
};
int main() {
// var = "abc";
// std::cout << var.get<const char *>() << std::endl;
}
if uncomment var = "abc" will get the fellowing error:
<source>: In instantiation of 'TinyAny& TinyAny::operator=(Tp&&) [with Tp = const char (&)[4]]':
<source>:40:11: required from here
<source>:17:9: error: new cannot be applied to a reference type
17 | new(data.get()) Tp(std::forward<Tp>(val));
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:17:44: error: invalid conversion from 'const char*' to 'char' [-fpermissive]
17 | new(data.get()) Tp(std::forward<Tp>(val));
| ~~~~~~~~~~~~~~~~^~~~~
| |
| const char*
The type of val is const char*, but I cannot understand which one is char? Or, how can I fix this error?
Source Code: https://gcc.godbolt.org/z/zW7fPc6G3

Answer 1

My finally implement:
#include <type_traits>
#include <utility>
class TinyAny
{
template<typename Tp>
class AnyData {
public:
static void create(void **data, Tp && val)
{
*data = new Tp(std::forward<Tp>(val));
}
static void deleter(void *data)
{
auto ptr = static_cast<Tp *>(data);
delete ptr;
}
};
public:
template <typename Tp__, typename Tp = std::decay_t<Tp__>>
TinyAny(Tp__&& val) : deleter(AnyData<Tp>::deleter) {
AnyData<Tp>::create(&data, std::forward<Tp>(val));
}
~TinyAny() {
deleter(data);
data = nullptr;
}
template <typename Tp>
TinyAny &operator = (Tp&& val)
{
TinyAny temp{std::forward<Tp>(val)};
swap(std::move(temp));
return *this;
}
template <typename Tp>
Tp get()
{
return *static_cast<Tp*>(data);
}
private:
TinyAny &swap(TinyAny && another) noexcept
{
std::swap(data, another.data);
std::swap(deleter, another.deleter);
return *this;
}
private:
void *data;
void (* deleter)(void *data);
};