Resetting unique_ptr to an array of characters

September 1, 2022

I’m porting an old C++ program to modern C++. The legacy program uses new and delete for dynamic memory allocation. I replaced new with std::unique_ptr, but I’m getting compilation error when I try to reset the unique_ptr.

Here is the striped down version of the program. My aim is to get rid of all the naked new.

#include <memory>

enum class Types {
    ONE,
    TWO,
};

// based on type get buffer length
int get_buffer_len(Types type) {
    if(type == Types::ONE) return 10;
    else if(type == Types::TWO) return 20;
    else return 0;
}

int main() {
    Types type = Types::ONE;
    std::unique_ptr<char[]> msg{};

    auto len = get_buffer_len(type);
    if(len > 0) {
        msg.reset(std::make_unique<char[]>(len));
    }

    // based on type get the actual message
    if(type == Types::ONE) {
        get_message(msg.get());
    }
}

I get the following compilation error:

error: no matching function for call to 'std::unique_ptr<char []>::reset(std::__detail::__unique_ptr_array_t<char []>)'
      |         msg.reset(std::make_unique<char[]>(len));
      |         ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

>Solution :

If you look at the reset function, it takes a ptr to memory, not another unique ptr:

// members of the specialization unique_ptr<T[]>
template< class U >
void reset( U ptr ) noexcept;

This function is designed to allow you to reset a unique pointer and simultaneously capture memory that you intend to manage with said unique_ptr. What you are looking to do is assign an r-value unique_ptr to ann existing unique_ptr (msg), for which c++ also has an answer:

unique_ptr& operator=( unique_ptr&& r ) noexcept;

Move assignment operator. Transfers ownership from r to *this as if by calling reset(r.release()) followed by an assignment of get_deleter() from std::forward(r.get_deleter()).

So you can instead just do: