c++ - How to return/copy values of an unique_ptr<unsigned char[]>?

Question

I have a simple class with one attribute std::unique_ptr<unsigned char[]> in C++. I want to have a function that converts string to std::unique_ptr<unsigned char[]>, other to convert float to std::unique_ptr<unsigned char[]>, and a third to return the attribute std::unique_ptr<unsigned char[]>. My header is compiling but the source CPP is not. Even the return attribute is not compiling.

#include <memory>
class SkinnyBuffer {
private:
    std::unique_ptr<unsigned char[]> _buff;
public:
    ~SkinnyBuffer();
    SkinnyBuffer();
    void setBuffValue(float f);
    void setBuffValue(std::string str);
    std::unique_ptr<unsigned char[]>* getBuffValue();
};

#include "utils/SkinnyBuffer.h"
SkinnyBuffer::~SkinnyBuffer() { }
SkinnyBuffer::SkinnyBuffer() { }
void SkinnyBuffer::setBuffValue(float f) {
    // How to implement it
    _buff = f;
}
void SkinnyBuffer::setBuffValue(std::string str) {
    _buff = std::unique_ptr<unsigned char[]>(str.data(), str.data() + str.length());
}
std::unique_ptr<unsigned char[]>* SkinnyBuffer::getBuffValue() {
    return &_buff;
}

Answer 1

std::unique_ptr is a non-copyable object. If you need a read-only access to it, you have two (main) options:

1. Return a reference to unique_ptr itself:

   const std::unique_ptr<unsigned char[]>& getBuffValue() const
   {
       return _buff;
   }
   

2. Return a const pointer to the managed array:

   const unsigned char* getBuffValue() const
   {
       return _buff.get();
   }
   

To assign a string to the buffer, you can do:

void setBuffValue(const std::string& str)
{
    _buff = std::make_unique<unsigned char []>(str.length() + 1);
    std::copy_n(str.c_str(), str.length() + 1, _buff.get());
}

Note that you have to copy the terminating null character to your buffer. Otherwise it will be almost useless for the outside world because its length will not be known to the user.

But do you really need std::unique_ptr<unsigned char[]>? std::vector seems to be more appropriate here.