assigning an unique_ptr with std::move not working

Question

i am quite new to c++ and smart pointers, but it is my understanding that i can assign an unique_ptr with std::move(another unique_ptr). when i try to assign a unique_ptr with std::move(..) it throws the following error:

binary '=': no operator found which takes a right-hand operand of type 'std::unique_ptr<T [],std::default_delete<_Ty>>'

the following code block is where the error is thrown, on the second last line.

template<typename T>
void wave::WaveReader::readSamples(std::istream& input, int nr_of_bytes)
{
    std::unique_ptr<T[]> a = std::make_unique<T[]>(this->data_chunk.subChunk2Size);
    input.read(reinterpret_cast<char*>(this->data.data.get()), this->data_chunk.subChunk2Size);
    this->data.length = (this->fmt.bitsPerSample / (nr_of_bytes * 8));
    this->data.width = this->data_chunk.subChunk2Size / (this->fmt.bitsPerSample / 8);
    this->data.data = std::move(a);
    std::cout << sizeof(T) << std::endl;
}

some extra code:

class WaveReader
{
private:
    RIFFCHUNK riff;
    FMTCHUNK fmt;
    DATACHUNK data_chunk;
    Array<int8_t> data;
    std::variant<Array<int8_t>,Array<uint16_t>,Array<uint32_t>> samples;
public:
    void read(std::string filepath);
private:
    void readSamples(std::istream& input);
    template<typename T>
    void readSamples(std::istream& input,int nr_of_bytes);
    std::string getDataChunkID(DATACHUNK& data);
    void read_RIFFCHUNK(std::istream& input, RIFFCHUNK& riff);
    void read_FMTCHUNK(std::istream& input, FMTCHUNK& fmt);
    void read_DATACHUNK(std::istream& input, DATACHUNK& data, FMTCHUNK& fmt);
};

the array class with the unique_ptr i am trying to assign to

template <typename T>
struct Array
{
    std::unique_ptr<T[]> data;
    uint32_t length;
    uint32_t width;
    Array(int width, int length) : width(width), length(length)
    {
        data = std::make_unique<T[]>(width);
    }
    Array() : width(0), length(0)
    {
        data = std::make_unique<T[]>(0);
    }
    T operator[](const Position& pos) { return data[(pos.x * length) + pos.y]; }
};

extra code: this code reads the file

void wave::WaveReader::read(std::string filename)
{
    std::ifstream file(filename, std::ios::binary);
    //read riff chunk
    read_RIFFCHUNK(file, this->riff);
    //read fmt chunk
    read_FMTCHUNK(file, this->fmt);
    //read data chunk, fills data object
    read_DATACHUNK(file, this->data_chunk, fmt);
    this->readSamples(file);
}

i use this code to determine what type the samples get stored in

void wave::WaveReader::readSamples(std::istream& input)
{
    //get Type
    switch (fmt.bitsPerSample)
    {
    case 8:
        readSamples<uint8_t>(input,1);
        break;
    case 16:
        readSamples<int16_t>(input, 2);
        break;
    case 24:
        readSamples<int32_t>(input, 3);
        break;
    case 32:
        readSamples<int32_t>(input, 4);
        break;
    default:
        abort(); // need to change this probably
    }
}

when i change std::unique_ptr in the array struct to a normal int it does work correctly. when i change it to a T it doesn't work.

Well, what is T inside readSamples? My guess is that it's not int8_t in your code. unique_ptr<int8_t> is not compatible with unique_ptr<int16_t>, for example. — alter igel

David Schwartz David Schwartz · Accepted Answer · 2020-05-07T20:32:31

When you do x = std::move(y) And y is of type std::unique_ptr<T>, then x must also be of type std::unique_ptr<T> (or two types whose pointers are implicitly convertible) or the assignment won't work. Your code, as shown, can only work if T is int8_t because that's what's on the left.

assigning an unique_ptr with std::move not working

1 Answers