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copy constructor for two dimensional vector


copy constructor for two dimensional vector

By : user2954811
Date : November 22 2020, 10:33 AM
it fixes the issue I want to write a copy constructor for two dimensional vector
code :
MyMatrix :: MyMatrix(const MyMatrix & mat):m_m(mat.m_m), m_n(mat.m_n), m_matrix(mat.m_matrix)
{
}


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vector::push_back insists on using copy constructor though a move constructor is provided

vector::push_back insists on using copy constructor though a move constructor is provided


By : fly haider
Date : March 29 2020, 07:55 AM
this will help Try adding "noexcept" to the declaration of the move constructor.
I can't quote the standard, but recent versions of gcc appear to require either that the copy constructor be public or that the move constructor be declared "noexcept". Regardless of the "noexcept" qualifier, if you make the copy constructor public, it will behave as you expect at run-time.
is vector reallocation and vector::push_back required by the standard to use placement new and copy constructor?

is vector reallocation and vector::push_back required by the standard to use placement new and copy constructor?


By : sysconfigk
Date : March 29 2020, 07:55 AM
this one helps. Yes and no. Vector, like all other containers, is required to use allocator_traits::construct and allocator_traits::destroy (§23.2.1/3) and to obtain all memory using the allocator (§23.2.1/7). If you provide a custom allocator, you can do just about anything. The default allocator is required to call ::operator new (§20.6.9.1/6) and to use placement new (§20.6.9.1/12).
When std::vector reallocate its memory array, is copy constructor or move constructor used?

When std::vector reallocate its memory array, is copy constructor or move constructor used?


By : Mukul
Date : March 29 2020, 07:55 AM
I wish did fix the issue. If the move-constructor exists and is noexcept then it is used. Otherwise the copy-constructor is used.
Using a move-constructor that might throw is undesirable as it might happen that some objects are moved to the new storage and then an exception prevents the rest of the objects being moved.
std::copy 3-dimensional array into 3-dimensional vector

std::copy 3-dimensional array into 3-dimensional vector


By : user3284271
Date : March 29 2020, 07:55 AM
around this issue A vector of vectors is not contiguous. Each vector, and you have 12 of them not including the vectors stitching the 12 together, holds it's own block of dynamically allocated memory. &pages[0][0][0] gives the address of the first column of the first row of the first page. Crom only knows where in memory the second column of the first row of the first page is, but the odds of it being immediately after the first one are astonishingly small. That's almost certainly the access violation.
With the vector of vectors approach, the best you can hope for is this something like:
code :
vsPage pages;
for (const auto & row : Array)
{
    vsRows r;
    for (const auto & col : row)
    {
        r.emplace_back(std::begin(col), std::end(col));
    }
    pages.emplace_back(r);

}
// Basic framework gleefully looted from jamesdlin
class ThreeDee
{
public:
    // zero initialized "empty" 3D matrix
    ThreeDee(size_t pages, size_t rows, size_t cols) :
            mPages(pages), mRows(rows), mCols(cols), mData(pages * rows * cols)
    {
    }


    // array initialized "empty" 3D matrix
    // the template params are deduced from the parameters of the array
    // but to get an array rather than a decayed pointer, we need to pass the array 
    // by reference
    template<size_t PAGES, size_t ROWS, size_t COLS>
    ThreeDee(string (&arr)[PAGES][ROWS][COLS]) :
            mPages(PAGES), mRows(ROWS), mCols(COLS), mData(PAGES * ROWS * COLS)
    {
        // I hate this, but I don't have anything better yet and I have to 
        // surrender the computer to a student with homework
        std::copy(&arr[0][0][0], &arr[0][0][0]+mData.size(), mData.data());
    }

    string& operator()(size_t page, size_t row, size_t col)
    {
        return mData[(page * mRows + row) * mCols + col]; // 3D to 1D mapping 
    }

    string operator()(size_t page, size_t row, size_t col) const
    {
        return mData[(page * mRows + row) * mCols + col];
    }

    size_t pages()const
    {
        return mPages;
    }
    size_t rows()const
    {
        return mRows;
    }
    size_t cols()const
    {
        return mCols;
    }

private:
    size_t mPages;
    size_t mRows;
    size_t mCols;
    std::vector<string> mData;
};
string Array[2][3][2];
ThreeDee pages(Array);
vector<unique_ptr<A> > in constructor - error: call to implicitly-deleted copy constructor

vector<unique_ptr<A> > in constructor - error: call to implicitly-deleted copy constructor


By : vinothini s
Date : March 29 2020, 07:55 AM
To fix the issue you can do Culprit is mpq_class. With simple dummy version it compiles fine (I commented out headers for gmp) :
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