diff --git a/diagonal.cpp b/diagonal.cpp
new file mode 100644
index 0000000..55cc556
--- /dev/null
+++ b/diagonal.cpp
@@ -0,0 +1,210 @@
+
+#ifndef __PROGTEST__
+#include <cassert>
+#include <cmath>
+#include <iostream>
+#include <iomanip>
+#include <string>
+#include <vector>
+#include <list>
+#include <forward_list>
+#include <algorithm>
+#include <memory>
+#include <stdexcept>
+using namespace std;
+#endif /* __PROGTEST__ */
+
+template <typename T_>
+class CDiagonalMatrix
+{
+public:
+    // constructor ( n, k )
+    CDiagonalMatrix( int n, int k )
+    : _n(n), _k(0) {
+        reshape(k);
+    }
+
+    // destructor, copy cons, operator = (if needed)
+    ~CDiagonalMatrix() {}
+    
+    // operator () ( row, col )
+    T_& operator()( int row, int col ) {
+        int diag = std::abs(row - col);
+        if ( diag >= _k || row >= _n || col >= _n )
+            throw std::out_of_range("");
+
+        if (!diag) {
+            return _data[diag][row]; //center
+        }
+
+        bool mid = row > col;
+        if (mid) {
+            return _data[diag][ (row - diag) ];
+        } else { //             offset      
+            return _data[diag][ (_n - diag) + (col - diag) ];
+        }
+    }
+    
+    // exists ( row, col ) const
+    bool exists( int row, int col ) const {
+        int diag = std::abs(row - col);
+        if ( diag >= _k || row >= _n || col >= _n )
+            return false;
+
+        return true;
+    }
+
+    // reshape ( k )
+    void reshape( int k ) {
+        if (!k || k > _n) throw std::out_of_range("");
+
+        if (k < _k) {
+            _data.resize( k );
+            _k = k;
+        } else {
+            while ( _k < k ) {
+                _data.emplace_back( _k ? ((_n - _k) * 2) : _n, T_() );
+                ++_k;
+            }
+        }
+
+    }
+
+private:
+    std::vector< std::vector< T_ > > _data;
+    int _n, _k;
+};
+
+#ifndef __PROGTEST__
+class CTestCnt
+{
+public:
+    CTestCnt(int val = 0)
+        : m_Val(val)
+    {
+        c_Inst++;
+    }
+    CTestCnt(const CTestCnt &src) noexcept
+        : m_Val(src.m_Val)
+    {
+        c_Inst++;
+    }
+    CTestCnt(CTestCnt &&src) noexcept
+        : m_Val(src.m_Val)
+    {
+        c_Inst++;
+    }
+    ~CTestCnt(void) noexcept
+    {
+        c_Inst--;
+    }
+    CTestCnt &operator=(CTestCnt src) noexcept
+    {
+        swap(src.m_Val, m_Val);
+        return *this;
+    }
+    inline static size_t c_Inst = 0;
+
+private:
+    int m_Val;
+};
+
+int main(void)
+{
+    CDiagonalMatrix<int> m1(5, 4);
+
+    for (int i = 0; i < 5; i++)
+        m1(i, i) = 10 + i;
+    for (int i = 0; i < 4; i++)
+    {
+        m1(i, i + 1) = 20 + i;
+        m1(i + 1, i) = 30 + i;
+    }
+    for (int i = 0; i < 3; i++)
+    {
+        m1(i, i + 2) = 40 + i;
+        m1(i + 2, i) = 50 + i;
+    }
+    m1(0, 3) = 60;
+    m1(1, 4) = 61;
+    m1(3, 0) = 70;
+    m1(4, 1) = 71;
+
+    CDiagonalMatrix<int> m2(m1);
+
+    for (int i = 0; i < 5; i++)
+        assert(m2(i, i) == 10 + i);
+    for (int i = 0; i < 4; i++)
+    {
+        assert(m2(i, i + 1) == 20 + i);
+        assert(m2(i + 1, i) == 30 + i);
+    }
+    for (int i = 0; i < 3; i++)
+    {
+        assert(m2(i, i + 2) == 40 + i);
+        assert(m2(i + 2, i) == 50 + i);
+    }
+    assert(m2(0, 3) == 60);
+    assert(m2(1, 4) == 61);
+    assert(m2(3, 0) == 70);
+    assert(m2(4, 1) == 71);
+
+    m2(0, 0) = 100;
+    assert(m1(0, 0) == 10);
+    assert(m2(0, 0) == 100);
+
+    assert(m1.exists(4, 1) && m1(4, 1) == 71);
+    m1.reshape(2);
+    assert(m1.exists(0, 1) && m1(0, 1) == 20);
+    assert(!m1.exists(0, 2));
+    try
+    {
+        m1(0, 0) = m1(0, 2);
+        assert("Missing an exception" == nullptr);
+    }
+    catch (const std::out_of_range &e)
+    {
+        // assert(e.what() == "invalid index ( 0, 2 )"s);
+    }
+
+    m1.reshape(4);
+    assert(m1.exists(0, 1) && m1(0, 1) == 20);
+    assert(m1.exists(0, 2) && m1(0, 2) == 0);
+
+    try
+    {
+        m1.reshape(6);
+        assert("Missing an exception" == nullptr);
+    }
+    catch (const std::out_of_range &e)
+    {
+        // assert(e.what() == "invalid matrix size"s);
+    }
+
+    try
+    {
+        CDiagonalMatrix<int> m3(5, 6);
+        assert("Missing an exception" == nullptr);
+    }
+    catch (const std::out_of_range &e)
+    {
+        // assert(e.what() == "invalid matrix size"s);
+    }
+
+    CDiagonalMatrix<CTestCnt> m10(12, 1);
+    assert(CTestCnt::c_Inst == 12);
+    m10.reshape(4);
+    assert(CTestCnt::c_Inst == 72);
+    m10.reshape(2);
+    assert(CTestCnt::c_Inst == 34);
+    m10.reshape(11);
+    assert(CTestCnt::c_Inst == 142);
+    m10.reshape(12);
+    assert(CTestCnt::c_Inst == 144);
+    m10.reshape(8);
+    assert(CTestCnt::c_Inst == 124);
+
+    std::cout << "success!\n";
+    return EXIT_SUCCESS;
+}
+#endif /* __PROGTEST__ */
\ No newline at end of file