uint256: Update constructors to c++11, make ONE static

Replace the memset with C++11 value/aggregate initialisation of
the m_data array, which still ensures the unspecified values end
up as zero-initialised.

This then allows changing UINT256_ONE() from dynamically allocating an
object, to a simpler referencing a static allocation.

src/test/uint256_tests.cpp

@@ -278,4 +278,10 @@ BOOST_AUTO_TEST_CASE( operator_with_self )
     BOOST_CHECK(v == UintToArith256(uint256S("0")));
 }
 
+BOOST_AUTO_TEST_CASE( check_ONE )
+{
+    uint256 one = uint256S("0000000000000000000000000000000000000000000000000000000000000001");
+    BOOST_CHECK_EQUAL(one, uint256::ONE);
+}
+
 BOOST_AUTO_TEST_SUITE_END()

src/uint256.cpp

@@ -80,7 +80,4 @@ template std::string base_blob<256>::ToString() const;
 template void base_blob<256>::SetHex(const char*);
 template void base_blob<256>::SetHex(const std::string&);
 
-uint256& UINT256_ONE() {
+const uint256 uint256::ONE(1);
-    static uint256* one = new uint256(uint256S("0000000000000000000000000000000000000000000000000000000000000001"));
-    return *one;
-}

src/uint256.h

@@ -20,10 +20,11 @@ protected:
     static constexpr int WIDTH = BITS / 8;
     uint8_t m_data[WIDTH];
 public:
-    base_blob()
+    /* construct 0 value by default */
-    {
+    constexpr base_blob() : m_data() {}
-        memset(m_data, 0, sizeof(m_data));
+
-    }
+    /* constructor for constants between 1 and 255 */
+    constexpr explicit base_blob(uint8_t v) : m_data{v} {}
 
     explicit base_blob(const std::vector<unsigned char>& vch);
 
@@ -111,7 +112,7 @@ public:
  */
 class uint160 : public base_blob<160> {
 public:
-    uint160() {}
+    constexpr uint160() {}
     explicit uint160(const std::vector<unsigned char>& vch) : base_blob<160>(vch) {}
 };
 
@@ -122,8 +123,10 @@ public:
  */
 class uint256 : public base_blob<256> {
 public:
-    uint256() {}
+    constexpr uint256() {}
+    constexpr explicit uint256(uint8_t v) : base_blob<256>(v) {}
     explicit uint256(const std::vector<unsigned char>& vch) : base_blob<256>(vch) {}
+    static const uint256 ONE;
 };
 
 /* uint256 from const char *.
@@ -147,6 +150,6 @@ inline uint256 uint256S(const std::string& str)
     return rv;
 }
 
-uint256& UINT256_ONE();
+inline const uint256& UINT256_ONE() { return uint256::ONE; }
 
 #endif // BITCOIN_UINT256_H