Merge #20016: uint256: 1 is a constant

4cc7171c98 wallet: no need for duplicate storage for ABANDON_HASH constant (Anthony Towns)
82cf4641f4 scripted-diff: Replace UINT256_ONE() with uint256::ONE (Anthony Towns)
183f308fff uint256: Update constructors to c++11, make ONE static (Anthony Towns)

Pull request description:

  `UINT256_ONE()` returns a reference to a global; mark it as const to be sure someone doesn't accidently modify it.

ACKs for top commit:
  promag:
    ACK 4cc7171c98
  MarcoFalke:
    re ACK 4cc7171c98, only change is some constexpr shuffling 🛁
  kallewoof:
    ACK 4cc7171c98

Tree-SHA512: 7f399658bfd9ffa4075bc2349049476d842b9579a67518fb7151f56eab36907ef24b1474ee1e89bdc69fe181abe7295dfe19e33b3623d43cec71fc00e356e347

src/script/interpreter.cpp

@@ -1375,7 +1375,7 @@ uint256 SignatureHash(const CScript& scriptCode, const T& txTo, unsigned int nIn
     if ((nHashType & 0x1f) == SIGHASH_SINGLE) {
         if (nIn >= txTo.vout.size()) {
             //  nOut out of range
-            return UINT256_ONE();
+            return uint256::ONE;
         }
     }
 

src/test/sighash_tests.cpp

@@ -28,7 +28,7 @@ uint256 static SignatureHashOld(CScript scriptCode, const CTransaction& txTo, un
 {
     if (nIn >= txTo.vin.size())
     {
-        return UINT256_ONE();
+        return uint256::ONE;
     }
     CMutableTransaction txTmp(txTo);
 
@@ -58,7 +58,7 @@ uint256 static SignatureHashOld(CScript scriptCode, const CTransaction& txTo, un
         unsigned int nOut = nIn;
         if (nOut >= txTmp.vout.size())
         {
-            return UINT256_ONE();
+            return uint256::ONE;
         }
         txTmp.vout.resize(nOut+1);
         for (unsigned int i = 0; i < nOut; i++)

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() {
-    static uint256* one = new uint256(uint256S("0000000000000000000000000000000000000000000000000000000000000001"));
-    return *one;
-}
+const uint256 uint256::ONE(1);

src/uint256.h

@@ -20,10 +20,11 @@ protected:
     static constexpr int WIDTH = BITS / 8;
     uint8_t m_data[WIDTH];
 public:
-    base_blob()
-    {
-        memset(m_data, 0, sizeof(m_data));
-    }
+    /* construct 0 value by default */
+    constexpr base_blob() : 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 +150,4 @@ inline uint256 uint256S(const std::string& str)
     return rv;
 }
 
-uint256& UINT256_ONE();
-
 #endif // BITCOIN_UINT256_H

src/wallet/scriptpubkeyman.cpp

@@ -655,7 +655,7 @@ std::unique_ptr<CKeyMetadata> LegacyScriptPubKeyMan::GetMetadata(const CTxDestin
 
 uint256 LegacyScriptPubKeyMan::GetID() const
 {
-    return UINT256_ONE();
+    return uint256::ONE;
 }
 
 /**

src/wallet/wallet.cpp

@@ -324,8 +324,6 @@ std::shared_ptr<CWallet> CreateWallet(interfaces::Chain& chain, const std::strin
     return wallet;
 }
 
-const uint256 CWalletTx::ABANDON_HASH(UINT256_ONE());
-
 /** @defgroup mapWallet
  *
  * @{

src/wallet/wallet.h

@@ -275,7 +275,7 @@ private:
     /** Constant used in hashBlock to indicate tx has been abandoned, only used at
      * serialization/deserialization to avoid ambiguity with conflicted.
      */
-    static const uint256 ABANDON_HASH;
+    static constexpr const uint256& ABANDON_HASH = uint256::ONE;
 
 public:
     /**