Add += and -= operators to ValueProxy

* Simplify updating some MPT fields in View
gregtatcam · Oct 15, 2024 · 9e3cfef · 9e3cfef
1 parent af29f41
commit 9e3cfef
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Showing 2 changed files with 43 additions and 8 deletions.
diff --git a/include/xrpl/protocol/STObject.h b/include/xrpl/protocol/STObject.h
@@ -500,6 +500,17 @@ class STObject::Proxy
     assign(U&& u);
 };
 
+// Constraint += and -= ValueProxy operators
+// to value types that support arithmetic operations
+template <typename TArg, typename TCls>
+concept CanAddSubstr =
+    (std::is_arithmetic_v<TArg> &&
+     (std::is_same_v<
+          std::make_unsigned_t<TArg>,
+          std::make_unsigned_t<typename TCls::value_type>> ||
+      (std::is_same_v<TArg, STAmount> &&
+       std::is_same_v<typename TCls::value_type, STAmount>)));
+
 template <class T>
 class STObject::ValueProxy : private Proxy<T>
 {
@@ -515,6 +526,16 @@ class STObject::ValueProxy : private Proxy<T>
     std::enable_if_t<std::is_assignable_v<T, U>, ValueProxy&>
     operator=(U&& u);
 
+    // Convenience operators for value types supporting
+    // arithmetic operations
+    template <class U>
+    requires CanAddSubstr<U, T> ValueProxy&
+    operator+=(U const& t);
+
+    template <class U>
+    requires CanAddSubstr<U, T> ValueProxy&
+    operator-=(U const& t);
+
     operator value_type() const;
 
 private:
@@ -732,6 +753,24 @@ STObject::ValueProxy<T>::operator=(U&& u)
     return *this;
 }
 
+template <typename T>
+template <class U>
+requires CanAddSubstr<U, T> STObject::ValueProxy<T>&
+STObject::ValueProxy<T>::operator+=(U const& t)
+{
+    this->assign(this->value() + t);
+    return *this;
+}
+
+template <class T>
+template <class U>
+requires CanAddSubstr<U, T> STObject::ValueProxy<T>&
+STObject::ValueProxy<T>::operator-=(U const& t)
+{
+    this->assign(this->value() - t);
+    return *this;
+}
+
 template <class T>
 STObject::ValueProxy<T>::operator value_type() const
 {

diff --git a/src/xrpld/ledger/detail/View.cpp b/src/xrpld/ledger/detail/View.cpp
@@ -569,6 +569,8 @@ Rate
 transferRate(ReadView const& view, MPTID const& issuanceID)
 {
     // fee is 0-50,000 (0-50%), rate is 1,000,000,000-2,000,000,000
+    // For example, if transfer fee is 50% then 10,000 * 50,000 = 500,000
+    // which represents 50% of 1,000,000,000
     if (auto const sle = view.read(keylet::mptIssuance(issuanceID));
         sle && sle->isFieldPresent(sfTransferFee))
         return Rate{1'000'000'000u + 10'000 * sle->getFieldU16(sfTransferFee)};
@@ -1897,11 +1899,7 @@ rippleCreditMPT(
         return tecOBJECT_NOT_FOUND;
     if (uSenderID == issuer)
     {
-        sleIssuance->setFieldU64(
-            sfOutstandingAmount,
-            sleIssuance->getFieldU64(sfOutstandingAmount) +
-                saAmount.mpt().value());
-
+        (*sleIssuance)[sfOutstandingAmount] += saAmount.mpt().value();
         view.update(sleIssuance);
     }
     else
@@ -1937,9 +1935,7 @@ rippleCreditMPT(
         auto const mptokenID = keylet::mptoken(mptID.key, uReceiverID);
         if (auto sle = view.peek(mptokenID))
         {
-            sle->setFieldU64(
-                sfMPTAmount,
-                sle->getFieldU64(sfMPTAmount) + saAmount.mpt().value());
+            (*sle)[sfMPTAmount] += saAmount.mpt().value();
             view.update(sle);
         }
         else