diff --git a/lib/valueflow.cpp b/lib/valueflow.cpp
index 8a3500772..fb13b9bca 100644
--- a/lib/valueflow.cpp
+++ b/lib/valueflow.cpp
@@ -345,6 +345,20 @@ static ValueFlow::Value castValue(ValueFlow::Value value, const ValueType::Sign
     return value;
 }
 
+static void combineValueProperties(const ValueFlow::Value &value1, const ValueFlow::Value &value2, ValueFlow::Value *result)
+{
+    if (value1.isKnown() && value2.isKnown())
+        result->setKnown();
+    else if (value1.isInconclusive() || value2.isInconclusive())
+        result->setInconclusive();
+    else
+        result->setPossible();
+    result->condition = value1.condition ? value1.condition : value2.condition;
+    result->varId = (value1.varId != 0U) ? value1.varId : value2.varId;
+    result->varvalue = (result->varId == value1.varId) ? value1.varvalue : value2.varvalue;
+    result->errorPath = (value1.errorPath.empty() ? value2 : value1).errorPath;
+}
+
 /** set ValueFlow value and perform calculations if possible */
 static void setTokenValue(Token* tok, const ValueFlow::Value &value, const Settings *settings)
 {
@@ -359,6 +373,48 @@ static void setTokenValue(Token* tok, const ValueFlow::Value &value, const Setti
     if (!parent)
         return;
 
+    if (value.isContainerSizeValue()) {
+        // .empty, .size, +"abc", +'a'
+        if (Token::Match(parent, "+")) {
+            for (const ValueFlow::Value &value1 : parent->astOperand1()->values()) {
+                for (const ValueFlow::Value &value2 : parent->astOperand2()->values()) {
+                    ValueFlow::Value result;
+                    result.valueType = ValueFlow::Value::ValueType::CONTAINER_SIZE;
+                    if (value1.isContainerSizeValue() && value2.isContainerSizeValue())
+                        result.intvalue = value1.intvalue + value2.intvalue;
+                    else if (value1.isContainerSizeValue() && value2.isTokValue() && value2.tokvalue->tokType() == Token::eString)
+                        result.intvalue = value1.intvalue + Token::getStrLength(value2.tokvalue);
+                    else if (value2.isContainerSizeValue() && value1.isTokValue() && value1.tokvalue->tokType() == Token::eString)
+                        result.intvalue = Token::getStrLength(value1.tokvalue) + value2.intvalue;
+                    else
+                        continue;
+
+                    combineValueProperties(value1, value2, &result);
+
+                    setTokenValue(parent, result, settings);
+                }
+            }
+        }
+
+
+        else if (Token::Match(parent, ". %name% (") && parent->astParent() == parent->tokAt(2) && parent->astOperand1() && parent->astOperand1()->valueType()) {
+            const Library::Container *c = parent->astOperand1()->valueType()->container;
+            const Library::Container::Yield yields = c ? c->getYield(parent->strAt(1)) : Library::Container::Yield::NO_YIELD;
+            if (yields == Library::Container::Yield::SIZE) {
+                ValueFlow::Value v(value);
+                v.valueType = ValueFlow::Value::ValueType::INT;
+                setTokenValue(const_cast<Token *>(parent->astParent()), v, settings);
+            } else if (yields == Library::Container::Yield::EMPTY) {
+                ValueFlow::Value v(value);
+                v.intvalue = !v.intvalue;
+                v.valueType = ValueFlow::Value::ValueType::INT;
+                setTokenValue(const_cast<Token *>(parent->astParent()), v, settings);
+            }
+        }
+
+        return;
+    }
+
     if (parent->str() == "(" && !parent->astOperand2() && Token::Match(parent,"( %name%")) {
         const ValueType &valueType = ValueType::parseDecl(parent->next(), settings);
         if (valueType.pointer)
@@ -471,13 +527,7 @@ static void setTokenValue(Token* tok, const ValueFlow::Value &value, const Setti
                 if (known || value1.varId == 0U || value2.varId == 0U ||
                     (value1.varId == value2.varId && value1.varvalue == value2.varvalue && value1.isIntValue() && value2.isIntValue())) {
                     ValueFlow::Value result(0);
-                    result.condition = value1.condition ? value1.condition : value2.condition;
-                    result.setInconclusive(value1.isInconclusive() | value2.isInconclusive());
-                    result.varId = (value1.varId != 0U) ? value1.varId : value2.varId;
-                    result.varvalue = (result.varId == value1.varId) ? value1.varvalue : value2.varvalue;
-                    result.errorPath = (value1.errorPath.empty() ? value2 : value1).errorPath;
-                    if (value1.valueKind == value2.valueKind)
-                        result.valueKind = value1.valueKind;
+                    combineValueProperties(value1, value2, &result);
                     const float floatValue1 = value1.isIntValue() ? value1.intvalue : value1.floatValue;
                     const float floatValue2 = value2.isIntValue() ? value2.intvalue : value2.floatValue;
                     switch (parent->str()[0]) {
diff --git a/test/testvalueflow.cpp b/test/testvalueflow.cpp
index 152b24908..ad63b1c3f 100644
--- a/test/testvalueflow.cpp
+++ b/test/testvalueflow.cpp
@@ -3379,7 +3379,7 @@ private:
         ASSERT(tokenValues(code, "s [").empty());
 
         code = "void f() {\n"
-               "  std::string s=\"abc\";\n" // size of s is 3
+               "  std::string s = \"abc\";\n" // size of s is 3
                "  s.size();\n"
                "}";
         ASSERT_EQUALS("", isKnownContainerSizeValue(tokenValues(code, "s . size"), 3));
@@ -3444,6 +3444,25 @@ private:
                "}";
         ASSERT(tokenValues(code, "ints [").empty());
 
+        // container size => yields
+        code = "void f() {\n"
+               "  std::string s = \"abcd\";\n"
+               "  s.size();\n"
+               "}";
+        ASSERT_EQUALS(4, tokenValues(code, "( ) ;").front().intvalue);
+
+        code = "void f() {\n"
+               "  std::string s;\n"
+               "  s.empty();\n"
+               "}";
+        ASSERT_EQUALS(1, tokenValues(code, "( ) ;").front().intvalue);
+
+        // Calculations
+        code = "void f() {\n"
+               "  std::string s = \"abcd\";\n"
+               "  x = s + s;\n"
+               "}";
+        ASSERT_EQUALS("", isKnownContainerSizeValue(tokenValues(code, "+"), 8));
     }
 };