[C++] Fix model of std::vector::size

Summary: It wasn't using code from `std::vector::empty` which recently was improved. Instead of inlining `std::vector::empty`, call it to know whether vector is empty or not.

Reviewed By: jvillard

Differential Revision: D5573379

fbshipit-source-id: e024a42

infer/models/cpp/include/infer_model/vector.h

@@ -237,10 +237,10 @@ class vector {
   const_reverse_iterator crend() const noexcept { return rend(); }
 
   size_type size() const noexcept {
-    if (beginPtr) {
-      return 10;
+    if (empty()) {
+      return 0;
     }
-    return 0;
+    return 10;
   }
 
   size_type capacity() const noexcept {}

infer/tests/codetoanalyze/cpp/errors/vector/loop.cpp

@@ -8,7 +8,7 @@
  */
 #include <vector>
 
-void foreach_access_ok(std::vector<int>& vec) {
+void foreach_access1_ok(std::vector<int>& vec) {
   if (vec.empty()) {
     // do nothing
   }
@@ -17,6 +17,13 @@ void foreach_access_ok(std::vector<int>& vec) {
   }
 }
 
+void foreach_access2_ok(std::vector<int>& vec) {
+  int s = vec.size();
+  for (const auto& elem : vec) {
+    auto r = elem;
+  }
+}
+
 void iterator_for_access_ok(std::vector<int>& vec) {
   if (vec.empty()) {
     // do nothing