Examples of errors detected by the V708 diagnostic

V708. Dangerous construction is used: 'm[x] = m.size()', where 'm' is of 'T' class. This may lead to undefined behavior.

Chromium

V708 CWE-758 Dangerous construction is used: 'm[x] = m.size()', where 'm' is of 'unordered_map' class. This may lead to undefined behavior. trace_log.cc 1343

std::unordered_map<std::string, int> thread_colors_;

std::string TraceLog::EventToConsoleMessage(....) {
  ....
  thread_colors_[thread_name] = (thread_colors_.size() % 6) + 1;
  ....
}

In case if the set thread_colors_ already contains the item associated with thread_name, there will be no problems. However, in case of its lack the program may act by two different scenarios depending on the version of the compiler, operating system, and so on, because the evaluation order of operator assignment operands is not defined.

LLVM/Clang

V708 [CWE-758] Dangerous construction is used: 'FeaturesMap[Op] = FeaturesMap.size()', where 'FeaturesMap' is of 'map' class. This may lead to undefined behavior. RISCVCompressInstEmitter.cpp 490

static void getReqFeatures(std::map<StringRef, int> &FeaturesMap,
                           const std::vector<Record *> &ReqFeatures) {
  for (auto &R : ReqFeatures) {
    StringRef AsmCondString = R->getValueAsString("AssemblerCondString");

    SmallVector<StringRef, 4> Ops;
    SplitString(AsmCondString, Ops, ",");
    assert(!Ops.empty() && "AssemblerCondString cannot be empty");

    for (auto &Op : Ops) {
      assert(!Op.empty() && "Empty operator");
      if (FeaturesMap.find(Op) == FeaturesMap.end())
        FeaturesMap[Op] = FeaturesMap.size();
    }
  }
}