V821. The variable can be constructed in a lower level scope.

The analyzer has detected a variable that could be created in a lower level scope. By changing the scope of an object, you can optimize the code's performance and memory consumption.

For example, when you allocate memory for a large array or create a "heavy" class object, and want this variable to be used only when a certain condition is met, it is better to place it in the block of a conditional statement.

Consider the following example:

void SetToolInfoText(ToolInfo &info, int nResource, int nItem)
{
  Text data(80);                            // <=
  if (nResource)
  {
    info.text = GetResource(nResource);
  }
  else
  {
    GetDataForItem(data, nItem);
    info.text = data.text();
  }
}

It is better to construct the 'data' object only in the 'else' branch.

Improved code:

void SetToolInfoText(ToolInfo &info, int nResource, int nItem)
{
  if (nResource)
  {
    info.text = GetResource(nResource);
  }
  else
  {
    Text data(80);                        // <=
    GetDataForItem(data, nItem);
    info.text = data.test();
  }
}

This diagnostics also detects cases of dynamic memory allocation that could be optimized by moving it to a lower level scope, for example:

void func(bool condition)
{
  int *arr = new int[1000];
  if (condition)
  {
    use(arr);
  }
  delete[] arr;
}

It is better to allocate (and free) memory only if the condition is true.

Improved code:

void func(bool condition)
{
  if (condition)
  {
    int *arr = new int[1000];
    use(arr);
    delete[] arr;
  }
}

Note that the analyzer tries to issue this warning only when moving a variable to a lower level scope gives a significant performance or memory gain. For this reason, the warning is not triggered by single variables of base types and objects created by the constructor without arguments (as tests show, this diagnostic almost always produces false positives in these cases).

This diagnostic is classified as: