Syntax for structure dereference operator (->) with array subscript operator ([])

Question

Suppose I have this composition:

class Dataset
{
  std::unique_ptr<Properties> properties;
  std::unique_ptr<Properties> & getProperties() { return properties; }

  Dataset & getDataset() { return *this }
  ....
}

class Properties
{
  Property & operator[](const std::string & s);
  ...
}

How do I call the operator[]? And why is the following a syntax error?

getDataset().getProperties()->["Key"] // syntax error

Answer 1

getDataset().getProperties()->operator[]("Key");

要么

(*getDataset().getProperties())["Key"];

Answer 2

我会比较喜欢：

(*getDataset().getProperties())["Key"];

Answer 3

There are at least two approaches

( *getDataset().getProperties() )["Key"]

getDataset().getProperties()->operator []( "Key" )