.. title:: clang-tidy - readability-string-compare

readability-string-compare
==========================

Finds string comparisons using the compare method.

A common mistake is to use the string's ``compare`` method instead of using the
equality or inequality operators. The compare method is intended for sorting
functions and thus returns a negative number, a positive number or
zero depending on the lexicographical relationship between the strings
compared. If an equality or inequality check can suffice, that is recommended.
This is recommended to avoid the risk of incorrect interpretation of the return
value and to simplify the code. The string equality and inequality operators
can also be faster than the ``compare`` method due to early termination.

Example
-------

.. code-block:: c++

  // The same rules apply to std::string_view.
  std::string str1{"a"};
  std::string str2{"b"};

  // use str1 != str2 instead.
  if (str1.compare(str2)) {
  }

  // use str1 == str2 instead.
  if (!str1.compare(str2)) {
  }

  // use str1 == str2 instead.
  if (str1.compare(str2) == 0) {
  }

  // use str1 != str2 instead.
  if (str1.compare(str2) != 0) {
  }

  // use str1 == str2 instead.
  if (0 == str1.compare(str2)) {
  }

  // use str1 != str2 instead.
  if (0 != str1.compare(str2)) {
  }

  // Use str1 == "foo" instead.
  if (str1.compare("foo") == 0) {
  }

The above code examples show the list of if-statements that this check will
give a warning for. All of them use ``compare`` to check equality or
inequality of two strings instead of using the correct operators.

Options
-------

.. option:: StringLikeClasses

   A string containing semicolon-separated names of string-like classes.
   By default contains only ``::std::basic_string``
   and ``::std::basic_string_view``. If a class from this list has
   a ``compare`` method similar to that of ``std::string``, it will be checked
   in the same way.

Example
^^^^^^^

.. code-block:: c++

  struct CustomString {
  public:
    int compare (const CustomString& other) const;
  }

  CustomString str1;
  CustomString str2;

  // use str1 != str2 instead.
  if (str1.compare(str2)) {
  }

If `StringLikeClasses` contains ``CustomString``, the check will suggest
replacing ``compare`` with equality operator.
