# Extra Clang Tools 18.1.8 documentation

## clang-tidy - performance-unnecessary-copy-initialization

# performance-unnecessary-copy-initialization

Finds local variable declarations that are initialized using the copy constructor of a non-trivially-copyable type but it would suffice to obtain a const reference.

The check is only applied if it is safe to replace the copy by a const reference. This is the case when the variable is const qualified or when it is only used as a const, i.e. only const methods or operators are invoked on it, or it is used as const reference or value argument in constructors or function calls.

Example:

    const string& constReference();
    void Function() {
      // The warning will suggest making this a const reference.
      const string UnnecessaryCopy = constReference();
    }

    struct Foo {
      const string& name() const;
    };
    void Function(const Foo& foo) {
      // The warning will suggest making this a const reference.
      string UnnecessaryCopy1 = foo.name();
      UnnecessaryCopy1.find("bar");

      // The warning will suggest making this a const reference.
      string UnnecessaryCopy2 = UnnecessaryCopy1;
      UnnecessaryCopy2.find("bar");
    }

## Options

AllowedTypes

  A semicolon-separated list of names of types allowed to be initialized by copying. Regular expressions are accepted, e.g. [Rr]ef(erence)?$ matches every type with suffix Ref, ref, Reference and reference. The default is empty. If a name in the list contains the sequence :: it is matched against the qualified typename (i.e. namespace::Type, otherwise it is matched against only the type name (i.e. Type).

ExcludedContainerTypes

  A semicolon-separated list of names of types whose methods are allowed to return the const reference the variable is copied from. When an expensive to copy variable is copy initialized by the return value from a type on this list the check does not trigger. This can be used to exclude types known to be const incorrect or where the lifetime or immutability of returned references is not tied to mutations of the container. An example are view types that don’t own the underlying data. Like for AllowedTypes above, regular expressions are accepted and the inclusion of :: determines whether the qualified typename is matched or not.