# Extra Clang Tools 21.1.6 documentation

## clang-tidy - performance-enum-size

# performance-enum-size

Recommends the smallest possible underlying type for an `enum` or `enum` class based on the range of its enumerators. Analyzes the values of the enumerators in an `enum` or `enum` class, including signed values, to recommend the smallest possible underlying type that can represent all the values of the `enum`. The suggested underlying types are the integral types `std::uint8_t`, `std::uint16_t`, and `std::uint32_t` for unsigned types, and `std::int8_t`, `std::int16_t`, and `std::int32_t` for signed types. Using the suggested underlying types can help reduce the memory footprint of the program and improve performance in some cases.

For example:

    // BEFORE
    enum Color {
        RED = -1,
        GREEN = 0,
        BLUE = 1
    };

    std::optional<Color> color_opt;

The Color `enum` uses the default underlying type, which is `int` in this case, and its enumerators have values of -1, 0, and 1. Additionally, the `std::optional<Color>` object uses 8 bytes due to padding (platform dependent).

    // AFTER
    enum Color : std::int8_t {
        RED = -1,
        GREEN = 0,
        BLUE = 1
    }

    std::optional<Color> color_opt;

In the revised version of the Color `enum`, the underlying type has been changed to `std::int8_t`. The enumerator RED has a value of -1, which can be represented by a signed 8-bit integer.

By using a smaller underlying type, the memory footprint of the Color `enum` is reduced from 4 bytes to 1 byte. The revised version of the `std::optional<Color>` object would only require 2 bytes (due to lack of padding), since it contains a single byte for the Color `enum` and a single byte for the `bool` flag that indicates whether the optional value is set.

Reducing the memory footprint of an `enum` can have significant benefits in terms of memory usage and cache performance. However, it’s important to consider the trade-offs and potential impact on code readability and maintainability.

Enums without enumerators (empty) are excluded from analysis.

Requires C++11 or above. Does not provide auto-fixes.

## Options

EnumIgnoreList

  Option is used to ignore certain enum types. It accepts a semicolon-separated list of (fully qualified) enum type names or regular expressions that match the enum type names. The default value is an empty string, which means no enums will be ignored.