Question

Whilst reading through the DirectWrite source code I came across the following struct:

/// <summary>
/// Line breakpoint characteristics of a character.
/// </summary>
struct DWRITE_LINE_BREAKPOINT
{
    /// <summary>
    /// Breaking condition before the character.
    /// </summary>
    UINT8 breakConditionBefore  : 2;

    /// <summary>
    /// Breaking condition after the character.
    /// </summary>
    UINT8 breakConditionAfter   : 2;

    /// <summary>
    /// The character is some form of whitespace, which may be meaningful
    /// for justification.
    /// </summary>
    UINT8 isWhitespace          : 1;

    /// <summary>
    /// The character is a soft hyphen, often used to indicate hyphenation
    /// points inside words.
    /// </summary>
    UINT8 isSoftHyphen          : 1;

    UINT8 padding               : 2;
};

Notice the strange ” : ” after each member declaration. I’m going to assume it’s a default initialisation value for the member variable.

I tried searching Google to confirm, but without knowing exactly what it’s called I didn’t get far (most of the results where to do with default initialisation).

What is the name of this technique?

Answer 1

Notice the strange ” : ” after each
member declaration. I’m going to
assume it’s a default initialisation
value for the member variable.

It’s not default initialization. It means breakConditionBefore is just 2 bit integer, isWhitespace is a 1 bit integer. And so on.

In DWRITE_LINE_BREAKPOINT, one 8-bit integer (i.e UINT8) is divided amongst 5 members, 3 of which are 2-bit integers, and 2 members are 1-bit integers.

Read about Bit-fields