What is the purpose of \protect? [duplicate]

Question

Possible Duplicate:
What is the difference between Fragile and Robust commands?

I've never understood the purpose of the command \protect? When and why should it be used?

Answer 1

To understand how to use \protect you need to understand the concept of fragile commands and to understand this concept, you need to understand what a moving argument is. Some commands called fragile commands causes an error when they use a moving argument. It's a command that expands into illegal TeX code during the save process. Some arguments are called moving arguments because they may be "moved" to other places in the document. A fragile command that appears in a moving argument must be preceded by a \protect command. For example,

• sectioning commands
• arguments of \caption commands
• commands that produce page headings

An example :

\caption{Beautiful picture from \protect \citeauthor{allan}
  \protect \shortcite{allan}}