What is the difference between xsl:copy and xsl:copy-of?

Question

I don't see the difference between xsl:copy and xsl:copy-of.

Which one should I use in which situation?

Answer 1

In short, xsl:copy is a shallow copy; xsl:copy-of is a deep copy.

When to use xsl:copy vs xsl:copy-of

• Use xsl:copy when you want to copy just the context item and have other plans for the children of the context item.
• Use xsl:copy-of when you want to copy XPath-selected nodes and their children, recursively.

Notes for xsl:copy

• The xsl:copy instruction copies the context item but none of its children nodes.
• This is a shallow copy.
• The xsl:copy instruction cannot have a @select XPath.

A very common use of xsl:copy can be found in the identity transformation:

<xsl:template match="@*|node()">
  <xsl:copy>
    <xsl:apply-templates select="@*|node()"/>
  </xsl:copy>
</xsl:template>

Notice that here the node itself is copied via xsl:copy and the children nodes are then transformed via xsl:apply-templates, giving other templates a chance to intervene in the transformation.

Notes for xsl:copy-of

• The xsl:copy-of instruction evaluates the XPath in its required @select attribute and copies the selected nodes and their children nodes, recursively.
• This is a deep copy.
• The xsl:copy-of instruction must have a @select XPath.

Notice that xsl:copy-of could have been used in the identity transformation, however the flexibility afforded by allowing other templates the chance to match during the recursion would have been lost.

Answer 2

xsl:copy is a shallow copy. Use it if all you want is to copy the current node (the "context item" in spec-speak). xsl:copy-of is a deep copy. Use it if you want to copy the complete node tree under the current node. For a more thorough and complete explanation read the spec that has been linked to from the first comment.