Is there a “glyph not found” character?

Question

Let\'s assume we have a text that contains a Unicode character that cannot be displayed because our font has no corresponding glyph. Usually, a placeholder is displayed inst

Answer 1

Unicode uses these terms:

• replacement glyph
• missing glyph
• interpretable but unrenderable character

The Unicode Standard (10.0) does not define how they have to look, but it suggests in chapter 5.3 [PDF] that implementations display

[…] distinctive glyphs that give some general indication of their type […]

to distinguish them from "unassigned code points". They give some examples:

The Unicode glossary entry says:

It often is shown as an open or black rectangle.

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tl;dr: There is no standardized look/glyph, it’s up to the implementation. To help users, implementations could display glyphs that indicate what type of character it is that can’t be displayed.

Answer 2

There is a notdef character that means the glyph is not found. But it has no charcode. You can use the charcodes of controll characters to insert a notdef character (like "", U+0002)