Why is the C++ scope resolution operator ::?

This is one of the few questions I didn't find an answer to in Design and Evolution of C++ by Stroustroup. Why is the C++ scope resolution operator ::, as opposed to just :?

I'm guessing it's becaus开发者_运维百科e : is already used to indicate the start of an initialization list in a constructor. Does anyone else agree, disagree, or have a definitive answer on this?

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It would create an ambiguity with goto labels, for one.

foo:bar;  // is foo a label or a namespace?