Definition and Declaration

1. Variable:

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int A;

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This isn't a Declaration, but also a Definition.

Since It is a defintion, it's must "Define only once".

----------------------Example------------------

If you write the this in a. h file, and included it everywhere ...

We know that ' #include ' means extracted the original file totally here, so the same Var int a have been defined as many Ti Mes as been included, which is invalid.

Ps:compile is OK, since it's local, local in the specific CPP file. When Complier was working on this file, he Dosen ' t know even a bit information the world outside.

So the generated file, O is also local.

However, Link is not OK. For multi definition error of int a.

2.Function:

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void f ();

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So you can declare it many times, everywhere. This is just a name. Comile and Link are both OK.

Define Variable in a. h file, "static" "extern" "Const" ... are needed.

Definition and Declaration