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                合規國際互聯網加速 OSASE為企業客戶提供高速穩定SD-WAN國際加速解決方案。 廣告
                [參考原文](http://stackoverflow.com/questions/1410563/what-is-the-difference-between-a-definition-and-a-declaration%20%E5%8F%82%E8%80%83%E5%8E%9F%E6%96%87) 變量聲明:編譯器被通知變量存在,不用為**變量申請內存單元**。 變量聲明包含變量標識符,變量類型,extern修飾 這些信息。不管是新建類型,對象,函數聲明,聲明是編譯器所需要的,聲明告訴編譯器需要接受對這個標識符的引用。聲明有如下形式。 ~~~ externint bar; externint g(int,int); // extern can be omitted for function declarations,函數申明可以省略 double f(int,double); // no extern allowed for type declarations,對于類型聲明省略extern也是行的 extern class foo; ~~~ 變量定義:編譯器為**變量申請內存**,就是變量定義。 包含變量類型,變量標識符這些信息;定義的本質是實例化或者說實現這個標識符,表現就是在內存中申請一個跟類型等量空間大小的內存空間。變量定義是連接器所需要的,連接器通過變量定義來引用到他們的實體內存空間。下面這些是上面聲明的定義: ~~~ int bar; int g(int lhs,int rhs){return lhs*rhs;} double f(int i,double d){return i+d;} class foo {}; ~~~
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