C++中definition與declaration的區別

翻了好幾篇關於definition與declaration和博客，都寫的很坑人 ，看來我得寫一篇了，避免很多新手一入門就被坑了。

definition是通過declaration完整的定義了實體，每個declaration都是definition，除了下面幾種情況。

1,存儲類標識符extern開頭的或者語言連接標識符開頭，但未初始化的。

extern const int a; // declares, but doesn't define a
extern const int b = 1;// defines b

extern C void f1(void(*pf)()); // declares a function f1 with C linkage,

2.沒有函數體的函數

int f(int); // declares, but doesn't define f

3.函數中未定義的參數

int f(int x); // declares, but doesn't define f and x
int f(int x) { // defines f and x
     return x+a;
}

4.類中聲明的靜態變量

struct S {    // defines S
    int n;        // defines S::n
    static int i; // declares, but doesn't define S::i
};
int S::i; // defines S::i

5.只申明了類的名字，主要用在forward declaration和elaborated type中

struct S; // declares, but doesn't define S
class Y f(class T p); // declares, but doesn't define Y and T (and also f and p)

6.模板參數

template // declares, but doesn't define T

7.預先申明的模板

template<> struct A; // declares, but doesn't define A

8.typedef聲明

typedef S S2; // declares, but doesn't define S2 (S may be incomplete)

9.alias申明

using S2 = S; // declares, but doesn't define S2 (S may be incomplete)

10.模糊enum的申明

enum Color : int; // declares, but doesn't define Col

11.Using-declaration

using N::d; // declares, but doesn't define d