声明:“在某处，存在一个foo。”

定义:“…就是这儿!”

c++标准3.1节:

声明将名称引入到翻译单元中，或者重新声明前一个翻译单元中引入的名称 声明。声明指定这些名称的解释和属性。

下一段声明(强调是我的)声明是一种定义，除非……

．.． 它声明一个函数而不指定函数体:

void sqrt(double);  // declares sqrt

．.． 它在类定义中声明一个静态成员:

struct X
{
    int a;         // defines a
    static int b;  // declares b
};

．.． 它声明了一个类名:

class Y;

．.． 它包含没有初始化式或函数体的extern关键字:

extern const int i = 0;  // defines i
extern int j;  // declares j
extern "C"
{
    void foo();  // declares foo
}

．.． Or是类型定义或using语句。

typedef long LONG_32;  // declares LONG_32
using namespace std;   // declares std

现在，理解声明和定义之间的区别很重要的一个重要原因是:一个定义规则。c++标准第3.2.1节:

任何翻译单元都不能包含任何变量、函数、类类型、枚举类型或模板的多个定义。

这听起来很俗气，但这是我能把这些条款直接记在脑子里的最好方法:

宣言:想象托马斯·杰斐逊发表演讲……“我在此声明这个foo在这个源代码中存在!!”

定义:想象一本字典，你正在查找Foo和它的实际意思。

从c++标准文档中添加定义和声明示例(来自3.1节声明和定义)

定义:

int a;                       // defines a
extern const int c = 1;      // defines c
int f(int x) { return x+a; } // defines f and defines x
struct S { int a; int b; };  // defines S, S::a, and S::b
struct X {                   // defines X
    int x;                   // defines non-static data member x
    static int y;            // DECLARES static data member y
    X(): x(0) { }            // defines a constructor of X
};
int X::y = 1;                // defines X::y
enum { up, down };           // defines up and down
namespace N { int d; }       // defines N and N::d
namespace N1 = N;            // defines N1
X anX;                       // defines anX

声明:

extern int a;                 // declares a
extern const int c;           // declares c
int f(int);                   // declares f
struct S;                     // declares S
typedef int Int;              // declares Int
extern X anotherX;            // declares anotherX
using N::d;                   // declares d

《K&R》(第二版)中有一些非常明确的定义;这有助于把它们放在一个地方，并作为一个整体来阅读:

"Definition" refers to the place where the variable is created or assigned storage; "declaration" refers to the places where the nature of the variable is stated but no storage is allocated. [p. 33] ... It is important to distinguish between the declaration of an external variable and its definition. A declaration announces the properties of a variable (primarily its type); a definition also causes storage to be set aside. If the lines int sp; double val[MAXVAL] appear outside of any function, they define the external variables sp and val, cause storage to be set aside, and also serve as the declaration for the rest of that source file. On the other hand, the lines extern int sp; extern double val[]; declare for the rest of the source file that sp is an int and that val is a double array (whose size is determined elsewhere), but they do not create the variables or reserve storage for them. There must be only one definition of an external variable among all the files that make up the source program. ... Array sizes must be specified with the definition, but are optional with an extern declaration. [pp. 80-81] ... Declarations specify the interpretation given to each identifier; they do not necessarily reserve storage associated with the identifier. Declarations that reserve storage are called definitions. [p. 210]

宣言

声明告诉编译器a 程序元素或名称存在。一个 声明引入一个或多个 命名到程序中。声明可以 在一个程序中出现一次以上。 因此，类，结构， 枚举类型等 用户定义的类型可以声明为 每个编译单元。

定义

定义指定哪些代码或数据 名称描述。名称必须为 在使用之前声明。