blocks|key|624463|text|在a.cpp中声明它为extern，在b.cpp中不使用extern：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|624464|a.h|624465|extern+const+int+n+;|code-block|syntax|javascript|624466|a.cpp|624467|#include+"a.h"
...
const+int+n=+8|624468|b.cpp：|624469|#include+"a.h"
...


int+main()
{++++++++
++++int+m+=+n;+
}|624470|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|T|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|V|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|W|8|@]|9|@]|A|$G|H]]|$1|M|3|N|5|6|7|X|8|@]|9|@]|A|$]]|$1|O|3|P|5|F|7|Y|8|@]|9|@]|A|$G|H]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

Declare it extern in a.cpp and just use without extern in b.cpp: 

a.h

<pre><code>extern const int n ;
</code></pre>

a.cpp

<pre><code>#include "a.h"
...
const int n= 8
</code></pre>

b.cpp:

<pre><code>#include "a.h"
...


int main()
{ 
 int m = n; 
}
</code></pre>

blocks|key|5070922|text|这是因为默认情况下，const暗示内部链接，因此您的“定义”在出现它的翻译单元之外不可见。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|5070923|在这种情况下，到目前为止最好的解决方案是将声明(extern+int+const+n;)放在一个头文件中，并将其包含在a.cpp和b.cpp中。链接是由编译器看到的第一个声明决定的，因此a.cpp中后面的定义将具有正确的(外部)链接。|5070924|或者，您可以在定义中强制链接：|5070925|extern+int+const+n+=+8;|code-block|syntax|javascript|5070926|尽管有extern，但这仍然是一个定义；任何在类定义之外带有初始化器的东西都是一个定义。|5070927|entityMap^0|A|5|0|O|J|1N|5|1T|5|2L|5|0|0|0|3|6|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|V|8|@$9|W|A|X|B|C]|$9|Y|A|Z|B|C]|$9|10|A|11|B|C]|$9|12|A|13|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|14|8|@]|D|@]|E|$]]|$1|J|3|K|5|L|7|15|8|@]|D|@]|E|$M|N]]|$1|O|3|P|5|6|7|16|8|@$9|17|A|18|B|C]]|D|@]|E|$]]|$1|Q|3|-4|5|6|7|19|8|@]|D|@]|E|$]]]|R|$]]

It's because <code>const</code> implies internal linkage by default, so
your "definition" isn't visible outside of the translation unit
where it appears.

In this case, by far the best solution is to put the declaration
(<code>extern int const n;</code>) in a header file, and include that in
both <code>a.cpp</code> and <code>b.cpp</code>. The linkage is determined by the
first declaration the compiler sees, so the later definition in
<code>a.cpp</code> will have the correct (external) linkage. 

Alternatively, you can force the linkage in the definition:

<pre><code>extern int const n = 8;
</code></pre>

Despite the <code>extern</code>, this is still a definition; anything with
an initializer outside of a class definition is a definition.

blocks|key|5554779|text|C%2B%2B中的const和constexpr变量有内部链接(因此在其他编译单元中无法访问)，如果它们没有同时声明为extern+(无论是在定义中还是在前面的声明中)。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|5554780|在C中，情况并非如此(好吧，C没有constexpr)，所以您的代码是有效的，而且您可以将extern放在定义中。|5554781|因此，如果你想编写C和C%2B%2B的代码(正如詹姆斯所指出的，这两个声明可能应该来自同一个头文件)：|5554782|//+a.cpp
extern+const+int+n;
const+int+n+=+8;

//+b.cpp
extern+const+int+n;

int+main()
{

++++int+m+=+n;+
}|code-block|syntax|javascript|5554783|如果你不这样做|5554784|//+a.cpp
extern+const+int+n+=+8;|5554785|也是可能的|5554786|entityMap^0|5|5|B|9|1J|6|0|H|9|19|6|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]|$9|11|A|12|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|13|8|@$9|14|A|15|B|C]|$9|16|A|17|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|18|8|@]|D|@]|E|$]]|$1|J|3|K|5|L|7|19|8|@]|D|@]|E|$M|N]]|$1|O|3|P|5|6|7|1A|8|@]|D|@]|E|$]]|$1|Q|3|R|5|L|7|1B|8|@]|D|@]|E|$M|N]]|$1|S|3|T|5|6|7|1C|8|@]|D|@]|E|$]]|$1|U|3|-4|5|6|7|1D|8|@]|D|@]|E|$]]]|V|$]]

<code>const</code> and <code>constexpr</code> variables in C++ have internal linkage (and thus aren't accessible in other compilation unit) if they aren't also declared <code>extern</code> (either in the definition or in a previous declaration).

In C, it isn't the case (well C hasn't <code>constexpr</code>) so your code is valid, and more you can put <code>extern</code> on a definition.

So if you want to write code which is both C and C++ (and the two declarations should probably come from the same header as James pointed out):

<pre><code>// a.cpp
extern const int n;
const int n = 8;

// b.cpp
extern const int n;

int main()
{

 int m = n; 
}
</code></pre>

if you don't

<pre><code>// a.cpp
extern const int n = 8;
</code></pre>

is also possible

blocks|key|1721430|text|To+share+a+const+object+among+multiple+files,+you+must+define+the+variable+as+extern.|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1721431|To+define+a+single+instance+of+a+const+variable,+we+use+the+keyword+extern+on+both+its+definition+and+declaration(s):|code-block|syntax|javascript|1721432|根据这些规则，您只需在定义中添加extern关键字。你已经在声明中有它了。|1721433|entityMap^0|0|2D|0|0|G|6|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|M|3|-4|5|6|7|V|8|@]|D|@]|E|$]]]|N|$]]

<code>To share a const object among multiple files, you must define the variable as extern.</code>

<pre><code>To define a single instance of a const variable, we use the keyword extern on both its definition and declaration(s):
</code></pre>

From these rules you just need to add the <code>extern</code> keyword in your definition. you already have it in declaration.

blocks|key|5554834|text|如果这里的其他答案不起作用，可能是因为您的定义位于不同的名称空间中……如果编译通过，并且您得到一个undefined+symbol链接器错误：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|5554835|5554836|检查未定义符号的命名空间；这是extern+const+int+n符号的有效命名空间，这是您在其中进行const+int+n+=+8定义的有效命名空间。|unordered-list-item|5554837|5554838|entityMap^0|1D|G|0|0|F|I|1F|F|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|P|8|@]|D|@]|E|$]]|$1|G|3|H|5|I|7|Q|8|@$9|R|A|S|B|C]|$9|T|A|U|B|C]]|D|@]|E|$]]|$1|J|3|-4|5|6|7|V|8|@]|D|@]|E|$]]|$1|K|3|-4|5|6|7|W|8|@]|D|@]|E|$]]]|L|$]]

If the other answers here do not do the trick, it may be the case that you have your definitions in different namespaces... if the compilation passes, and you get an <code>undefined symbol</code> linker error:

<ul>
<li>check the namespace of the undefined symbol; that's the effective namespace for the <code>extern const int n</code> declaration.</li>
<li>ensure that's your effective namespace where you make the <code>const int n = 8</code> definition.</li>
</ul>

My project consists of only two source files:

a.cpp:

<pre><code>const int n = 8;
</code></pre>

b.cpp:

<pre><code>extern const int n;

int main()
{
 // error LNK2001: unresolved external symbol "int const n" (?n@@3HB)
 int m = n; 
}
</code></pre>

I know there are several methods to make it work; however, I just wonder WHY it does't work?

Why does "extern const int n;" not work as expected?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我的项目只包含两个源文件：a.cpp：const int n = 8;b.cpp：extern const int n;int main(){    // error LNK2001: unresolved external symbol "int const n" (?n@@3HB)    int m = n; }我...

问为什么"extern const int n;“不能按预期工作？
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问为什么"extern const int n;“不能按预期工作？EN