Why can't extern compile variables decorated with const?

const Variables can be used by other files extern Do you quote ？ Why? ？

Let's start with a piece of code ：

// main.cc
#include<stdio.h>
//  Reference externally defined const_int Variable 
extern const int const_int;
int main()
{
    printf("const_int：%d\n",const_int);
    return 0;
}
// const.cc
//  Definition const  Variable 
const int const_int  = 10;

Compile Links ：

 $ g++ -o main main.cc const.cc
 /tmp/ccWHAXxB.o: In function `main':
main.cc:(.text+0x6): undefined reference to `const_int'
collect2: error: ld returned 1 exit status

We found a link problem , say const_int There are no defined references , But we are http://const.cc It's defined in the file .

Let's get rid of const Modifier compile again , Discovery is possible . From the above compilation problem , That leads to what we are going to talk about today . As for why it compiles, but , Finally, explain .

Of course you will find , according to C Code to compile , It can be compiled . I'll explain later .

Link properties

We all know ,C/C++ The compilation of code is usually precompiled , assembly , compile , link （ Reference resources hello How the program is generated ） There are usually variables with three link attributes ： External links , Internal link or no link .

Has a functional scope , Variables in block scope or function prototype scope are unlinked variables ; Variables with file scope can be internal links or external links . External link variables can be used in multiple files , Internal link variables can only be used in one compilation unit （ A source code file and the header file it contains ）.

About scope , You can also refer to 《 Global variables , Static global variables , local variable , Static local variables 》.
Said so much , Take a specific example ：

 #include<stdio.h>
 int external_link = 10;  //  File scope , External links 
 static internal_link = 20; //  File scope , internal link 
 int main()
 {
     int no_link = 30;   //  No links 
     printf("%d %d %d \n",external_link,internal_link,no_link);
     return 0;
 }

It's easy to distinguish between unlinked variables , As long as it is not a variable of file scope , Basically, there is no link attribute . Is the file scope variable an internal link or an external link ？ Just see if there is static Can be modified . Of course for C++, It also depends on whether there is const modification , Let's talk about it later .

How to know what link attribute a variable is ？

for instance , In the previous code , We are in accordance with the C Code to compile ：

 $ gcc -c const.c 
 $ nm const.o |grep const_int
 0000000000000000 R const_int

nm Command in 《linux Common commands - Development and debugging 》 There is a little introduction , It can be used to see ELF Symbol information of the file .

From the results here we can see const_int The front is R Embellished ,
R： The symbol is located in the read-only data area ,READONLY The meaning of

and The letter is capitalized , In fact, it also means that it has external link properties .

Let's see according to C++ The code to compile ：

 $ g++ -c const.c
 $ nm const.o |grep const_int
 0000000000000000 r _ZL9const_int

You can see , Its modifier is also r, But it is lowercase r, Lowercase letters indicate that the variable has internal link properties .

nm The command is very practical , But this article is not the focus .

const keyword

Speaking of const keyword , stay 《const How to use keywords 》 and 《C++ Medium const And C Medium const What's the difference ？》 Has been analyzed in , Let's talk about it briefly , By const Keyword modifies the variable , Indicates that it is read-only , Do not want to be modified .

extern keyword

extern Keywords can refer to external definitions , Presumably many friends are already familiar with , for instance , If you take the const Keywords are removed ,http://main.cc Medium extern It means , That means there is one const_int Variable , But it is defined elsewhere , So here extern Decorate it , So in the link phase , It will find its definition in other compilation units , And link .

Of course , Another function that is less concerned is , stay C++ in , It can change const Link properties of variables .

Yes , stay C++ in , It changed const_int Link properties for . We can modify it const.c Is as follows ：

 extern const int const_int  = 10;

And then check it out ：

  $ nm const.o |grep const_int
 0000000000000000 R const_int

Found no , The decoration in front of it becomes capitalized R 了 , So at this point , You compile again , You can compile , It's not a mistake , about C, It is originally an external link attribute , Therefore, no error will be reported at all .

extern There is another use ：
《C++ How to call C Interface 》？

Solve the problem

therefore , The common problem of link error is that the definition cannot be found , The reason is nothing more than ：

• Undefined
• Defined elsewhere , But it does not have external link properties
• Defined , Have external link properties , But there is a problem with the order of links

Because in C++ in , By const The decorated variable defaults to the internal link attribute , Because the link will not find the definition .

summary

This article starts with a compilation problem , Led to a lot of content , Include ：

• Scope --《 Global variables , Static global variables , local variable , Static local variables 》
• const keyword --《const How to use keywords 》
• extern keyword
• nm Look at the symbol table