GCC [7] - compilation checks the declaration of functions, and link checks the definition bugs of functions

Project scenario ：

• I have a problem recently , Let's simplify ：

// int xxx();
 
int main()
{
    
    xxx();
}

• Compile the , Natural compilation, but , The message is ：

[email protected]:~/taoge/cpp/test$ g++ -c main.cpp 
main.cpp: In function ‘int main()’:
main.cpp:5:9: error: ‘xxx’ was not declared in this scope
     xxx();
         ^
[email protected]:~/taoge/cpp/test$ 

• Open the above comment , obtain ：

int xxx();
 
int main()
{
    
    xxx();
}

• Compile the , Can you pass ？ have a look ：

[email protected]:~/taoge/cpp/test$ g++ -c main.cpp     
[email protected]:~/taoge/cpp/test$ 

• Link up , Find out xxx() Undefined .

[email protected]:~/taoge/cpp/test$ g++ main.o
main.o: In function `main': main.cpp:(.text+0x5): undefined reference to `xxx()'
collect2: error: ld returned 1 exit status

So as to confirm the problems in the title .
My wonder at that time was , xxx Function has no definition , Can actually compile . It does . To distinguish between compilation and linking .
Take a closer look at the above English Tips , Compile not pass , It is shown that not declared, And the link doesn't pass , It is shown that undefined. therefore , At a glance .