链接头文件c ++时对函数的未定义引用

Question

I'm trying to do the Chapter8's drill on the books of Bjarne stroustrup. I have followed all the steps but when running the programme i get two errors: undefined reference to 'print_foo' undefined reference to 'print(int)'. I use VSC.

Here are my files:

-----my.h-----

extern int foo;
void print_foo();
void print(int);

-------my.cpp--------

#include"std_lib_facilities.h"
#include"my.h"

void print_foo() 
{
    cout<<"foo = "<<foo<<'\n';
}

void print(int i)
{
    cout<<"i = "<<i<<'\n';
}

-------use.cpp--------

#include<iostream> 
#include"my.h" 

using namespace std;

inline void keep_window_open() {char cc;cin>>cc;}

int main()
{   
    int foo = 7;
    print_foo();
    print(99);

    return 0;
}

Answer 1

TL;DR:

compile the files use.cpp and my.cpp using g++ and declare foo outside main.

---

I was in this same exercise and had the same problem.

So here is my solution:

Generally the extensions in vscode use one single file, so you will need to use the terminal to specify the other files (in this case, my.cpp).

my.h

extern int foo;
void print_foo();
void print(int i);

my.cpp

#include "my.h"
#include "std_lib_facilities.h"

void print_foo()
{
    cout << foo << endl; // foo is defined in use.cpp
}

void print(int i)
{
    cout << i << endl;
}

use.cpp

#include "my.h"

int foo; // foo is declared outside main. Foo is a global var
int main()
{
    foo = 7; //definition of foo
    print_foo();

    print(99);
}

Note that foo is defined outside main because my.h calls extern int foo; .

Then, compile it using:

g++ use.cpp my.cpp