compile undefined and unused function in c++

Here is the sample code. I have a function print without definition and I used it in the print_in_A() in the struct A.

Why can this code compile and run?

void print(int x);


struct A {
    void print_in_A() {
        print(1);
    }
};

int main()
{
    A a;
    return 0;
}

And also, if I define an inline function like this, it can compile and run too. But if I remove the inline keyword, it fails to compile.

void print(int x);

inline void print_int(int a) {
    print(a);
}


int main()
{
    return 0;
}

DEMO - Without error when inline is used

DEMO - With error when inline is not used

Can anyone tell me why?