linking1.cpp

void bar();
void foo(){
    bar();
}

int main(){
   foo();
    return 0;
}
/******************************************************************/

$ g++ linking.cpp -save-temps

linking.o: In function `foo()':
linking.cpp:(.text+0x5): undefined reference to `bar()'
collect2: error: ld returned 1 exit status

$ nm linking.o
                 U _GLOBAL_OFFSET_TABLE_
                 U _Z3barv
0000000000000000 T _Z3foov
000000000000000c T main

linking2.cpp

void bar();
void foo(){
    //bar();
}

int main(){
    foo();
    return 0;
}

$ g++ linking.cpp -save-temps
no error

$ nm linking.o
0000000000000000 T _Z3foov
0000000000000007 T main

linking3-1.cpp

void bar();
void foo(){
    bar();
}

int main(){
    //foo();
    return 0;
}

$ g++ linking.cpp -save-temps -O1

linking.o: In function `foo()':
linking.cpp:(.text+0x5): undefined reference to `bar()'
collect2: error: ld returned 1 exit status

$ nm linking.o
                 U _GLOBAL_OFFSET_TABLE_
                 U _Z3barv
0000000000000000 T _Z3foov
000000000000000e T main

linking3-2.cpp

void bar();
static void foo(){
    bar();
}

int main(){
    //foo();
    return 0;
}

$ g++ linking.cpp -save-temps -O1
no error

$ nm linking.o
0000000000000000 T main

linking3-3.cpp

void bar();
static void foo(){
    bar();
}

int main(){
    //foo();
    return 0;
}

$ g++ linking.cpp -save-temps -O0

linking.o: In function `foo()':
linking.cpp:(.text+0x5): undefined reference to `bar()'
collect2: error: ld returned 1 exit status

$ nm linking.o
                 U _GLOBAL_OFFSET_TABLE_
                 U _Z3barv
0000000000000000 T _Z3foov
000000000000000c T main

linking4-1.cpp

/* linkingHelper.cpp content */
void bar(){}

/******************************************************************/
/* linking.cpp content */
void foo(){
    bar();
}

int main(){
    foo();
    return 0;
}

$ g++ linking.cpp linkingHelper.cpp -save-temps -O1
no error

$ nm linking.o
                 U _GLOBAL_OFFSET_TABLE_
                 U _Z3barv
0000000000000000 T _Z3foov
000000000000000e T main

linking4-2.cpp

/* linkingHelper.cpp content */
static void bar(){}

/******************************************************************/
/* linking.cpp content */
void foo(){
    bar();
}

int main(){
    foo();
    return 0;
}


$ g++ linking.cpp linkingHelper.cpp -save-temps -O1
linking.o: In function `foo()':
linking.cpp:(.text+0x5): undefined reference to `bar()'
linking.o: In function `main':
linking.cpp:(.text+0x13): undefined reference to `bar()'
collect2: error: ld returned 1 exit status