domiyanyue

int bar();

int foo(){
    return bar();
}

int bar(){
    return 1;
}

domiyanyue

g++ -c example.cpp -o example.o

domiyanyue

g++ -c main.cpp -o main.o
g++ -c util.cpp -o util.o
g++ main.o util.o -o a.out

domiyanyue

int foo();
int foo();
int foo(){
    return 1;
}

int main(){
    return foo();
}

domiyanyue

one_definition_rule# g++ -c error1.cpp
error1.cpp: In function ‘int foo()’:
error1.cpp:5:5: error: redefinition of ‘int foo()’
 int foo(){
     ^~~
error1.cpp:1:5: note: ‘int foo()’ previously defined here
 int foo() {
     ^~~

domiyanyue

muti_file# g++ -c main.cpp
muti_file# g++ -c main.cpp -o main.o
muti_file# g++ -c util.cpp -o util.o
muti_file# g++ main.o util.o -o a.out
util.o: In function `foo()':
util.cpp:(.text+0x0): multiple definition of `foo()'
main.o:main.cpp:(.text+0x0): first defined here
collect2: error: ld returned 1 exit status

domiyanyue

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

domiyanyue

no_header_file# g++ -c main.cpp -o main.o
no_header_file# g++ -c util.cpp -o util.o
no_header_file# g++ main.o util.o -o a.out

domiyanyue

#include "util.h"
int main(){
   return foo();  
}

domiyanyue

struct Time {
    int hour;
    int minute;

    Time(int h, int m) : hour(h), minute(m){}
};

bool isDayTime(Time t);