LD_PRELOAD help - c++

Question

I'm trying to use LD_PRELOAD.
original.cpp
void myPuts() {
puts ("Hello myPuts");
}
int main() {
myPuts();
return 0;
}
hacked.cpp
void myPuts() {
std::cout >> "Hello hacked myPuts";
}
I compile original.cpp:
g++ original.cpp
And hacked.cpp:
g++ -shared -fPIC hacked.cpp
I try:
LD_PRELOAD=./hacked.so ./original.out
The string "Hello hacked myPuts" should be seen, by "Hello myPuts" appears.
(If I try to "overwrite" the puts function, it works correctly)
What am I missing?

Answer 1

From man ld.so
LD_PRELOAD
A whitespace-separated list of additional, user-specified, ELF shared libraries to be loaded before all others. This can be used to selectively override functions in other shared libraries.
If myPuts was in shared library linked to main application it would work, but not when
myPuts exists in the application and does not resolved in an external library.

Answer 2

You should have:
main.cpp
int main() {
myPuts();
return 0;
}
original.cpp
void myPuts() {
puts ("Hello myPuts");
}
hacked.cpp
void myPuts() {
std::cout << "Hello hacked myPuts";
}
Compiling all:
g++ -shared -fPIC original.cpp -o liboriginal.so
g++ -shared -fPIC hacked.cpp -o libhacked.so
g++ main.cpp -loriginal -o main.out
And using:
LD_PRELOAD=./libhacked.so ./main.out