C++ error: undefined reference to function - c++

Question

I have three files:
Two .cpp files (Reader.cpp and algo.cpp) and one header file (algo.h).
In Reader.cpp file, its function PacketHandler calls a function present in algo.cpp.
Reader.cpp
void PacketHandler(Packet* sniff_packet, void* user) {
std::string payload;
RawLayer* raw_payload = sniff_packet->GetLayer<RawLayer>();
if (raw_payload) {
/* Summarize some data */
cout << "[+] ------- [+]" << endl;
TCP* tcp_layer = sniff_packet->GetLayer<TCP>();
cout << "[#] TCP packet from source port: " << dec << tcp_layer->GetSrcPort() << endl;
cout << "[#] With Payload: " << endl;
payload = raw_payload->GetStringPayload();
SPPM(payload); // Function present in algo.cpp
}
}
void main() { // }
algo.h
#ifndef algo_H_
#define algo_H_
#include <string>
using namespace std;
void SPPM (std::string input);
#endif
algo.cpp
#include "algo.h"
#define byte uint8_t
using namespace std;
void SPPM(std::string input){ //definition }
When I compile the code using g++, I get this error:
In function `PacketHandler(Crafter::Packet*, void*)':
/home/maleeha/libcrafter_latest/libcrafter-master/libcrafter/pcap_reader.cpp:32: undefined reference to `SPPM(std::basic_string<char, std::char_traits<char>, std::allocator<char> >)'collect2: ld returned 1 exit status
I am executing my code this way:
g++ -c Reader.cpp -o Reader.o -g -lcrafter
g++ -c algo.cpp -o algo.o
g++ -o program Reader.o algo.o -lcrafter
./program Packets.pcap
Why is this error coming?