What is the C++ equivalent of this C code - c++

Question

I'm trying to learn how to access c++ library from c, I understood I've to use extern "C", in order to test it, I started with the below working c code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void c_free_string(const char *str){
free(str);
}
char *concat(const char *str1, const char *str2)
{
char *res;
const char del[] = ", ";
res = malloc(strlen(str1) + strlen(str2) + strlen(del) + 1);
if (!res) {
fprintf(stderr, "malloc() failed: insufficient memory!\n");
return EXIT_FAILURE;
}
strcpy(res, str1);
strcat(res, del);
strcat(res, str2);
printf("Result: '%s'\n", res);
return res;
}
int main(void) {
const char str1[] = "First";
const char str2[] = "Second";
char* s = concat(str1, str2);
printf("Result: '%s'\n", s);
c_free_string(s);
return EXIT_SUCCESS;
}
I wanted to write the equivalent c++ code of the above
I want to be ensure using the correct declaration to be able to do c binding with the lib generated from this c++ code.
I started with the belowm but stuck:
#include <iostream>
#include <sstream>
extern "C"
void c_free_string(const char *str){
free(str);
}
extern "C"
char *concat(const char *str1, const char *str2)
{
std::cout << "Hello from C++";
stringstream ss;
ss << str1 << ", " << str2;
string res = ss.str();
std::cout << "Result: "<<res;
return res;
}

Answer 1

Thanks for the comments, the answer looks to be:
#include <iostream>
#include <sstream>
extern "C"
void c_free_string(char *str){
free(str);
}
extern "C"
char *concat(const char *str1, const char *str2)
{
printf("welcome from c++\n");
char *res;
const char del[] = ", ";
res = (char*) malloc(strlen(str1) + strlen(str2) + strlen(del) + 1);
if (!res) {
fprintf(stderr, "malloc() failed: insufficient memory!\n");
exit(EXIT_FAILURE);
}
strcpy(res, str1);
strcat(res, del);
strcat(res, str2);
printf("Result: '%s'\n", res);
return res;
}
UODATE
A cleaner version based on Paul McKenzie feedback:
#include <iostream>
#include <string>
#include <cstring>
extern "C"
void c_free_string(char *str){
delete [] str;
}
extern "C"
char *concat(const char *str1, const char *str2)
{
const char* del = ", ";
int len1 = strlen(str1);
int len2 = strlen(del);
int len3 = strlen(str2);
char *res = new char [len1 + len2 + len3 + 1]{};
strcpy(res, str1);
strcpy(res + len1, del);
strcpy(res + len1 + len2, str2);
return res;
}
int main()
{
char *test = concat("abc", "123");
std::cout << test;
c_free_string(test);
}