How concatenate a string and a const char?
string a = "hello ";
const char *b = "world";

const char *C;

string a = "hello ";
const char *b = "world";
a += b;
const char *C = a.c_str();

string a = "hello ";
const char *b = "world";
const char *C = (a + b).c_str();

#include <string>
 ///...
 std::string str="foo";
 std::string str2=str+" bar";
 str+="bar";

#include <sstream>
//...
std::string str1="hello";
std::stringstream ss;
ss << str1 << "foo" << ' ' << "bar" << 1234;
std::string str=ss.str();

my_c_func(str1.c_str());

char *cp=std::malloc(str1.size()+1);
std::copy(str1.begin(), str2.end(), cp);
cp[str1.size()]='';