//older version
atoi( str.c_str() )
//------------------------------------------//
// C++11
// string -> integer
int i = std::stoi(s);
// string -> float
float f = std::stof(s);
// string -> double
double d = std::stod(s);
//------------------------------------------//
#include <sstream>
// string -> integer
std::istringstream ( str ) >> i;
// string -> float
std::istringstream ( str ) >> f;
// string -> double
std::istringstream ( str ) >> d;
//------------------------------------------//
// Boost's lexical_cast
#include <boost/lexical_cast.hpp>
#include <string>
std::string str;
try {
    int i = boost::lexical_cast<int>( str.c_str());
    float f = boost::lexical_cast<int>( str.c_str());
    double d = boost::lexical_cast<int>( str.c_str());
    } catch( boost::bad_lexical_cast const&amp;amp;amp; ) {
        // Error management
}

char* strtoch(string str){
    char *cstr = new char[str.length() + 1];
    strcpy(cstr, str.c_str());
    return cstr;
}
//------------------------------------------//
std::string x = "hello world";
const char* p_c_str = x.c_str();
const char* p_data = x.data();
char* p_writable_data = x.data(); // for non-const x from C++17
char* p_x0_rw = &x[0];  // compiles if x is not const...
//------------------------------------------//
std::vector<char> cstr(str.c_str(), str.c_str() + str.size() + 1);
// or
std::string str;
std::vector<char> writable(str.begin(), str.end());
writable.push_back('\0');

#include <stdlib.h>
int atoi(const char *nptr);
long atol(const char *nptr);
long long atoll(const char *nptr);
long long atoq(const char *nptr);

// read the value as an ascii code
char a = 'a';
// to
int ia = (int)a;
// or
int ia = a;
// or
int num = static_cast<int>(letter); // if letter='a', num=97
//------------------------------------------//
//convert the character '0' -> 0, '1' -> 1, etc
char a = '4';
int ia = a - '0'; // a - '0' is equivalent to ((int)a) - ((int)'0')

int i = 49;
char c = i; // c='1' 49=1
char c = (char)i; // c='1' 49=1
char c = '0'+i; // c='a' 48+49=97=a

int i = 49;
char buffer[33];
char *c = itoa(i,buffer,10);

#include <string>
int i = 49;
std::string s = std::to_string(i);
// or
auto s = std::to_string(i);
// or
char *intStr = itoa(i);
string str = string(intStr);
// or
#include <sstream>
stringstream ss;
ss << a;
string str = ss.str();
// or
#include <sstream>
template <typename T>
std::string NumberToString ( T Number )
{
   std::ostringstream ss;
   ss << Number;
   return ss.str();
}
// or
#include <boost/lexical_cast.hpp>
int num = 4;
std::string str = boost::lexical_cast<std::string>(num);