std::string name = "John";
int age = 21;

std::string name = "John";
int age = 21;
std::string result;

// 1. with Boost
result = name + boost::lexical_cast<std::string>(age);

// 2. with C++11
result = name + std::to_string(age);

// 3. with FastFormat.Format
fastformat::fmt(result, "{0}{1}", name, age);

// 4. with FastFormat.Write
fastformat::write(result, name, age);

// 5. with the {fmt} library
result = fmt::format("{}{}", name, age);

// 6. with IOStreams
std::stringstream sstm;
sstm << name << age;
result = sstm.str();

// 7. with itoa
char numstr[21]; // enough to hold all numbers up to 64-bits
result = name + itoa(age, numstr, 10);

// 8. with sprintf
char numstr[21]; // enough to hold all numbers up to 64-bits
sprintf(numstr, "%d", age);
result = name + numstr;

// 9. with STLSoft's integer_to_string
char numstr[21]; // enough to hold all numbers up to 64-bits
result = name + stlsoft::integer_to_string(numstr, 21, age);

// 10. with STLSoft's winstl::int_to_string()
result = name + winstl::int_to_string(age);

// 11. With Poco NumberFormatter
result = name + Poco::NumberFormatter().format(age);

auto result = name + std::to_string( age );

std::stringstream ss;
ss << age;
std::cout << name << ss.str() << std::endl;

char buffer[128];
snprintf(buffer, sizeof(buffer), "%s%d", name.c_str(), age);
std::cout << buffer << std::endl;

#include <iostream>
#include <sstream>

std::ostringstream o;
o << name << age;
std::cout << o.str();

#include <iostream>
#include <string>
#include <sstream>
using namespace std;
string itos(int i) // convert int to string
{
    stringstream s;
    s << i;
    return s.str();
}

string s = name + std::to_string(age);

std::string name = "John";
int age = 21;

name += std::to_string(age);

std::cout << name;

John21

sprintf(outString,"%s%d",name,age);

#include <string>
#include <sstream>
using namespace std;
string concatenate(std::string const& name, int i)
{
    stringstream s;
    s << name << i;
    return s.str();
}

#include <sstream>

template <class T>
inline std::string to_string (const T& t)
{
   std::stringstream ss;
   ss << t;
   return ss.str();
}

std::string szName = "John";
   int numAge = 23;
   szName += to_string<int>(numAge);
   cout << szName << endl;

template <typename L, typename R> std::string operator+(L left, R right) {
  std::ostringstream os;
  os << left << right;
  return os.str();
}

std::string foo("the answer is ");
int i = 42;
std::string bar(foo + i);
std::cout << bar << std::endl;

the answer is 42

#include <string>
#include <sstream>
#include <bits/stdc++.h>
#include <iostream>
using namespace std;

int main() {
    string name = "John";
    int age = 21;

    string answer1 = "";
    // Method 1). string s1 = to_string(age).

    string s1=to_string(age); // Know the integer get converted into string
    // where as we know that concatenation can easily be done using '+' in C++

    answer1 = name + s1;

    cout << answer1 << endl;

    // Method 2). Using string streams

    ostringstream s2;

    s2 << age;

    string s3 = s2.str(); // The str() function will convert a number into a string

    string answer2 = "";  // For concatenation of strings.

    answer2 = name + s3;

    cout << answer2 << endl;

    return 0;
}

CString nameAge = "";
nameAge.Format("%s%d", "John", 21);

#include <sstream>
#define MAKE_STRING(tokens) /****************/
    static_cast<std::ostringstream&>(
        std::ostringstream().flush() << tokens
    ).str()
    /**/

int main() {
    int i = 123;
    std::string message = MAKE_STRING("i = " << i);
    std::cout << message << std::endl; // prints: "i = 123"
}

QString string = QString("Some string %1 with an int somewhere").arg(someIntVariable);
string.append(someOtherIntVariable);

#include <boost/format.hpp>
#include <string>
int main()
{
    using boost::format;

    int age = 22;
    std::string str_age = str(format("age is %1%") % age);
}

#include <boost/spirit/include/karma.hpp>
#include <iterator>
#include <string>
int main()
{
    using namespace boost::spirit;

    int age = 22;
    std::string str_age("age is ");
    std::back_insert_iterator<std::string> sink(str_age);
    karma::generate(sink, int_, age);

    return 0;
}

#include <iostream>
#include <locale>
#include <string>

template <class Facet>
struct erasable_facet : Facet
{
    erasable_facet() : Facet(1) { }
    ~erasable_facet() { }
};

void append_int(std::string& s, int n)
{
    erasable_facet<std::num_put<char,
                                std::back_insert_iterator<std::string>>> facet;
    std::ios str(nullptr);

    facet.put(std::back_inserter(s), str,
                                     str.fill(), static_cast<unsigned long>(n));
}

int main()
{
    std::string str = "ID: ";
    int id = 123;

    append_int(str, id);

    std::cout << str; // ID: 123
}

#include <sstream>

std::ostringstream s;
s << "John " << age;
std::string query(s.str());

std::string query("John " + std::to_string(age));

#include <boost/lexical_cast.hpp>

std::string query("John " + boost::lexical_cast<std::string>(age));

char intToChar(int num)
{
    if (num < 10 && num >= 0)
    {
        return num + 48;
        //48 is the number that we add to an integer number to have its character equivalent (see the unsigned ASCII table)
    }
    else
    {
        return '*';
    }
}

string intToString(int num)
{
    int digits = 0, process, single;
    string numString;
    process = num;

    // The following process the number of digits in num
    while (process != 0)
    {
        single  = process % 10; // 'single' now holds the rightmost portion of the int
        process = (process - single)/10;
        // Take out the rightmost number of the int (it's a zero in this portion of the int), then divide it by 10
        // The above combination eliminates the rightmost portion of the int
        digits ++;
    }

    process = num;

    // Fill the numString with '*' times digits
    for (int i = 0; i < digits; i++)
    {
        numString += '*';
    }


    for (int i = digits-1; i >= 0; i--)
    {
        single = process % 10;
        numString[i] = intToChar ( single);
        process = (process - single) / 10;
    }

    return numString;
}

string name = "John";
int age = 5;
char temp = 5 + '0';
name = name + temp;
cout << name << endl;

Output:  John5

auto result = fmt::format("{}{}", name, age);

auto result = std::format("{}{}", name, age);