How to concatenate a std::string and an int

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I thought this would be really simple, but it’s presenting some difficulties. If I have

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

How do I combine them to get a single string "John21"?

24 s
24

In alphabetical order:

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);
  1. is safe, but slow; requires Boost (header-only); most/all platforms
  2. is safe, requires C++11 (to_string() is already included in #include <string>)
  3. is safe, and fast; requires FastFormat, which must be compiled; most/all platforms
  4. (ditto)
  5. is safe, and fast; requires the {fmt} library, which can either be compiled or used in a header-only mode; most/all platforms
  6. safe, slow, and verbose; requires #include <sstream> (from standard C++)
  7. is brittle (you must supply a large enough buffer), fast, and verbose; itoa() is a non-standard extension, and not guaranteed to be available for all platforms
  8. is brittle (you must supply a large enough buffer), fast, and verbose; requires nothing (is standard C++); all platforms
  9. is brittle (you must supply a large enough buffer), probably the fastest-possible conversion, verbose; requires STLSoft (header-only); most/all platforms
  10. safe-ish (you don’t use more than one int_to_string() call in a single statement), fast; requires STLSoft (header-only); Windows-only
  11. is safe, but slow; requires Poco C++ ; most/all platforms

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