How to concatenate a std::string and an int?

Question

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

Answer 1

It seems to me that the simplest answer is to use the sprintf function:

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

Answer 2

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

Answer 3

If you are using MFC, you can use a CString

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

Managed C++ also has a string formatter.

Answer 4

This is the easiest way:

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

Answer 5

Here is an implementation of how to append an int to a string using the parsing and formatting facets from the IOStreams library.

#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
}

Answer 6

In C++20 you'll be able to do:

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

In the meantime you can use the {fmt} library, std::format is based on:

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

Disclaimer: I'm the author of the {fmt} library and C++20 std::format.