How to add characters to strings in C++?

Question

I am writing an IP calculator program to convert an IP adress into the broadcast adress, network adress etc.

I need to convert the adress into 8-digit biniary first, I already used itoa for it decimal to biniary conversion, but I still need to make it always 8 digit.

I want to use a switch to do it. First the program counts the number of digits in the binary form, and then, using switch (I put it into a comment, I need to figure out this proble first) and if it adds enough zeros in front of the number (actually a string at this point) so it would always be 8 characters long.

I want to use string e1('00',e); instruction to add two zeros to the string, but it doesn't work. e is the original string, e1 is the new 8 character string. It doesn't want to compile, stops at this line (string e1('00',e);) and gives:

error: no matching function for call to 'std::__cxx11::basic_string<char>::
basic_string(int, std::__cxx11::string)'|

My code:

#include <iostream>
#include <string>
#include <bits/stdc++.h>
using namespace std;
/**
* C++ version 0.4 std::string style "itoa":
* Contributions from Stuart Lowe, Ray-Yuan Sheu,

* Rodrigo de Salvo Braz, Luc Gallant, John Maloney
* and Brian Hunt
*/
std::string itoa(int value, int base)
{

    std::string buf;

    // check that the base if valid
    if (base < 2 || base > 16)
        return buf;

    enum
    {
        kMaxDigits = 35
    };
    buf.reserve(kMaxDigits); // Pre-allocate enough space.

    int quotient = value;

    // Translating number to string with base:
    do
    {
        buf += "0123456789abcdef"[std::abs(quotient % base)];
        quotient /= base;
    } while (quotient);

    // Append the negative sign
    if (value < 0)
        buf += '-';

    std::reverse(buf.begin(), buf.end());
    return buf;
}

int main()
{
    cout << "Dzien dobry." << endl
         << "Jest OK!\n";
    int a, b, c, d, i, j, k, l, m, n, o, p, r, s, t, u, w, v, x, y, z;

    cout << "Wprowadz pierwszy oktet: ";
    cin >> a;
    cout << "Wprowadz drugi oktet: ";
    cin >> b;
    cout << "Wprowadz trzeci oktet: ";
    cin >> c;
    cout << "Wprowadz czwarty oktet: ";
    cin >> d;
    cout << "Wyswietlam adres IP w postaci dziesietnej: " << a << "." << b << "." << c << "." << d << "\n";
    char res[1000];
    itoa(a, res, 2);
    string e(res);
    itoa(b, res, 2);
    string f(res);
    itoa(c, res, 2);
    string g(res);
    itoa(d, res, 2);
    string h(res);
    //
    x = e.size();
    cout << x << "\n";
    /*
    if (x<8)
    {
        switch(x)
        {
            case 1: string e(1);
            break;
            case 2: string e(3);
            break;
            case 3: string e(2);
            break;
            case 4: string e(0);
            break;
            case 5: string e(4);
            break;
            case 6: string e(7);
            break;
            case 7: string e(0);
            break;
            default: cout << "error";
        }
    }

*/
    string e1('00', e);
    cout << e1 << "\n";
    cout << "Wyswietlam adres IP w postaci dwojkowej: " << e1 << "." << f << "." << g << "." << h;

    return 0;
}

Answer 1

Single quotation marks are for single characters, double quotation marks are for strings, i.e. more than one character, so you need "00".

There is no std::string constructor for the parameters you provide, among other methods, you can use:

string e1;
e1.append("00").append(e);

Or

string e1 = "00";
e1 += e;

Aditional notes:

Avoid using #include <bits/stdc++.h>

Avoid using using namespace std;

Answer 2

Just use + or +=:

std::string foo = "Hello";
std::string bar = foo + " world";

std::cout << bar; // Prints Hello world

You can also modify a string in-place with +=:

std::string foo = "Hello";
foo += " world"; // foo is "Hello world" now

Note that this invalidates any existing iterators you had that pointed to characters inside foo. You can get the new begin and end iterators by just calling begin and end again.