如何向C++中的字符串添加字符？

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, ie 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.