String to Base64

#include <iostream>

char* strToBase64(char* input, size_t size_in)
{
    char base64char[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
    char padding = '=';

    unsigned int storage = 0;
    // storage unit for converting 3 * 8 bits to 4 * 6 bits

    size_t size_out = (size_in % 3) ? 4 * ((size_in / 3) + 1) : 4 * (size_in / 3);
    // rounding up integer division of 4 * (n / 3) to calculate size of Base64 string

    char* base64str = new char[size_out + 1]();
    // allocate enough memory to hold the Base64 string

    for (int i = 0; i < (size_out / 4); ++i)
    {
        for (int c = 0; c < 3; ++c) { storage = (storage << CHAR_BIT) | input[(i * 3) + c]; }
        // place 3 * 8 bits into storage unit

        for (int c = 0; c < 4; ++c) { base64str[(i * 4) + c] = base64char[((storage << c * 6) >> 18) & 63]; }
        // place 4 * 6 bits into Base64 string

        for (int c = 0; c < (((size_in % 3) != 0) ? (3 - (size_in % 3)) : 0); ++c) { base64str[size_out - 1 - c] = padding; }
        // add padding to string
    }
    return base64str;
}

int main()
{
    char inputstr[] = "Hello World!";
    // Your input string here

    char* output = strToBase64(inputstr, (sizeof(inputstr) - 1));
    // - 1 is only required if the string is null-terminated!

    std::cout << output << '\n';
    // Base64 string gets printed here

    delete[] output;
    // delete base64str created in strToBase64()

    return 0;
}