在C ++中将整数填充/嵌入到字符串中

Question

i have some data that has to match a certain packet protocol. i'm getting said data as a std::string.

i need to take some integer values and put them in the string in certain locations. i don't want to convert the integer to string values; i want to copy the actual byte values in.

so for example, if my string is "abcdefg", and i have a 2 byte short with the value of 25 that i want to put into the 1st position, i don't want "a25defg". i want "a" + 0x00 + 0x19 + "defg" (forgive me for that. it was the best way i could think to illustrate what i want).

is there a slick way to do this using regular C++ strings? boost isn't a possibility for me here due to system req's.

from what i see, stringstream may help, but all of the examples i see are converting the number/short/integer into the ascii version of it, and that's not what i want.

i can easily do this with char arrays, but i was wondering if i can do it with strings.

thanks.

Answer 1

Its not recommended, but you can just reinterpret it:

short something = 25;
string something_string(reinterpret_cast<unsigned char*>(&something),
                        sizeof(something));
// something_string == 19 00

You need to take care of endianess though, (like hton )

Answer 2

You say it's easy to do with char arrays, but it's just as easy with std::strings!

std::string str = "abcdef";
short val = 25;
str[1] = val>>8;
str[2] = val&255;

That works the way you want. If the protocol is big-endian.

Edit:
This works only if you want to replace existing chars in the string. To create a new string from scratch, or add a value at the end, see Dani's answer.

Answer 3

template<size_t N>
void cram_helper( unsigned long long value, char* const dst )
{
    char* p = dst + N;
    while (p > dst) { *--p = value & 0xFF; value >>= 8; }
}

template<typename T>
void cram( T t, char* dst )
{
    cram_helper<sizeof(T)>(t, dst);
}

char buffer[] = { "abcdefg" };
short s = 25;
cram(s, buffer+1);

string buffer1 = "abcdefg";
cram(s, &buffer[1]);

---

template<size_t N, typename Container>
void cram_helper( unsigned long long value, Container& dst, int initial_index )
{
    char index = initial_index + N;
    while (index > initial_index) { dst[--index] = value & 0xFF; value >>= 8; }
}

template<typename T, typename Container>
void cram( T t, Container& dst, size_t initial_index = 0 )
{
    cram_helper<sizeof(T)>(t, dst, initial_index);
}

std::string buffer2 = "abcdefg";
short s = 25;
cram(s, buffer2, 1);