Is there a better way of copying a string to a character array with things already in it...

Question

messageBuffer[0] = 1;
messageBuffer[1] = 0;
for (int i = 2; i < (userName.size() + 2); i++)
{
    messageBuffer[i] = userName[(i - 2)];
}

userName is a string. I was just wondering if there is already a function that exists that I haven't found yet. I have tried looking on cpluscplus but nothing that I see. Thanks for all the help guys =) I really appreiciate it. This site is awesome!

Answer 1

strcpy(messageBuffer + 2, userName.c_str());

standard disclaimers about making sure you have enough memory apply

messageBuffer should be 3 characters bigger than the string (one for \0)

a bit of reference about the function here

Answer 2

Yes, you can use strcpy, memcpy, memmove or std::copy to do this. Just pass the address of messageBuffer[2] as the destination. strcpy( &messageBuffer[2], userName.begin() );

Answer 3

strncpy

EDIT: See @jaif answer for "C++" way.

Answer 4

Use strncpy

Answer 5

What about using strcpy (or strncpy to prevent buffer overflows)?
Make sure the length of messageBuffer allows copying userName and just
strcpy(messageBuffer + 2, userName);

Answer 6

Use strncpy() for ANSI strings (char*), wcsncpy() for Unicode (wchar_t*) strings.

Answer 7

You have two options, the unsafe and the safe way.

Unsafe:

// bad idea, string lengths are not checked and if src is longer than 
// memory available for dest, you will stomp over random memory
strcpy(dest, src);

Safer:

// Much safer, you can specify how many characters to copy (lesser of src length and
// dest length - 1 and add a null terminator '\0' to dest if the string was truncated).
strncpy(dest, src, num_characters);

Answer 8

Use strcpy and check the buffer size by hand.

strncpy is a little safer, but dangerous in other way. If the buffer is too small, strncpy does not terminate the string with \0 which will cause an error somewhere else in the program.

If you want to use strncpy, then be sure to verify that the output is \0-terminated. Usually when people use strncpy, they forget to do this which is why I recommend strcpy. C and C++ programmers can usually spot the missing buffer size check when using strcpy.

Answer 9

If possible, use a std::vector with std::copy

std::vector<char> messageBuffer;
messageBuffer.reserve(userName.size() + 2); // optional
messageBuffer.push_back(1);
messageBuffer.push_back(0);
std::copy(userName.begin(), userName.end(), std::back_inserter(messageBuffer));
theC_API(&messageBuffer[0]);

Maybe not the fastest, but no chance of miscalculations.

Answer 10

Better way by using vector is

std::vector<char> messageBuffer;
messageBuffer.resize(userName.size()+2);
strcpy(&messageBuffer[0],usernamr.c_str());
std::copy(userName.begin(), userName.end(), std::back_inserter(messageBuffer));
theC_API(&messageBuffer[0]);

Answer 11

C++ strings are not guaranteed by the standard to be contiguous, which means that all the suggestions so far using strncpy are either unsafe (if they copy from something like &userName[0] or potentially inefficient (if they copy from userName.c_str(), which may imply an unnecessary copy)

The correct C++ solution is to use std::copy. Fast and safe.

std::copy(userName.begin(), userName.end(), messageBuffer+2);

As a general rule, if you find yourself messing around with C string functions in a C++ program, you are doing it wrong.