C++ std:string memory model

The following code (part of a request-response loop in a networked server) works most of the time, but sometimes fails, in that the client will report it has gotten some weird other string (seemingly开发者_开发技巧 random bytes from locations nearby in memory in this functions, or null bytes).

string res = "";
if (something) {
    res = "ok";
}

if (res.length() > 0) {
    send_data((void*) res.c_str(), res.length());
}

In my mind, it would seem that both "" and "ok" are constant std:strings, and res is a pointer to either one of them, and as such the whole thing should work, but apparently that's not the case, so can someone please explain to me what happens here?

---

You probably forgot to send the null-terminator to denote the end of the string:

send_data((void*) res.c_str(), res.length()+1);

---

Your code is okay, I suppose there's some memory corruption in your program.

"" and "ok" are actually zero-terminated buffers of type 'const char *', not strings. When you assign them to your string all their data is copied inside string internal buffer, not including last char which is zero, so

res = "";

will clear internal string buffer, and res.length() will become 0.

res.c_str() will return the address of that buffer, not the address of "" or "ok" literals.