Question

If I have a std::vector buffer that has room for 100 characters and I call a C function that will write a NULL terminated string like writestring(&buffer[0], buffer.size()) from what I’ve read that’s ok if there is already data in buffer. But if the vector is empty the behavior is undefined. How do I get around that, can I write something to the buffer like “qwerty” and then call writestring() so that I know the behavior is always defined? What kind of storage do you guys use when calling C functions? I just read about .data but I don’t know how common or portable that is.

Thanks

Answer 1

C++

std::vector<char> v;
v.resize(100);
writestring(&v[0], v.size());

C

void writestring(char*p, size_t len) {
  strncpy(p, "Doofi", len);
}

if the vector is empty the behavior is undefined. How do I get around that?

By passing a non-empty buffer. Use the .resize() method to quickly fill the vector, or construct it to be the right size initially: std::vector<int> v(100);.

What kind of storage do you guys use when calling C functions?

std::vector is a fine choice for this application. To C, it looks just like an array or a malloc’d region. But, realize that C’s pointer is only valid as long as the vector is neither reallocated or destroyed.