Can I detach a std::vector<char> from the data it contains?

Question

I'm working with a function which yields some data as a std::vector<char> and another function (think of legacy APIs) which processes data and takes a const char *, size_t len. Is there any way to detach the data from the vector so that the vector can go out of scope before calling the processing function without copying the data contained in the vector (that's what I mean to imply with detaching).

Some code sketch to illustrate the scenario:

// Generates data
std::vector<char> generateSomeData();

// Legacy API function which consumes data
void processData( const char *buf, size_t len );

void f() {
  char *buf = 0;
  size_t len = 0;
  {
      std::vector<char> data = generateSomeData();
      buf = &data[0];
      len = data.size();
  }

  // How can I ensure that 'buf' points to valid data at this point, so that the following
  // line is okay, without copying the data?
  processData( buf, len );
}

Answer 1

void f() { 
  char *buf = 0; 
  size_t len = 0; 
  std::vector<char> mybuffer; // exists if and only if there are buf and len exist
  { 
      std::vector<char> data = generateSomeData(); 
      mybuffer.swap(data);  // swap without copy
      buf = &mybuffer[0]; 
      len = mybuffer.size(); 
  } 

  // How can I ensure that 'buf' points to valid data at this point, so that the following 
  // line is okay, without copying the data? 
  processData( buf, len ); 
} 

Answer 2

I wouldn't recommend it, but:

char* ptr = NULL;
int len = -1;
{
    vector<char> vec;
    /* ... fill vec with data ... */
    vec.push_back(NULL); // dont forget to null terminate =)
    ptr = &vec.front();
    len = vec.size();
    // here goes...
    memset((void*)&vec, 0, sizeof(vec));
}
// vec is out of scoop, but you can still access it's old content via ptr.
char firstVal = ptr[0];
char lastVal = ptr[len-1];
delete [] ptr; // don't forget to free

Voila!

This code is actually pretty safe as vec's destructor will call delete [] 0;, which is a safe operation (unless you got some strange implementation of stl).