The STL class vector is designed to be usable as a C style array. The elements in a non-empty vector are guaranteed to be stored contiguously so it is possible to use the address of the first element in the container as a pointer to an array of elements. The best way to do this is by '&v[0]' where v is a vector of some object with a C compatible type. Other methods of treating a vector as an array are implementation defined and not portable.

   
   extern "C" void functionTakingArrayOfInt( int i[] );
   extern "C" void functionTakingPointerToArrayOfInt( int* pvi );
   
   void goodWayToUseCFunctionWithVector( vector< int >& vec )
   {
      assert( false == vec.empty() && "this doesnt work with empty vectors!" );
      functionTakingArrayOfInt( &vec[ 0 ] );            // ok
      functionTakingPointerToArrayOfInt( &vec[ 0 ] );   // ok
   }
   
   void badWayToUseCFunctionWithVector( vector< int >& vec )
   {
      functionTakingArrayOfInt( vec.begin() );    // may not work as intended!
   }

Effective STL Item 16;