writing directly to std::string internal buffers

Question

I was looking for a way to stuff some data into a string across a DLL boundary. Because we use different compilers, all our dll interfaces are simple char*.

Is there a correct way to pass a pointer into the dll function such that it is able to fill the string buffer directly?

string stringToFillIn(100, '\0');
FunctionInDLL( stringToFillIn.c_str(), stringToFillIn.size() );   // definitely WRONG!
FunctionInDLL( const_cast<char*>(stringToFillIn.data()), stringToFillIn.size() );    // WRONG?
FunctionInDLL( &stringToFillIn[0], stringToFillIn.size() );       // WRONG?
stringToFillIn.resize( strlen( stringToFillIn.c_str() ) );

The one that looks most promising is &stringToFillIn[0] but is that a correct way to do this, given that you'd think that string::data() == &string[0]? It seems inconsistent.

Or is it better to swallow an extra allocation and avoid the question:

vector<char> vectorToFillIn(100);
FunctionInDLL( &vectorToFillIn[0], vectorToFillIn.size() );
string dllGaveUs( &vectorToFillIn[0] );

Answer 1

I'm not sure the standard guarantees that the data in a std::string is stored as a char*. The most portable way I can think of is to use a std::vector, which is guaranteed to store its data in a continuous chunk of memory:

std::vector<char> buffer(100);
FunctionInDLL(&buffer[0], buffer.size());
std::string stringToFillIn(&buffer[0]);

This will of course require the data to be copied twice, which is a bit inefficient.