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 ther

Answer 1

I'd not construct a std::string and ship a pointer to the internal buffers across dll boundaries. Instead I would use either a simple char buffer (statically or dynamically allocated). After the call to the dll returns, I'd let a std::string take over the result. It just feels intuitively wrong to let callees write in an internal class buffer.

Answer 2

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.

Answer 3

You can use char buffer allocated in unique_ptr instead vector:

// allocate buffer
auto buf = std::make_unique<char[]>(len);
// read data
FunctionInDLL(buf.get(), len);
// initialize string
std::string res { buf.get() };

You cannot write directly into string buffer using mentioned ways such as &str[0] and str.data():

#include <iostream>
#include <string>
#include <sstream>

int main()
{
    std::string str;
    std::stringstream ss;
    ss << "test string";
    ss.write(&str[0], 4);       // doesn't working
    ss.write(str.data(), 4);    // doesn't working
    std::cout << str << '\n';
}

Live example.