C++ stream 至 memory

Question

how can I create std::ostream and std::istream objects to point to a piece of memory I allocated and manage (I don't want the stream to free my memory).

I was looking at using rdbuf()->pubsetbuf() to modify one of the other streams - say sstringstream . However I think streambuf used by stringstream will free the buffer afterwards?

Basically I'm trying to serialize some things to shared memory.

Thanks.

Answer 1

Take a look at the bufferstream class in theBoost.Interprocess library:

The bufferstream classes offer iostream interface with direct formatting in a fixed size memory buffer with protection against buffer overflows.

Answer 2

#include <iostream>
#include <streambuf>

//...
size_t length = 100;
auto pBuf = new char[length]; // allocate memory

struct membuf: std::streambuf // derive because std::streambuf constructor is protected
{
   membuf(char* p, size_t size) 
   {
       setp( p, p + size); // set start end end pointers
   }
   size_t written() {return pptr()-pbase();} // how many bytes were really written?
};

membuf sbuf( pBuf, length ); // our buffer object
std::ostream out( &sbuf );   // stream using our buffer

out << 12345.654e10 << std::endl;
out.flush();

std::cout << "Nr of written bytes: " << sbuf.written() << std::endl;
std::cout << "Content: " << (char*)pBuf << std::endl;

//...
delete [] pBuf; // free memory 

Answer 3

A minimal memory buffer has to implemented only the overflow function of std::streambuf and keep track of the get pointers of the streambuf . This is done viasetp and pbump .

You will also need to add an underlying memory buffer, but this can be done rather easy. cppreference.com has a good example of an implementation based on an std::array in the already mentioned page for the overflow function. While this is a good start to learn, you might want to have a memory buffer that can resize. You can try to implement it based on an std::vector (like I did here ).

I ended up writing a memory buffer based on std::realloc to get the most performance while also giving me the possibility to hand the raw pointer over to C libaries. You can find my implementation here .