return my_ostringstream.str().c_str();

[jabbah]

I want to use a stringstream internally, but to conform to an api I want to return a char*.

Only this does not work:

#include <iostream>
#include <string>
#include <sstream>
using namespace std;
 
class A
{
    ostringstream my_ostringstream;
public:
    void generate()
    {
        my_ostringstream << "do something";
    }
    const char* get()
    {
        return my_ostringstream.str().c_str();
    }
};
 
void main()
{
    A a;
    a.generate();
    cout << "data: " << a.get() << "\n";
}
 

using ms visual studio 2005 I get a "debug assertion failed! [...] _BLOCK_TYPE_IS_VALID(pHead->nBlockUse)" at runtime.

Ok, so maybe my_ostringstream.str() creates a local copy and then returns a pointer to a local object? Is that the case? And anyways, can somebody give me a hint on how to resolve that?

[weaknessforcats]

Temporary objects go out of scope at the end of a statement so the pointers to them become garbage.

Just don't rely on a compiler temporary.

I changed your code to allow for a member variable.

class A
{
    stringstream my_ostringstream;
    string temp;
public:
    void generate()
    {
        my_ostringstream << "do something";
    }
    const char* get()
    {
        temp = my_ostringstream.str();
        return temp.c_str();
    }
};
 

[jabbah]

I changed your code to allow for a member variable.

Ok, that works. thanks for that.


A related question: Is there a similar way if we are given a simple function instead of a class? for example:

const char* getadata()
{
  ostringstream my_ostringstream;
  my_ostringstream << "do something";
  return my_ostringstream.str().c_str();
}
 

Ok, I do understand that this implementation cant possibly work, since it suffers from the same deficiency as my initial post. However, what I want is to return a char* and not force the caller to delete the returned value.
Maybe, I guess, ... thats just not possible, right?

[weaknessforcats]

Ok, I do understand that this implementation cant possibly work, since it suffers from the same deficiency as my initial post. However, what I want is to return a char* and not force the caller to delete the returned value.
Maybe, I guess, ... thats just not possible, right?

You would need to allocate memory on the heap for the char* string and copy the std::string to that memory. There is a string::copy() for that purpose.

Then return the allocated address. Later someone else can delete it.

[jabbah]

Later someone else can delete it.

That is, someone else *has* to delete it, right.

[weaknessforcats]

Yes, they do.

Unless...

You use a smart pointer that knows when to delete itself.

There is an article (and template code) for a handle in the C/C++ HowTos forum article on Handle classes.

[jabbah]

thanks weaknessforcats