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:

[CODE=cpp]
#include <iostream>
#include <string>
#include <sstream>
using namespace std;

class A
{
ostringstream my_ostringstrea m;
public:
void generate()
{
my_ostringstrea m << "do something";
}
const char* get()
{
return my_ostringstrea m.str().c_str() ;
}
};

void main()
{
A a;
a.generate();
cout << "data: " << a.get() << "\n";
}
[/CODE]


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

Ok, so maybe my_ostringstrea m.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.

[code=cpp]
class A
{
stringstream my_ostringstrea m;
string temp;
public:
void generate()
{
my_ostringstrea m << "do something";
}
const char* get()
{
temp = my_ostringstrea m.str();
return temp.c_str();
}
};
[/code]

[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:

[code=cpp]
const char* getadata()
{
ostringstream my_ostringstrea m;
my_ostringstrea m << "do something";
return my_ostringstrea m.str().c_str() ;
}
[/code]
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