Converting a void* to a std::string

Question

After perusing the web and messing around myself, I can't seem to convert a void*'s target (which is a string) to a std::string. I've tried using sprintf(buffer, "%p", *((int *)point)); as recommended by this page to get to a C string, but to no avail. And sadly, yes, I have to use a void*, as that's what SDL uses in their USEREVENT struct.

The code I'm using to fill the Userevent, for those interested, is:

std::string filename = "ResumeButton.png";
SDL_Event button_press;
button_press.type = BUTTON_PRESS;
button_press.user.data1 = &filename;
SDL_PushEvent(&button_press);

Any ideas?

EDIT: Thanks for all the responses, I just needed to cast the void* to a std::string*. Silly me. Thank you guys so much!

What is STL, what is USEREVENT in STL? C++ has no such thing. — GManNickG, Jun 19 '10 at 19:44
What are you trying to do? You have a void* pointer which points to What? — Pavel Radzivilovsky, Jun 19 '10 at 19:51
I think he means SDL UserEvent http://linux.die.net/man/3/sdl_userevent — Stephen, Jun 19 '10 at 19:54
@Lewis : you've got a problem here. By the time your eventhandler runs, `filename` is out of scope and `user.data1` is pointing to garbage. You'll probably segfault (although, it _might_ work as expected... for awhile... until it segfaults). Look at my answer to see how to prevent that. — Stephen, Jun 19 '10 at 20:16
You're absolutely right, now that you point that out. Thanks so much! — Lewis, Jun 19 '10 at 20:46

score 34 · Accepted Answer · answered Jun 19 '10 at 19:58

You just need to dynamically allocate it (because it probably needs to outlive the scope you're using it in), then cast it back and forth:

// Cast a dynamically allocated string to 'void*'.
void *vp = static_cast<void*>(new std::string("it's easy to break stuff like this!"));

// Then, in the function that's using the UserEvent:
// Cast it back to a string pointer.
std::string *sp = static_cast<std::string*>(vp);
// You could use 'sp' directly, or this, which does a copy.
std::string s = *sp;
// Don't forget to destroy the memory that you've allocated.
delete sp;

Note that this answer has an edge over the other(s), since it prevents the pointer's target from going out of scope. Thanks again! — Lewis, Jun 19 '10 at 20:45

score 12 · Answer 2 · answered Jun 19 '10 at 19:57

Based on your comment "What I meant was to convert what the void* is pointing to (which is a string) into a string."

Assuming you have this:

std::string str = ...;
void *ptr = &str;

You can just cast back to the string:

std::string *pstr = static_cast<std::string *>(ptr);

Note that it is on you to verify that ptr actually points to a std::string. If you are mistaken and it points to something else, this will cause undefined behavior.

score 5 · Answer 3 · answered Jun 19 '10 at 19:43

5

If you trying to format the address as text you can use a stringstream:

std::stringstream strm;
strm << ptr;
std::string str = strm.str(); 

// str will now have something like "0x80004567"

If that's not what you are interested in, please clarify your question.

answered Jun 19 '10 at 19:43

R Samuel Klatchko

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Sorry, I wasn't clear at all. What I meant was to convert what the void* is pointing to (which is a string) into a string. – Lewis Jun 19 '10 at 19:54

score 0 · Answer 4 · answered Jun 19 '10 at 19:59

0

If the void is a const char*, then you can just call the std::string constructor with it, i.e.

const char* cakes = something;
std::string lols = std::string(cakes);

answered Jun 19 '10 at 19:59

Puppy

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You can generally not assume this to be the case, so your suggestion would likely lead to a segmentation violation. Please consider removing it. – einpoklum Aug 01 '16 at 07:17
I tested this out of curiosity. It caused segmentation violation. – Alexandre Severino Jan 09 '22 at 23:26

Converting a void* to a std::string

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