String stream comes up empty

Question

I've been trying to append an integer to a string, and I've tried several solutions here, including:

std::stringstream ss;

ss << gvHours << "\'" << gvMinutes << "\"" << gvSeconds << ":" << gvTicks;

std::string output(ss.str());
return output.c_str();

and

std::stringstream ss;
std::string output = "";

ss << gvHours << "\'" << gvMinutes << "\"" << gvSeconds << ":" << gvTicks;

output = ss.str();
return output.c_str();

The first one gives me an empty string, and the second gives me an unreadable character that Notepad++ displays as "SOH" in a black box. Does anyone have any idea what I'm doing wrong?

See http://stackoverflow.com/questions/6441218/can-a-local-variables-memory-be-accessed-outside-its-scope. It's likely the cause of your problem. And depending on how you view the string, it could be empty simply because it consists of invisible characters. For example, SOH is the "Start of Header" control character (ASCII code 2). — chris, Sep 12 '15 at 20:19

score 4 · Answer 1 · answered Sep 12 '15 at 20:20

4

Yes, this part

return output.c_str();

is broken.

When you take c_str() from a std::string, you are just getting a c-style pointer to the contents of that string. But you are not getting ownership of the data underlying that string. As soon as you return from the function, that stringstream and string are destroyed (as they are local variables) and the c_str() is a dangling pointer. So the whole thing is undefined behavior.

answered Sep 12 '15 at 20:20

Chris Beck

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So then, should I make a pointer to a new const char* and return that instead? – Kelvin Shadewing Sep 12 '15 at 20:28
@KelvinShadewing, Returning a `std::string` is the most obvious advice to give, but I assume you have a reason for not doing that. – chris Sep 12 '15 at 20:31

Kelvin Shadewing · Answer 2 · 2015-09-12T21:48:29.827

1

I fixed it. Here's the code I got that works:

std::stringstream ss;
std::string output;

ss << gvHours << "\'" << gvMinutes << "\"" << gvSeconds << ":" << gvTicks;

output = ss.str();
return output;

And in the function where it was needed:

fprintf(gvLog, ">: %s\n", timeString().c_str());

edited Sep 12 '15 at 21:48

answered Sep 12 '15 at 20:31

Kelvin Shadewing

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Note that you have no way of deleting this dynamically allocated string object. You leak one string per call. – chris Sep 12 '15 at 20:33
Edited. That should fix the memory leak, I think. – Kelvin Shadewing Sep 12 '15 at 21:48
Awesome. Thanks for your help! – Kelvin Shadewing Sep 12 '15 at 22:04

String stream comes up empty

2 Answers2