C++ stringstream concatenate query

Question

The code is supposed to concatenate argv[1] with .txt , and with _r.txt .

std::stringstream sstm;
std::stringstream sstm_r;

sstm<<argv[1]<<".txt";
sstm_r<<argv[1]<<"_r.txt";

const char* result = sstm.str().c_str();
const char* result_r = sstm_r.str().c_str();

fs.open(result);
fs_r.open(result_r);

cout<<result<<endl;
cout<<result_r<<endl;

But what it does is , when i enter "abc" as argv[1] , it gives me , result as "abc_r.tx0" and result_r also same "abc_r.tx0" .What is the correct way to do this and why is this wrong .

Something's fishy in your question. The code has ".txt" while the text has ".tx0". Are you sure you're providing correct/up-to-date information in the question? — Alexey Frunze, Sep 12 '12 at 13:11
`sstm.str()` is a temporary string and so is `sstm.str().c_str()`. The're both only valid until the end of expression, yet you're storing the c_str pointer and later accesing it. You've got undefined behaviour. — jrok, Sep 12 '12 at 13:11
@rajat how can `c_str()` be legacy `C` if `std::string` didn't exist in C? — Luchian Grigore, Sep 12 '12 at 13:15
ohk , it might not be . i was not sure , thanks for editing . — rajat, Sep 12 '12 at 13:15

score 8 · Accepted Answer · answered Sep 12 '12 at 13:12

8

The std::string instances to which the pointers returned by c_str() are associated will be destroyed leaving result and result_r as dangling pointers, resulting in undefined behaviour. You need to save the std::string instances if you want to use c_str():

const std::string result(sstm.str());

fs.open(result.c_str());  /* If this is an fstream from C++11 you
                             can pass a 'std::string' instead of a
                             'const char*'. */

answered Sep 12 '12 at 13:12

hmjd

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Sry this , does not work well . It gives me a trailing question mark at the end of the filename . what it causing this problem . – rajat Sep 12 '12 at 16:15
@rajat, assuming `argv[1]` is populated the code is correct. See http://ideone.com/alBG9 for demo. – hmjd Sep 12 '12 at 16:22
it can't depend on how argv[1] is populated because i am getting question mark at the end of the file name not in between as i am embedding .txt like this sstm< – rajat Sep 12 '12 at 16:28
could you please try and see if you are getting the same problem if you write a file . – rajat Sep 12 '12 at 16:29

score 3 · Answer 2 · edited May 23 '17 at 10:34

3

Work with the strings like this:

{
  const std::string& tmp = stringstream.str();
  const char* cstr = tmp.c_str();
}

This is taken from another exchange here.

edited May 23 '17 at 10:34

Community

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answered Sep 12 '12 at 13:14

Brent Arias

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C++ stringstream concatenate query

2 Answers2