C++: Converting stringstream to char* runtime error

Question

I am trying to convert in C++ a stringstream of "1.txt" so that it is equal to a char* value of "1.txt". I need the raw char* as an argument for a function, so it can't be const char or anything else. When I run it, I get a blank output. Why, and how do I fix it?

#define SSTR(x) dynamic_cast< std::stringstream & >( (std::stringstream() << std::dec << x ) ).str()
int booknum = 1;
std::stringstream stringstream;
stringstream << SSTR(booknum) << ".txt";
std::vector<std::string> argv;
std::vector<char*> argc;
std::string arg;
std::string arg3;
while (stringstream >> arg) argv.push_back(arg);
for (auto i = argv.begin(); i != argv.end(); i++)
   argc.push_back(const_cast<char*>(i->c_str()));
argc.push_back(0);
int arg4 = argc.size();
for (int i = 0; i < arg4; i++)
    std::cout << &arg3[i] << std::endl;

Do you always write like this or only for SO? Format your code, give sensible names to the variables and maybe someone will help you. — Anton Savin, Mar 16 '15 at 19:11
why do you go such length to just convert from one type to another? — Jahid, Mar 16 '15 at 19:15
I used the approach here http://stackoverflow.com/questions/11209311/c-from-stringstream-to-char — Eric Morse, Mar 16 '15 at 19:24
@EricMorse: I don't see anywhere on that page anything even remotely similar to your `SSTR` macro. — Mooing Duck, Mar 16 '15 at 19:25
@Mooing Duck: The easiest method to convert string to int since my compiler doesn't have the itoa command. http://stackoverflow.com/questions/5590381/easiest-way-to-convert-int-to-string-in-c/5590404#5590404 — Eric Morse, Mar 16 '15 at 19:34
@Mooing Duck: I get error: 'to_string' is not a member of 'std' when doing #include with std::string b = std::to_string(3); — Eric Morse, Mar 16 '15 at 19:58
oh I figured out what was wrong. I wasn't using -std=c++11 when compiling — Eric Morse, Mar 16 '15 at 20:07

score 3 · Accepted Answer · answered Mar 16 '15 at 19:15

3

That seems very complicated, instead of e.g.

std::ostringstream oss;
oss << booknum << ".txt";
std::string s = oss.str();
char* pString = new char[s.length() + 1];
std::copy(s.c_str(), s.c_str() + s.length() + 1, pString);

yourFunctionThatTakesCharPtr(pString);

delete[] pString;

answered Mar 16 '15 at 19:15

Some programmer dude

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You could even replace `pString` with a `std::string` – Mooing Duck Mar 16 '15 at 19:16
@MooingDuck It depends on what the function does, as it's stated in the question that it takes a non-const pointer. – Some programmer dude Mar 16 '15 at 19:17
Then pass in `s.data()`. Oh, you mean it could take a pointer by reference and `delete[]` `new[]` it again? Always possible I suppose. – Mooing Duck Mar 16 '15 at 19:17
1

@MooingDuck I mean that the string inside a `std::string` object should be considered constant when using raw pointers, using non-constant C-style pointer allows the function to modify the string which might not work well with the `std::string` object. For example, what if the function truncates the string by setting the string terminator, that won't be reflected by the `std::string` object. – Some programmer dude Mar 16 '15 at 19:19

score 0 · Answer 2 · answered Mar 16 '15 at 19:34

Here's a sample conversion code:

#include <iostream>
#include <sstream>
#include <typeinfo>  //it's just to print the resultant type to be sure


int main() {int booknum=1;
    std::stringstream ss;
    ss << booknum<<"1.txt";
    char* x=new char[ss.str().length()+1];
    ss >> x;
    std::cout<<typeid(x).name();
return 0;}

Output:

Pc

C++: Converting stringstream to char* runtime error

2 Answers2