How to convert string to char* using stringstream?

Question

I have a function like this:

template <typename T>
void parse_to_T(const std::string& str, T* result) {
    std::stringstream ss;
    ss << str;
    ss >> *result;
}

this function is mean to convert the string to the specified type.

it is work to parse the string to int, float or char.

parse_to_T<int>(...);
parse_to_T<float>(...);
parse_to_T<char>(...);

but when meet char*, segment fault ...

I use the function like this:

int int_val;
string m = "1";
parse_to_T<int>(m, &int_val); // works
char* str_val = NULL;
parse_to_T<char*>(m, &str_val); // segmentfault

How to imply this function to make it work?

(convert the string to specified type like int, double, char, char*)?

Hey, I don't know how to explain my use case, but I will try:

To simple, the question is, given a file, for each line, data may have these types :

int
float
char
char*
an array T[num] (T is int, float, char or char*, or any build_in type.)

imple a parse function to parse this file.

and this problem is an exam ...

thanks and I found the error now.

char* str_val = NULL;
parse_to_T<char*>(m, &str_val); // segmentfault;
char* str_val;
parse_to_T<char*>(m, &str_val); // segmentfault;
char* str_val = new char[256];
parse_to_T<char*>(m, &str_val); // works !!

then the error is I didn't allocate memory to the ptr...

I don't suppose you would find functions like the [`std::stod`](http://en.cppreference.com/w/cpp/string/basic_string/stof) and [`std::stoi`](http://en.cppreference.com/w/cpp/string/basic_string/stol) families more appealing? It seems to me you're trying to reinvent a wheel here. — WhozCraig, Feb 28 '17 at 05:27
@WhozCraig I really know this function works. But my problem is to imple a common function to convert it to specified type.......... — 李浩然, Feb 28 '17 at 05:31
Your question really just doesn't make any sense. Why would you want to convert a string to a char* using stringstream? What possible use case could there be for that? — David Schwartz, Feb 28 '17 at 05:35
Replace `char* str_val = NULL;` with `char* str_val;`. Or just use `char* str_val = m.c_str();` — ETech, Feb 28 '17 at 05:41
@ETech Replace char* str_val = NULL with char* str_val; it doesn't work, still segmentfault. — 李浩然, Feb 28 '17 at 05:47
@ETech: No, and no. The first is undefined behavior, the second will not compile. — Benjamin Lindley, Feb 28 '17 at 05:47
@DavidSchwartz I modified my question and explain it , does it make sense now ? — 李浩然, Feb 28 '17 at 05:49
@n.m. convert a string to string ? ..but that's not what I wan't ... I wanna this function support convert string to char* ... — 李浩然, Feb 28 '17 at 05:51
You are trying to use char * pointer without any memory allocation and/or size definition. It seems that you wish to make universal transformer for all types. So, before using char * - allocate memory for m characters or use char[] array. And of course - it shall remove universal effect. — ETech, Feb 28 '17 at 05:52
Correct, you don't want to convert a string to a string. You don't need the function supporting types you don't use, so just don't use `char*` and don't worry about the function not supporting it. — n. m. could be an AI, Feb 28 '17 at 05:54
Brute forcing this all into one function is possible, but problematic. You could make a specialization that handles `char *` in a much more intelligent manner, but I join the crowd wondering why bother? — user4581301, Feb 28 '17 at 05:55
@李浩然 Nope, still completely baffled. A `char *` is a C++ pointer type. A pointer can't possibly be stored in a file and be useful since it's only meaningful inside a process. A file can have strings in it. — David Schwartz, Feb 28 '17 at 18:05

score 3 · Accepted Answer · edited Mar 24 '17 at 14:37

This segfault is because stringstream does not allocate memory to hold the result of the operation when extracting values into a char* array. It tries to put the values into the memory pointed at by the lhs operand. You must allocate memory yourself.

Here is a simple example:

#include <string>
#include <sstream>
#include <iostream>

template <typename T>
void parse_to_T(const std::string& str, T* result) {
    std::stringstream ss;
    ss << str;
    std::cout << ss.str() << std::endl;
    ss >> *result;
    std::cout << *result << std::endl;
}

int main() {
   char* buffer = new char[256];
   /* Don't do this in real code. If the input is larger than
      the size of the buffer it will end very, very badly.
      always have a way of allocating the correct amount
      of memory.
   */
   int int_val;
   std::string m = "1";
   parse_to_T<int>(m, &int_val);
   char* str_val = NULL;
   parse_to_T<char*>(m, &buffer);
   delete[] buffer;
   return 0;
}

You could include a template specialization for the char* datatype that does the allocation based on the amount of data in the stream (calling stringstream.str().size() should work) but the user would have to free the returned memory.

Be careful with fixed-lengths arrays: if input is larger then it will overflow or you may have a security issue. Last is true even if the specification of input states a maximum length, it can always be compromised. — cbuchart, Feb 28 '17 at 05:50
No need for `new`. Allocate buffer as a temporary and let the compiler deal with it. — user4581301, Feb 28 '17 at 05:52
There's also no need for the both insertion and extraction. `std::istringstream ss(str);` , then the extraction, should be all that is needed. — WhozCraig, Feb 28 '17 at 06:00

cbuchart · Answer 2 · 2017-02-28T05:44:05.967

1

First, your destination variable str_val has no allocated memory, so it is normal you have a segmentation fault.

On the other hand, it would be better if you use std::string instead. If you need the raw pointer const char* then you can use std::string::c_str for so. If you have to modify it then just create a copy.

edited Feb 28 '17 at 05:44

answered Feb 28 '17 at 05:38

cbuchart

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How to convert string to char* using stringstream?

2 Answers2