Function with return value char* returning garbage

Question

I'm trying to build a utility class that I can re-use, that converts a std::string to a char*:

char* Foo::stringConvert(std::string str){
      std::string newstr = str;
      // Convert std::string to char*
      boost::scoped_array<char> writable(new char[newstr.size() + 1]);
      std::copy(newstr.begin(), newstr.end(), writable.get());
      writable[newstr.size()] = '\0'; 
      // Get the char* from the modified std::string
      return writable.get();
}

The code works when I tried to load the output from within the stringConvert function, however when used in other parts of my application, this function returns garbage.

For example:

Foo foo;
char* bar = foo.stringConvert(str);

The above code returns garbage. Is there any workaround for this kind of issue?

It would have been easier to answer the question if you hadn't omitted the code. — Andreas Brinck, Jun 25 '12 at 08:55
@cppcoder boost::scoped_array writable(new char[newstr.size() + 1]); — quarks, Jun 25 '12 at 09:04
You might want to have a look at http://stackoverflow.com/questions/347949 — Tony Delroy, Jun 25 '12 at 09:25
@TonyDelroy Actually the shown in the link is basically the same with my code. I will update the code in the question for clarification — quarks, Jun 25 '12 at 09:54
@xybrek: you haven't understood it... when you return only the .get() result, the scoped_array destructor still deletes the memory you newed while the function returns... the char* addresses that invalid memory region.... — Tony Delroy, Jun 25 '12 at 13:56
@TonyDelroy Yah I know about that, because my code works when not passed as a function of a class. — quarks, Jun 25 '12 at 18:02

score 2 · Answer 1 · answered Jun 25 '12 at 08:51

2

I'm going to assume writable is an object with automatic duration that destructs the char* it contains in the destructor - that's your problem - whatever writable.get() returns is no longer valid.

Just return a std::string instead, why on earth do you need a raw char *?

answered Jun 25 '12 at 08:51

Luchian Grigore

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I can't use the c_str() function because, I will get this error: cannot convert parameter 2 from 'const char *' to 'LPSTR'(char*) – quarks Jun 25 '12 at 09:03
@xybrek why is `bar` a `char*`? Are you modifying the string? Do you need to modify the string? – Luchian Grigore Jun 25 '12 at 09:05
char* is the parameter that needs to be passed on one of the function in my app – quarks Jun 25 '12 at 09:08
I mean the library I'm using with my app needs char* as parameter – quarks Jun 25 '12 at 09:09
@xybrek the you need to use strcpy. – Luchian Grigore Jun 25 '12 at 09:10

score 1 · Accepted Answer · answered Jun 25 '12 at 08:57

1

Why don't just use std::string.c_str()? This is a library method that does just what you need.

answered Jun 25 '12 at 08:57

undefined

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I can't use the c_str() function because, I will get this error: cannot convert parameter 2 from 'const char *' to 'LPSTR'(char*) – quarks Jun 25 '12 at 09:03
You can look here: http://stackoverflow.com/questions/342772/convert-lptstr-to-char looks similiar to your case. – undefined Jun 25 '12 at 09:54

Function with return value char* returning garbage

2 Answers2