How to put char array into std::string

Question

I allocate a char array then I need to return it as a string, but I don't want to copy this char array and then release its memory.

        char* value = new char[required];
        f(name, required, value, NULL); // fill the array
        strResult->assign(value, required);
        delete [] value;

I don't want to do like above. I need put the array right in the std string container. How I can do that?

Edit1:

I understood that I should not and that the string is not designed for this purpose. MB somebody know another container implementation for char array with which I can do that?

"*MB somebody know another container implementation for char array with which I can do that?*" `std::vector`... — ildjarn, Feb 10 '12 at 22:27

ildjarn · Answer 1 · 2012-02-10T19:46:24.953

6

In C++11, the following is guaranteed to work:

std::string strResult(required, '\0');
f(name, required, &strResult[0], NULL);

// optionally, to remove the extraneous trailing NUL (assuming f NUL-terminates):
strResult.pop_back();

return strResult;

In C++03 that's not guaranteed to work, but it is addressed by Library Issue 530 which most standard library implementations have had implemented for years, so it's probably safe, but ultimately is implementation-dependent.

edited Feb 10 '12 at 19:46

answered Feb 10 '12 at 19:40

ildjarn

62,044
9
127
211

It does not work I got a fatal error from my application when I did that. – itun Feb 10 '12 at 19:48
1

@itun : Then you're obviously using one of those non-C++11 implementations I referred to. ;-] – ildjarn Feb 10 '12 at 19:48
I know I use the last MinGW without C++11 support flag. – itun Feb 10 '12 at 19:52

score 3 · Answer 2 · answered Feb 10 '12 at 19:38

3

Instead of passing value into the function, pass in &s[0], where s is a std::string of the right length.

answered Feb 10 '12 at 19:38

Stuart Golodetz

20,238
4
51
80

You mean `value` instead of `required`. But it won't work. the std::string's buffer is read-only. – Mr Lister Feb 10 '12 at 19:39
+1 but I guess you mean instead of passing _value_ into the function. – rasmus Feb 10 '12 at 19:39
@MrLister A string is not read only, unless it is declared as const. The conversion from &s[0] to char* is fine. – rasmus Feb 10 '12 at 19:41
Hm, I'll have to do some serious testing and internet searching. – Mr Lister Feb 10 '12 at 19:44
Yes, I meant `value`, wasn't concentrating sorry. – Stuart Golodetz Feb 10 '12 at 19:47
@rasmus : `&s[0]` _is_ a `char*`, there's no conversion. – ildjarn Feb 10 '12 at 19:50
I did some searching and I found that it's _possible_, but _not recommended_ to pass the address of the string's buffer to a function. See my edited answer. – Mr Lister Feb 10 '12 at 20:01

score 2 · Accepted Answer · edited May 23 '17 at 12:03

You shouldn't. std::strings were not designed to expose their internal data to be used as a buffer.
It's not guaranteed that the string's buffer address won't change during execution of an external function, especially not if the function allocates or deallocates memory itself.
And it's not guaranteed that the string's buffer is contiguous.

See, for example, here.

The example in your post is the better way to do it.

score 0 · Answer 4 · edited Feb 10 '12 at 19:46

0

You're gonna have to copy it to a string. AFAIK std::string does not let you access it's internals directory to do any sort of an address based direct assignment.

std::string s(charArry);

edited Feb 10 '12 at 19:46

Stuart Golodetz

20,238
4
51
80

answered Feb 10 '12 at 19:40

Sid

7,511
2
28
41

How to put char array into std::string

4 Answers4