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<char>... — ildjarn
– ildjarn, Commented Feb 10, 2012 at 22:27

ildjarn · Accepted Answer · 2012-02-10 19:46:24Z

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, 2012 at 19:46

answered Feb 10, 2012 at 19:40

ildjarn

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3 Comments

itun Over a year ago

It does not work I got a fatal error from my application when I did that.

ildjarn Over a year ago

@itun : Then you're obviously using one of those non-C++11 implementations I referred to. ;-]

itun Over a year ago

I know I use the last MinGW without C++11 support flag.

Stuart Golodetz · Accepted Answer · 2012-02-10 19:38:15Z

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, 2012 at 19:38

Stuart Golodetz

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7 Comments

Mr Lister Over a year ago

You mean value instead of required. But it won't work. the std::string's buffer is read-only.

rasmus Over a year ago

+1 but I guess you mean instead of passing value into the function.

rasmus Over a year ago

@MrLister A string is not read only, unless it is declared as const. The conversion from &s[0] to char* is fine.

Mr Lister Over a year ago

Hm, I'll have to do some serious testing and internet searching.

Stuart Golodetz Over a year ago

Yes, I meant value, wasn't concentrating sorry.

|

Community · Accepted Answer · 2017-05-23 12:03:33Z

2

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.

edited May 23, 2017 at 12:03

CommunityBot

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answered Feb 10, 2012 at 19:38

Mr Lister

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Comments

Stuart Golodetz · Accepted Answer · 2012-02-10 19:46:21Z

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, 2012 at 19:46

Stuart Golodetz

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answered Feb 10, 2012 at 19:40

Sid

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