How to construct a C++ string from sequence of bytes?

Question

Given

const void * data = ...;
size_t size = ...;

std::string message(???)

How to construct std::string from raw pointer to data and size of the data? data may contain NUL characters.

If the data could contain zero characters, then it is not a zero-terminated string. In which case it is not clear what you want to get in your std::string. — Don Reba
– Don Reba, Commented Sep 8, 2015 at 13:10
@DonReba std::string is just a string, not a zero-terminated string — vladon
– vladon, Commented Sep 8, 2015 at 13:12
The follow-up question is, do you really need an std::string, or would std::vector<char> be more suitable? — juanchopanza
– juanchopanza, Commented Sep 8, 2015 at 13:13
@juanchopanza what's the benefit of std::vector<char> over std::string (aside of readability)? — vladon
– vladon, Commented Sep 8, 2015 at 13:19
@vladon It has a simpler interface which is easier to understand. And it makes the intent clear. It represents a char buffer, not a readable string object. It also doesn't need a \0 tagged on at the end. So it really depends on your use-case. — juanchopanza
– juanchopanza, Commented Sep 8, 2015 at 13:20

ForEveR · Accepted Answer · 2015-09-08 13:32:53Z

12

string constructor can work with char*, that contains \0, if size is right.

Constructs the string with the first count characters of character string pointed to by s. s can contain null characters. The length of the string is count. The behavior is undefined if s does not point at an array of at least count elements of CharT.

So just use

std::string message(static_cast<const char*>(data), size);

edited Sep 8, 2015 at 13:32

answered Sep 8, 2015 at 13:03

ForEveR

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juanchopanza · Accepted Answer · 2015-09-08 13:44:10Z

7

You can cast data to const char*, then use the std::string two iterator constructor.

const char* sdata = static_cast<const char*>(data);
std::string message(sdata, sdata + size);

Note that is all you need is a byte buffer, it might be simpler and clearer to use an std::vector<unsigned char> instead.

const unsigned char* sdata = static_cast<const unsigned char*>(data);
std::vector<unsigned char> message(sdata, sdata + size);

edited Sep 8, 2015 at 13:44

answered Sep 8, 2015 at 13:04

juanchopanza

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juanchopanza Over a year ago

@vladon No problem. Also note the other answer uses a different constructor. The one I use is common among standard library containers, but std::string has a larger interface.

Catalin · Accepted Answer · 2015-09-08 13:05:41Z

1

As long as size indicates the correct number, casting is fine. You can have NULL anywhere std::string and you don't even need to NULL terminate your buffer for that matter.

answered Sep 8, 2015 at 13:05

Catalin

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edmz · Accepted Answer · 2015-09-08 13:35:11Z

1

And of course, if you already have a string object

myString.insert(0, static_cast<const char*>(data), size);
                ^^ -- starting index

which calls

basic_string& insert( size_type index, const CharT* s, size_type count );

§

Inserts the first count characters from the character string pointed to by s at the position index. s can contain null characters.

edited Sep 8, 2015 at 13:35

answered Sep 8, 2015 at 13:22

edmz

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