For regular C strings, a null character '\0' signifies the end of data.
What about std::string, can I have a string with embedded null characters?
- 1See std::string equivalent for data with NULL characters?Matthew Flaschen– Matthew Flaschen2010-05-16 22:46:16 +00:00Commented May 16, 2010 at 22:46
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4 Answers
Yes you can have embedded nulls in your std::string.
Example:
std::string s; s.push_back('\0'); s.push_back('a'); assert(s.length() == 2); Note: std::string's c_str() member will always append a null character to the returned char buffer; However, std::string's data() member may or may not append a null character to the returned char buffer.
Be careful of operator+=
One thing to look out for is to not use operator+= with a char* on the RHS. It will only add up until the null character.
For example:
std::string s = "hello"; s += "\0world"; assert(s.length() == 5); The correct way:
std::string s = "hello"; s += std::string("\0world", 6); assert(s.length() == 11); Storing binary data more common to use std::vector
Generally it's more common to use std::vector to store arbitrary binary data.
std::vector<char> buf; buf.resize(1024); char *p = &buf.front(); It is probably more common since std::string's data() and c_str() members return const pointers so the memory is not modifiable. with &buf.front() you are free to modify the contents of the buffer directly.
4 Comments
Pavel Minaev
In C++9x
&s.front() is also modifiable and guaranteed to point at a contiguous buffer. While there was no such guarantee in C++03, there are no known C++ implementations for which it didn't hold true in practice (which is partly why it was added to C++0x so quickly).
nneonneo
Note that as of C++11,
.c_str() and .data are synonyms. In particular, this means that the string returned by .data must have a null terminator appended.
Keith Thompson
@PavelMinaev: I presume "C++9x" was a typo for "C++0x" (which became C++11 some time after you posted your comment).
n.caillou
s.append("\0world", 6); is better than s += std::string("\0world", 6);Yes. A std::string is just a vector<char> with benefits.
However, be careful about passing such a beast to something that calls .c_str() and stops at the 0.
2 Comments
Notinlist
The first is not true, as I recently learned. Vector's swap preserves iterators and references to contents, string's not necessarily. stackoverflow.com/questions/25201758/…
Lightness Races in Orbit
@Notinlist: It has a different name, too! Oh the horror
You can, but why would you want to? Embedding NUL in an std::string is just asking for trouble, because functions to which you pass an std::string may very well use it's c_str() member, and most will assume that the first NUL indicates the end of the string. Hence this is not a good idea to do. Also note that in UTF-8, only '\0' will result in a 0, so even for i18n purposes, there is no justification for embedding NULs.
answered May 16, 2010 at 22:56
Michael Aaron Safyan
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Lightness Races in Orbit
No, it's silly. "Don't use the full range of
std::string's functionality, because you might pass the result of c_str() to C-string functions without also passing a length", really? Well, if you never do that, you'll be fine...Yep this is valid.
You can have a null character in the middle of the string.
However, if you use a std::string with a null character in the middle with a c string function your in undefined behaviour town - and nobody wants to be there!!!:
int n = strlen( strWithNullInMiddle.c_str() ); // Boom!!! answered May 16, 2010 at 22:43
Robben_Ford_Fan_boy
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Matthew Flaschen
strlen will just return the number of characters before the first null. It might be unanticipated behavior, but it's not undefined.