Is the capacity size of string always a multiple value of 15?
for example: In all cases the capacity is 15
string s1 = "Hello";
string s2 = "Hi";
string s3 = "Hey";
or is it random?
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1It'll depend on your STL implementation - which compiler version / STL are you using?Rup– Rup2011-02-17 17:52:12 +00:00Commented Feb 17, 2011 at 17:52
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FYI, A quick look at the STL code on VS2008 shows the initial size is 16; maybe the size is implementation specific, but I think there's always an initial capacity bigger than zero.Max– Max2011-02-17 18:08:14 +00:00Commented Feb 17, 2011 at 18:08
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I'm on VS2005 VC++ 8.0 but recently I have updated to latest Windows SDK.user3234– user32342011-02-17 18:09:41 +00:00Commented Feb 17, 2011 at 18:09
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@Max: it's probably saving 1 for the ending \0 .Eugen Constantin Dinca– Eugen Constantin Dinca2011-02-17 18:18:40 +00:00Commented Feb 17, 2011 at 18:18
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A good reference on this topic: learncpp.com/cpp-tutorial/17-3-stdstring-length-and-capacityAJG85– AJG852011-02-17 19:41:31 +00:00Commented Feb 17, 2011 at 19:41
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3 Answers
Is the capacity size of string always a multiple value of 15?
No; the only guarantee about the capacity of a std::string is that s.capacity() >= s.size().
A good implementation will probably grow the capacity exponentially so that it doubles in size each time a reallocation of the underlying array is required. This is required for std::vector so that push_back can have amortized constant time complexity, but there is no such requirement for std::string.
In addition, a std::string implementation can perform small string optimizations where strings smaller than some number of characters are stored in the std::string object itself, not in a dynamically allocated array. This is useful because many strings are short and dynamic allocation can be expensive. Usually a small string optimization is performed if the number of bytes required to store the string is smaller than the number of bytes required to store the pointers into a dynamically allocated buffer.
Whether or not your particular implementation performs small string optimizations, I don't know.
4 Comments
user3234
How can i find the std::string implementation detail?
James McNellis
@user3234: Find your implementation's
<string> header and take a look. Is there a particular reason that you need to know or are you just curious?user3234
I don't see anything in
<string> header file. I need to append a character \n at the end of my std::string but I want to be sure that it doesn't need reallocation Or otherwise I can allocate memory on heap without needing std::string.James McNellis
@user3234:
std::string has a reserve member function that behaves just as std::vector::resize does. If you need to append another character but size() == capacity(), there's really no way around a reallocation.Implementation specific - std::String usually allocates a small starting string, 16bytes is common. It's a compromise between not having to do a realloc and move for very short strings and not wasting space
Comments
It's an implementation detail on which you're not supposed to rely on; to see exactly how std::string grow in your implementation you can have a look at the sources of its CRT. In general it has an exponential grow.
