Default capacity of std::string?

Question

When I create a std::string using the default constructor, is ANY memory allocated on the heap? I'm hoping the answer does not depend on the implementation and is standardized. Consider the following:

std::string myString;

Solution 1

Unfortunately, the answer is no according to N3290.

Table 63 Page 643 says:

• data() a non-null pointer that is copyable and can have 0 added to it
• size() 0
• capacity() an unspecified value

The table is identical for C++03.

Solution 2

No, but, and I don't know of any implementation that does allocate memory on the heap by default. Quite a few do, however, include what's called the short string optimization (SSO), where they allocate some space as part of the string object itself, so as long as you don't need more than that length (seems to be between 10 and 20 characters as a rule) it can avoid doing a separate heap allocation at all.

That's not standardized either though.

Solution 3

It is implementation dependent. Some string implementations use a small amount of automatically allocated storage for small strings, and then dynamically allocate more for larger strings.

Comments

[Spire]

Nitpick: The "null" in "null terminator" refers to an ASCII NUL character, which is spelled differently from the null pointer NULL.

[visitor]

In VC++ this should be covered by the small string optimization. G++ uses ref-counting: all empty strings are represented by the same statically allocated piece of memory. Which implementations actually do involve a dynamic allocation for empty strings?

[velkyel]

that is not literally ref-counting. References to this memory with '\0' are not counted at all.