C++ vector max_size();

Kundan picture Kundan · Sep 28, 2010 · Viewed 56.4k times · Source

On 32 bit System.

  1. std::vector<char>::max_size() returns 232-1, size of char — 1 byte
  2. std::vector<int>::max_size() returns 230-1, size of int — 4 byte
  3. std::vector<double>::max_size() returns 229-1, size of double — 8 byte

can anyone tell me max_size() depends on what?

and what will be the return value of max_size() if it runs on 64 bit system.

Answer

Anthony Williams picture Anthony Williams · Sep 28, 2010

max_size() is the theoretical maximum number of items that could be put in your vector. On a 32-bit system, you could in theory allocate 4Gb == 2^32 which is 2^32 char values, 2^30 int values or 2^29 double values. It would appear that your implementation is using that value, but subtracting 1.

Of course, you could never really allocate a vector that big on a 32-bit system; you'll run out of memory long before then.

There is no requirement on what value max_size() returns other than that you cannot allocate a vector bigger than that. On a 64-bit system it might return 2^64-1 for char, or it might return a smaller value because the system only has a limited memory space. 64-bit PCs are often limited to a 48-bit address space anyway.