[cfe-dev] libc++: max_size() of a std::vector

[Csaba Raduly]

Hi Francois,
The return type of size() is size_t, which is unsigned, so it can
represent values up to 2^n-1.

http://en.cppreference.com/w/cpp/container/vector/max_size says:

"This value is typically equal to
std::numeric_limits<size_type>::max(), and reflects the theoretical
limit on the size of the container. "

Csaba


On Sat, Feb 14, 2015 at 6:21 PM, Francois Fayard
<fayard.francois at icloud.com> wrote:
> Hi,
>
> I am puzzled by the result of std::vector<char>::max_size() on the n = 32 and n = 64 bits system I have tested. The result is 2^n - 1. Let me explain why I am puzzled.
>
>
>
> Every implementation of std::vector<T> that I know of, libc++ included, has three members of type T*: begin_, end_, capacity_. begin_ points to the first value of the vector and end_ points to the one after the last. The size() method is computes end_ - begin_. But the result of this difference is of type std::ptrdiff_t which is a signed integer of n bits on every implementation that I know of. Therefore, this type can not store the integer 2^n - 1, but only up to 2^(n - 1) - 1.
>
>
>
> I would expect this last number for max_size(). Is it a bug or something that I overlooked?
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