Why is the size of "std::vector" 16 Byte?

Question

I'm using memcpy to copy the content of std:vectors<> to primitve Arrays. For datatypes like int, float, double etc. it worked well. As I started to copy a boolvector I encountered a problem namely I got strange values.

First I started to make an test output for a float vector:

std::vector<float> test1 (3,0);

cout << "Sizeof test1[0] : " << sizeof(test1[0]) << endl
     << "Memoryaddress 0: " << &test1[0] << endl
     << "Memoryaddress 1: " << &test1[1] << endl
     << "Memoryaddress 2: " << &test1[2] << endl;

The output is:

Sizeof test1[0]: 4
Memoryaddress 0: 02793820
Memoryaddress 1: 02793824
Memoryaddress 2: 02793828

And this is what I expect. The float size is 4 Byte and the distance to the next float value is 4 Byte. When I do this for bool the output looks like this:

std::vector<bool> test (3,0);

cout << "Sizeof test[0]: " << sizeof(test[0]) << endl
     << "Memoryaddress 0: " << &test[0] << endl
     << "Memoryaddress 1: " << &test[1] << endl
     << "Memoryaddress 2: " << &test[2] << endl;

The output is:

Sizeof test[0]: 16
Memoryaddress 0: 011EF94C
Memoryaddress 1: 011EF93C
Memoryaddress 2: 011EF92C

Why is the size of bool 16 Byte? It seems like a total overkill to me. Are there explanations for this?

Your title is misleading, you never actually check the size of `std::vector`. — jrok, Apr 04 '13 at 10:58
You can't take the `sizeof` something that's smaller than one character. The `sizeof` operator returns the size in integer multiples of the size of a character. — David Schwartz, Apr 04 '13 at 10:59
@DavidSchwartz: True, but that doesn't explain why it's 16 bytes rather than 1. — Mike Seymour, Apr 04 '13 at 11:15
@MikeSeymour: The point is to show that since no answer would make sense given the OP's assumptions, those assumptions can't be right. It makes no sense to expect this to work. — David Schwartz, Apr 04 '13 at 11:19
@DavidSchwartz: Unless the assumption is that `test[0]` is a reference to `bool`, just as `test1[0]` is a reference to `float` - quite a reasonable assumption if you don't know about the `vector` specialisation. `bool` itself is not "smaller than one character", and one would be surprised if it were larger than that. — Mike Seymour, Apr 04 '13 at 11:24

Mike Seymour · Accepted Answer · 2013-04-04T11:36:18.043

Unlike other specialisations of vector, vector<bool> does not manage a dynamic array of bool objects. Instead, it is supposed to pack the boolean values into a single bit each.

Since individual bits are not addressable, test[0] cannot simply be a reference to bool. Instead, it is an class type vector<bool>::reference that can be converted to bool (to get the value), and assigned from bool (to modify the vector element).

This means that vector<bool> doesn't entirely meet the requirements of a standard container, and can't be used if you need references or pointers to its elements. If you do require a "real" container with addressable elements, consider vector<char> or deque<bool> instead.

@akristmann You can also take a look at http://en.cppreference.com/w/cpp/utility/bitset which *might* be suitable for you. — Red XIII, Apr 04 '13 at 15:02

score 4 · Answer 2 · answered Apr 04 '13 at 10:58

std::vector<bool> is a specialized version of vector, which is optimizes for space.

The storage is not necessarily an array of bool values, but the library implementation may optimize storage so that each value is stored in a single bit.
Elements are not constructed using the allocator object, but their value is directly set on the proper bit in the internal storage.

More information: http://www.cplusplus.com/reference/vector/vector-bool/

Why is the size of "std::vector" 16 Byte?

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