std::size() and sizeof()

[chipp]

i wanna ask, why std::size() and sizeof() results for array is different while std::size() is said as sizeof()'s replacement..?
e.g.:

std::array <int, 3> num {1, 2, 9};
std::cout << std::size(num); //result: 3 

using sizeof():

std::array <int, 3> num {1, 2, 9};
std::cout << sizeof (num); //result: 12 

[mbozzi]

std::size does not replace sizeof.

The former gives the number of elements in a container.
The latter gives the compile-time constant size of a type or object in bytes.

[chipp]

ic... looks like i was mistakenly took them has the same functions...
i wanna ask also, if num is std::vector why sizeof(num) is 16..?

[Pointee]

Because sizeof(a vector) means how much space the vector itself needs(which means, not related to the elements). The vector may holds a pointer that is 8 bytes long, and two ints that are 4*2=8bytes long.

[chipp]

which means, not related to the elements

@point: so you mean, it doesn't matter what type vector object is, the size always 4 bytes?

[George P]

it doesn't matter what type vector object is, the size always 4 bytes?

Should be easy enough to test (VS 2019, x86 32-bit) Release compile):

#include <iostream>
#include <vector>

int main()
{
   std::vector<char>   cVec { 'a', 'B', 'z' };
   std::vector<double> dVec { 1.0, 2.3, 3.4 };

   std::cout << sizeof(cVec) << ' ' << sizeof(dVec) << '\n';
}

12 12

x86 Debug compile:

16 16

x64 Release compile:

24 24

x64 Debug compile:

32 32

Well, not ALWAYS 4 bytes, but there is a definite pattern that shows the sizeof a vector doesn't matter what the POD data type is. The bitness and type of compile makes a difference.

Just for giggles what about a simple C++ class? What is the sizeof that? (x86 Release)

#include <iostream>
#include <vector>

class Test
{
public:
    Test() {};
   ~Test() {};

private:
   char data { 0 };
};

int main()
{
   Test t;
   std::cout << sizeof(t) << '\n';

   std::vector<Test> cVec;

   for (size_t i{ }; i < 3; ++i)
   {
      cVec.emplace_back(Test{});
   }

   std::cout << sizeof(cVec) << '\n';
}

1
12

[dhayden]

so you mean, it doesn't matter what type vector object is, the size always 4 bytes?

The size is implementation dependent. That means it might be one value for one template type/compiler/operating system and a different value on another. You can't rely on it being anything specific.

[chipp]

ok, thanks for the answers, guys... i'll call it done, then...