C++ sizeof Vector is 24?

Question

I was just messing around and learning about vectors as well as structs, and at one point, I tried outputting the size of a vector in bytes. Here's the code:

#include <iostream> #include <vector>  struct Foo{     std::vector<int> a; };  int main() {     using std::cout; using std::endl;         Foo* f1 = new Foo;      f1->a.push_back(5);     cout << sizeof(f1->a) << endl;     cout << sizeof(f1->a[0]) << endl;      delete[] f1; } 

The output is 24 and 4.

Obviously the second line printed 4, because that is the size of an int. But why exactly is the other value 24? Does a vector take up 24 bytes of memory? Thanks!

Answer 1

While the public interface of std::vector is defined by the standard, there can be different implementations: in other words, what's under the hood of std::vector can change from implementation to implementation.

Even in the same implementation (for example: the STL implementation that comes with a given version of Visual C++), the internals of std::vector can change from release builds and debug builds.

The 24 size you see can be explained as 3 pointers (each pointer is 8 bytes in size on 64-bit architectures; so you have 3 x 8 = 24 bytes). These pointers can be:

• begin of vector
• end of vector
• end of reserved memory for vector (i.e. vector's capacity)