What does std::vector look like in memory?

Question

I read that std::vector should be contiguous. My understanding is, that its elements should be stored together, not spread out across the memory. I have simply acce

Answer 1

In terms of the actual structure, an std::vector looks something like this in memory:

struct vector {    // Simple C struct as example (T is the type supplied by the template)
  T *begin;        // vector::begin() probably returns this value
  T *end;          // vector::end() probably returns this value
  T *end_capacity; // First non-valid address
  // Allocator state might be stored here (most allocators are stateless)
};

Relevant code snippet from the libc++ implementation as used by LLVM

Printing the raw memory contents of an std::vector:
(Don't do this if you don't know what you're doing!)

#include 
#include 

struct vector {
    int *begin;
    int *end;
    int *end_capacity;
};

int main() {
    union vecunion {
        std::vector stdvec;
        vector           myvec;
        ~vecunion() { /* do nothing */ }
    } vec = { std::vector() };
    union veciterator {
        std::vector::iterator stditer;
        int                       *myiter;
        ~veciterator() { /* do nothing */ }
    };

    vec.stdvec.push_back(1); // Add something so we don't have an empty vector

    std::cout
      << "vec.begin          = " << vec.myvec.begin << "\n"
      << "vec.end            = " << vec.myvec.end << "\n"
      << "vec.end_capacity   = " << vec.myvec.end_capacity << "\n"
      << "vec's size         = " << vec.myvec.end - vec.myvec.begin << "\n"
      << "vec's capacity     = " << vec.myvec.end_capacity - vec.myvec.begin << "\n"
      << "vector::begin()    = " << (veciterator { vec.stdvec.begin() }).myiter << "\n"
      << "vector::end()      = " << (veciterator { vec.stdvec.end()   }).myiter << "\n"
      << "vector::size()     = " << vec.stdvec.size() << "\n"
      << "vector::capacity() = " << vec.stdvec.capacity() << "\n"
      ;
}