get wrong size from 2d vector pointer in c++

Question

I create a 2d vector, however when I read the size from this 2d vector pointer, it gets wrong size. I have no idea about, can anyone explain this?

#include <stdio.h>
#include <vector>

void read_size(std::vector<std::vector<float>> *vec)
{
    printf("read size %d, %d\n", vec->size(), vec[0].size());
}


int main()
{
    std::vector<std::vector<float>> vec;
    vec.resize(100);
    vec[0].resize(50);
    printf("main %d, %d\n", vec.size(), vec[0].size());
    read_size(&vec);
    return 0;
}

The output is

main 100, 50
read size 100, 100

I think both size should be 100, 50

Answer 1

vec->size() is equiavlent to (*vec).size().

vec[0] is equivalent to *(vec + 0), which is equivalent to *vec.

Therefore, vec->size() and vec[0].size() are the same things.

You should first dereference the pointer vec in the function read_size to access to its elements.

This can be done like (*vec)[0].size(), vec[0][0].size(), or vec->at(0).size().