How to get the begin pointer of std::vector with zero size?

Question

How to get the begin pointer of std::vector with zero size?

Manda Rajo 141

Hi, how to fix that error in the function begin()? I'm using Visual Studio 2015. Possibly the original source code was made on Linux or CodeBlocks IDE, and possibly it doesn't show that error?

#include "stdafx.h"
#include <vector>
#include <conio.h>

template <typename T>
struct emc_vector {
    size_t size() const { return m_data.size(); }
    T *begin() {
        return &m_data[0]; // <-- "Error: Expression vector subscript out of range" because m_data.size() is 0.
    }
    T *end() {
        if (size()) {
          //return &m_data[size()]; [error] Expression vector subscript out of range
            return &m_data[size() - 1] + 1;
        }
        else {
            return begin();
        }
    }
    void insert(T *at, const T& value = T()) {
        m_data.insert(m_data.begin() + size_t(at - begin()), value);
    }
    void push_back(const T &value) {
        m_data.push_back(value);
    }
private:
    std::vector<T> m_data;
};


int main()
{
    emc_vector<char> vec; // Its size is 0.
    //vec.push_back('a');
    vec.insert(vec.begin(), 'x'); // <-- This is the goal, but it crashes.

    _getch();
    return 0;
}

Igor Tandetnik 1,116 Reputation points

2022-04-15T16:44:15.76+00:00

Instead of &m_data[0] use m_data.data(). data() member function is guaranteed to return some pointer even if the vector is empty. It could be a null pointer, or a non-null pointer that is not dereferenceable. It's also guaranteed that data() + size() is always valid.

2 answers

Your answer

Igor Tandetnik 1,116 Reputation points

2022-04-15T16:44:15.76+00:00

Instead of &m_data[0] use m_data.data(). data() member function is guaranteed to return some pointer even if the vector is empty. It could be a null pointer, or a non-null pointer that is not dereferenceable. It's also guaranteed that data() + size() is always valid.

Answer 1

Viorel 126.9K

Check a workaround:

T* begin( )
{
    return size( ) ? &m_data[0] : nullptr;
}

Or use an approach that is probably more recommended:

using iterator = typename std::vector<T>::iterator;

iterator begin( )
{
    return m_data.begin( );
}

void insert( iterator at, const T& value = T( ) )
{
    m_data.insert( at, value );
}

0 comments

Answer 2

Manda Rajo 141

Thanks, I believe that your first method will work.

Maybe it's not recommended to write like this (in the function end()):

return &m_data[size() - 1] + 1; // Maybe it's not safe because +1 into a non existing item is wrong? Nobody knows.

Maybe the idea from the following link is imperfect because it crashes just like I explained, https://github.com/graphitemaster/glsl-parser/blob/main/util.h

So, I decide to use an iterator as you said one day, but for the moment, I use std::vector.

Igor Tandetnik 1,116 Reputation points

2022-04-15T16:39:39.697+00:00

This is very obviously not going to work when size() == 0. size() returns an unsinged integer, so size()-1 is going to wrap around to a huge number, and then m_data[size() - 1] would exhibit undefined behavior by way of accessing an index way way out of bounds. + 1 part is not a problem, or rather, is not an immediate problem - the code is already broken before it gets to that part.

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How to get the begin pointer of std::vector with zero size?

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