将 std::vector::data 传递给期望类型**（双指针）的函数

Question

As the title describes, I am trying to pass the pointer to the data of a std::vector into a function expecting a double pointer. Take as an example the code below. I have an int pointer d which is passed to myfunc1 as &d (still not sure if call it the pointer's reference or what), where the function changes its reference to the beginning of an int array filled with 1,2,3,4 . However, if I have a std::vector of ints and try to pass &(vec.data()) to myfunc1 the compiler throws the error lvalue required as unary '&' operand . I have already tried something like (int *)&(vec.data()) as per this answer , but it does not work.

Just for reference, I know I can do something like myfunc2 where I directly pass the vector as reference and the job is done. But I want to know if it's possible to use myfunc1 with the std::vector's pointer.

Any help will be very much appreciated.

#include <iostream>
#include <vector>


using std::cout;
using std::endl;
using std::vector;

void myfunc1(int** ptr)
{
    int* values = new int[4];
    // Fill all the with data
    for(auto& i:{0,1,2,3})
    {
        values[i] = i+1;
    }

    *ptr = values;
}

void myfunc2(vector<int> &vec)
{
    int* values = new int[4];
    // Fill all the with data
    for(auto& i:{0,1,2,3})
    {
        values[i] = i+1;
    }

    vec.assign(values,values+4);
    delete values;
}

int main()
{
    // Create int pointer
    int* d;

    // This works. Reference of d pointing to the array
    myfunc1(&d);

    // Print values
    for(auto& i:{0,1,2,3})
    {
        cout << d[i] << " ";
    }
    cout << endl;

    // Creates the vector
    vector<int> vec;

    // This works. Data pointer of std::vector pointing to the array
    myfunc2(vec);

    // Print values
    for (const auto &element : vec) cout << element << " ";
    cout << endl;

    // This does not work
    vector<int> vec2;
    vec2.resize(4);

    myfunc1(&(vec2.data()));

    // Print values
    for (const auto &element : vec2) cout << element << " ";
    cout << endl;

    return 0;
}

EDIT: What my actual code does is to read some binary files from disk, and load parts of the buffer into the vector. I was having troubles getting the modified vector out of a read function, and this is what I came up with that allowed me to solve it.

Answer 1

When you write: myfunc1(&(vec2.data()));

You are getting the address of a rvalue. The pointed int* is so a temporary that is destroyed right after the call.

This is why you get this error.

But, as @molbdnilo said, in your myfunc1() function, you are reassigning the pointer (without caring to destroy previously allocated memory by the way).
But the std::vector already manages its data memory on its own. You cannot and you must not put your hands on it.

---

What my actual code does is to read some binary files from disk, and load parts of the buffer into the vector.

A solution could be to construct your std::vector by passing the iterator to the beginning and the iterator to the end of the desired part to extract in the constructor's parameters.

For example:

int * buffer = readAll("path/to/my/file"); // Let's assume the readAll() function exists for this example

// If you want to extract from element 5 to element 9 of the buffer
std::vector<int> vec(buffer+5, buffer+9);

If the std::vector already exists, you can use the assign() member function as you already did in myfunc2() :

vec.assign(buffer+5, buffer+9);

Of course in both cases, you have to ensure that you are not trying to access an out of bounds element when accessing the buffer.

Answer 2

The problem is that you cannot take the address of data() , since it is only a temporary copy of the pointer, so writing to a pointer to it makes not that much sense. And that is good that way. You DO NOT want to pass data() to this function since it would overwrite the pointer with a new array and that would break the vector. You can remove one * from the function and only assign to it and not allocate the memory there. This will work, but make sure to allocate the memory in the caller (with resize , just reserve will result un undefined behavior, since data() is only a pointer to the beginning of the valid range [data(), data() + size()) . The range [data(), data() + capacity ()) is not necessary valid.