how can i implement a function which can passin vector and int *?

Question

I want to write a function which can modify element for all container which space is continueous, like, vector, int*. char* ....

here is my code:

#include <vector>
#include <string>
#include <iostream>

using namespace std;

template <typename T>
void test(T* t, size_t size) {
  for (size_t i = 0; i < size; ++i) {
    // *(t + i * sizeof(size_t)) += 2;
    *(t + i * sizeof(size_t)) = *(t + i * sizeof(size_t)) + 2;
    // cout << *reinterpret_cast<int*>(t + sizeof(size_t) * i) << endl;
  }                                                                                                                                                                                                        
}

int main() {
  std::vector<size_t> a = {1,2,3,4};
  test(&a[0], 4);  // print result is: 3, 2,3,4, which only modify the first element
  int b[4] = {1,2,3,4};
  test(b, 4); // print result is: 3, 2,3,4, which only modify the first element
  for (size_t i = 0; i < a.size(); ++i) {
    cout << a[i] << " ";
  }
  for (size_t i = 0; i < 4; ++i) {
    cout << b[i] << " ";
  }
}

please see the notes in code, i think when i use *(t + i * sizeof(size_t)), it will find the next int position, but it failed, can anyone help on this?

"I want to write a function which can modify element for all container which space is continueous, like, vector, int*. char*" Then do what the standard library does, and accept a pair of iterators (pointers are a kind of iterator). — Karl Knechtel, Sep 22 '20 at 02:05

score 2 · Accepted Answer · answered Sep 22 '20 at 02:10

2

If you want to add 2 to each element of a collection, C++ already provides a way to do that:

std::vector<size_t> a = {1,2,3,4};
std::transform(a.begin(), a.end(), a.begin(), [](int i){ return i + 2; });

This has a few obvious advantages over your code:

it works with non-contiguous collections (as well as contiguous ones)
it separates the iteration from the operation carried out one each item
it's already debugged and heavily tested

answered Sep 22 '20 at 02:10

Jerry Coffin

476,176
80
629
1,111

@xyhuang I think `std::for_each` is the better candidate if you weren't planning to put them in another container. – Ranoiaetep Sep 22 '20 at 05:14

cigien · Answer 2 · 2020-09-22T01:59:51.283

1

There's no need to multiply the offset by sizeof(size_t) in this expression:

*(t + i * sizeof(size_t))

Since the type of the pointer is known to be T, adding i to the pointer will point it at the next object. So you can just do:

*(t + i)

Of course, the usual way to write that is:

t[i]

Also, this call is incorrect:

std::vector<size_t> a = {1,2,3,4};
test(&a[0], 4);

If you want to treat the vector as an array like you're doing, you need to use the data method like this:

test(a.data(), 4);

Here's a demo.

edited Sep 22 '20 at 01:59

answered Sep 22 '20 at 01:52

cigien

57,834
11
73
112

thanks a lot, i think it's a simple question, just forget the simple way. thank you very much – xyhuang Sep 22 '20 at 01:58
@xyhuang See the update. Your call using vector is wrong. – cigien Sep 22 '20 at 02:01
one more question, when i call *(t + i * sizeof(size_t)) ，i think it will visit unknown memory address, why it is not crashed(like segment fault) – xyhuang Sep 22 '20 at 02:01
@xyhuang Refer to [this](https://stackoverflow.com/questions/671703/array-index-out-of-bound-in-c) to find out reason of not crash. – rustyhu Sep 22 '20 at 03:13

how can i implement a function which can passin vector and int *?

2 Answers2