Populate vector using an array

Question

I'm learning how to use vectors in C++ using this post and they gave the example below on populating a vector using an array. The syntax is a bit confusing though.

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

int main() {
    // Array of string objects
    std::string arr[] = {
        "first",
        "sec",
        "third",
        "fourth"
    };

    // Vector with a string array
    std::vector < std::string > vecOfStr(arr,arr +sizeof(arr) / sizeof(std::string)); //This line is confusing
    for (std::string str: vecOfStr)
        std::cout << str << std::endl;
}

As far as my understanding goes, right after the variable name for the vector there comes the size of the vector. I would imagine that would have simply been sizeof(arr) / sizeof(std::string) but instead it's arr,arr +sizeof(arr) / sizeof(std::string). Can anyone tell me why?

std::vector < std::string > vecOfStr(std::begin(arr), std::end(arr)); — WhozCraig
– WhozCraig, Commented Jan 11, 2020 at 14:56
You want to look at overloading. std::vector's constructor is overloaded, which means that, depending on the number and types of arguments it received, the behaviour is different. — Fureeish
– Fureeish, Commented Jan 11, 2020 at 15:25

Fureeish · Accepted Answer · 2020-01-11 15:23:50Z

6

std::vector has multiple constructors which you can choose from. One of the constructors indeed receives the vector's length. In your example, a different constructor is being used. One which receives two iterators.

You pass the constructor two pointers of std::string which serve as iterators.

The addition and division operators are just pointers arithmetics.

edited Jan 11, 2020 at 15:23

Fureeish

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answered Jan 11, 2020 at 15:13

Gils

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Comments

Jesper Juhl · Accepted Answer · 2020-01-11 14:50:38Z

3

Use a std::array. Then you can populate the vector as

std::vector<std::string> vecOfStr(arr.begin(), arr.end());

Get out of the habit of using C-style arrays.

Also; if you insist on using C-style arrays, then, at least, use std::size() if you need the number of elements.

answered Jan 11, 2020 at 14:50

Jesper Juhl

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2 Comments

SRR Over a year ago

I'm just following the tutorial, I'm guessing C-style arrays are bad?

Gils Over a year ago

@S.Ramjit Usually, you want to avoid using c-style arrays in c++ code. If you still need to use it, use 'std::array' instead, like Jesper suggested.

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