Indexing std::vector works with true values, but does not work with false values

Question

I want to run a simple program in C++ in which there are two vectors - one is a std::vector<int> and the other is a std::vector<bool> of equal length. The value of the boolean vector at an index decides whether the value of the integer vector will be printed or not. Here is a copy of the program I am trying to run:

#include<bits/stdc++.h>
using namespace std;

int main(){
    vector<int> arr{1, 2, 3, 4, 5};
    vector<bool> b(true, arr.size());

    for(int i=0; i<arr.size(); i++){
        if(b[i])
            cout<<arr[i]<<endl;
    }
    return 0;
}

The above program runs as expected. But there is an Address Boundary Error encountered when I change the values from true to false in the std::vector initialization line. Precisely, the error is:

fish: Job 1, './test_vector_bool' terminated by signal SIGSEGV (Address boundary error)

I am already aware of some of the major pitfalls of using vector<bool> from these threads:

What I gathered from these threads is we should not be using reference for vector<bool> in cpp. But all I need to know is why is the vector with true working and initialization with false failing?

`b(true, arr.size())` - > `b(arr.size(), true)` The size comes before the default value — StoryTeller - Unslander Monica, Jul 01 '21 at 09:34
Does this answer your question? [How to initialize a vector in C++](https://stackoverflow.com/questions/8906545/how-to-initialize-a-vector-in-c) — Top-Master, Jul 01 '21 at 09:49

rawrex · Accepted Answer · 2021-07-01T09:42:09.540

3

The line vector<bool> b(true, arr.size()); will create a vector, with 1 element, of value arr.size(). Thus, the loop will get out of bounds.

It's crucial to remember that when we construct a container this way, we say: "How much of what".

In this case: "We need arr.size() elements with value true":

vector<bool> b(arr.size(), true);

For more information, you may want to check the documentation on vector constructors, the third one in particular, "fill constructor".

edited Jul 01 '21 at 09:42

answered Jul 01 '21 at 09:34

rawrex

4,044
2
8
24

I feel so stupid to have messed this up. Once in a *fool* moon, we do forget basic things. I came back to delete this question, but since you took the pain to answer this stupid question I am leaving it as it is marking your answer as solution. – Deeptendu Santra Jul 01 '21 at 09:53
@DeeptenduSantra happens to the best of us! It's all good, you should feel free to manage the post as you consider it should be managed. Best of luck to you! – rawrex Jul 01 '21 at 09:56
I completely missed the fact because there should be some out of bounds error when I run the code in the question, the result is 1 2 3 4 5. It should return boundary error in that case too. – Deeptendu Santra Jul 01 '21 at 10:07
You might want to explain the (not shown) line `vector b(false, arr.size());` instead of (or in addition to) `vector b(true, arr.size());`, as the former is what triggered the SIGSEGV signal. – JaMiT Jul 02 '21 at 02:47
@rawrex *"You should feel free to manage the post as you consider it should be managed."* -- nice sentiment, but unfortunately not realistic. A user cannot delete their own question if it has an answer with upvotes. – JaMiT Jul 02 '21 at 02:50
@JaMiT *"A user cannot delete their own question if it has an answer with upvotes"* - was not aware of that. Thanks! Will consider taking actions based on the fresh input shortly. – rawrex Jul 02 '21 at 02:52

Indexing std::vector works with true values, but does not work with false values

1 Answers1