Segmentation fault in C++ adding item to a vector

Question

Lately I have set myself to learn C++, and while working on a bigger project, I have tried to use 'vectors'. But every-time I try passing it a value, it exits with a segmentation fault.

Here is my terminal output:

#include <iostream>
#include <vector>
using namespace std;
int main(){
    vector<int> test;
    cout << "hello world" << endl;
    test[0] = 0;
    return 0;
}

me@my-MacBook-Pro Desktop % g++ test.cpp -o o && ./o
hello world
zsh: segmentation fault  ./o

#include <iostream>
#include <vector>
using namespace std;
int main(){
    vector<int> test;
    cout << "hello world" << endl;
   //test[0] = 0;
    return 0;
}

me@my-MacBook-Pro Desktop % g++ test.cpp -o o && ./o
hello world
me@my-MacBook-Pro Desktop %

You have to resize the vector first (or pass the size to the ctor). — wohlstad, May 01 '22 at 13:52
Perhaps you want to talk push_back() to add a value to the vector. — Jeremy Friesner, May 01 '22 at 13:53
Does this answer your question? [Fixing Segmentation faults in C++](https://stackoverflow.com/questions/3718998/fixing-segmentation-faults-in-c) — user438383, May 01 '22 at 13:53

Oblivion · Answer 1 · 2022-05-01T14:06:40.523

4

The segfault is because of out of bound access. you need to set the size in the ctor

vector<int> test(1);

or push_back:

vector<int> test;
test.push_back(0);

edited May 01 '22 at 14:06

answered May 01 '22 at 13:55

Oblivion

7,176
2
14
33

A 3rd option is to use `resize(1)` on an already constructed `vector`. – wohlstad May 01 '22 at 13:56
But if I want to leave a slot empty ? – Loeffelan May 01 '22 at 14:01
@Loeffelan Then you should use `std::map` – SHR May 01 '22 at 14:04
1

@Loeffelan I think there is XY problem here. You should first explain what you are trying to do. – Oblivion May 01 '22 at 14:05

Bfyuvf · Accepted Answer · 2022-05-01T14:07:18.910

Size it vector<int> test = {0,1,2,3,4}; or vector<int> test(5)

But you might want to use push_back in this situation

#include <iostream>
#include <vector>
using namespace std;

    int main(){
    vector<int> test;
    cout << "hello world" << endl;
    test.push_back(0);
    cout << test[0];
    return 0;
}

Basically adds an item at the end.

Can also use maps with the keys be ints if you want to be able to just [] it or leave in spaces (which from what i saw is what your trying to do)

#include <iostream>
#include <unordered_map>
using namespace std;

int main(){
    unordered_map<int, int> test;
    cout << "hello world" << endl;
    test[0] = 0;
    cout << test[0];
    return 0;
}

Segmentation fault in C++ adding item to a vector

2 Answers2