Debug Assertion Failed: Expression vector subscript out of range-CodePudding

I dont understand why it says subscript out of range when I have reserved the space in the vector. I have created a short form of my code to explain what the problem is better:

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

using namespace std;

class A {
public:
    vector<vector<string>> foo;
    thread* aThread;

    A() {
        foo.reserve(10); //makes sure we have space...
        aThread = new thread([this]() {
            for (int i = 0; i < 10; i  ) {
                foo[i].push_back("Hello"); // Debug assertion failed. :(
            }
        });
    }
};

int main()
{
    A a;
    a.aThread->join();

    for (int i = 0; i < 10; i  ) {
        for (int j = 0; j < a.foo.size(); j  ) {
            cout << a.foo[i][j] << " ";
        }
        cout << endl;
    }
    return 0;
}

Here it gives the error as soon as I am trying to add the element into my foo vector inside the thread. I cannot figure out what is wrong. Please help.

CodePudding user response：

foo.reserve(10)

Reserves space for elements in foo, but it does not populate any of the elements with an empty std::vector.

You can change it to:

foo.resize(10);

Which will reserve the space and create the empty vector< string > elements, so that you can access them.