Vector as a Private member of a Class defined with pointer: no error but strange behavior

I don't understand why the following code doesn't work: no error, it just send me the following results:

Hello world! Process returned -1073741819 (0xC0000005) execution time : 1.656 s Press any key to continue.

I defined listInt as a vector in private member. Is it coming from the pointer that I use to define the Class1 object?

    #include "Class1.h"
    using namespace std;

    Class1::Class1()
    {
        int age(10);
        listInt[0]=age;
        cout <<"address Vector1: "<<&listInt[0] << " -  value1:"<<listInt[0] <<  endl;

    }
    Class1::~Class1()
    {
        //delete listInt;
    }
    void Class1::ClassPrintOut() const
    {

    cout << listInt[0] <<  endl;

    }

Class1.cpp

    #ifndef CLASS1_H_INCLUDED
    #define CLASS1_H_INCLUDED
    #include <vector>
    #include <iostream>

    class Class1
    {
        public:
    Class1();
    ~Class1();
    void ClassPrintOut() const;


        private:
    std::vector<int> listInt;


    };
    #endif

Main.cpp

    #include <iostream>
    #include "Class1.h"
    using namespace std;


    int main()
    {
        cout << "Hello world!" << endl;
        Class1 *test(0);
    test =new Class1();
    test->ClassPrintOut();

    delete test;
    return 0;

    }

You did not explicitly initialize your std::vector<int> listInt member. This means the default constructor of std::vector<int> will be used. It initializes a vector with initial size of 0, thus accessing it with index 0 is invalid.

To give a initial size to your list, you should do:

Class1::Class1() : listInt(/* the initial size */) { ... }

This gives you a vector with size of /* the initial size */ and all elements initialized to zero.

If you want to change the size by dynamically adding elements, use push_back or emplace_back:

listInt.push_back(/* element to add */);
listInt.emplace_back(/* element to add */);

Both append an element to the end of the vector and increase the vector's size by one.

The difference is that emplace_back constructs the new element in-place using the parameters you passed to it, but push_back always copies (or move) the element. In your case (std::vector<int>), they make no difference.