How to initialize vectors in a class's contructor?

November 18, 2022

I am trying to create a class that has a member variable of type std::vector<std::pair<int, int>>.

Below is an example class that recreates my original class:

class temp {
private:
    vector<pair<int, int>> map;
public:
    temp() {
        vector<pair<int, int>> map(3, { 0, 0 });
    }

    void foo() {
        //Do something
    }
};

when I add the following line of code in the constructor of the above class, it throws no error:

cout<<map[0].first<<" "<<map[0].second<<'\n';

However, when I add the same line of code in the function foo and call it like so:

temp map{};
map.foo();

it throws the following exception:
exception thrown when the last line of code is added to foo()

What am I doing wrong?

>Solution :

In your temp constructor you define a brand new and totally separate variable with the name map.

What you should use is a constructor initializer list:

temp()
    : map(3)
{
    // Empty
}

The above will initialize the vector using the constructor taking a size argument. This specific example means that map will be initialized with three default-constructed elements.