Vector Pushback Not Working Inside of a Constructor Class - c++

Question

Problem
(Apologies if I use the wrong terminology, I am used to Java constructors, and have just recently tried implementing them in C++)
I have a constructor class called FileHandler that will make a new instance of a FileHandler with two vectors inside of it (The constructor has been dulled down, as there is a lot of extraneous code not pertinent to this issue):
class FileHandler {
public:
std::vector<std::string> fullPath, fileNames;
FileHandler() {
//Creates two vectors
fullPath;
fileNames;
}
As far as I am aware, this should create two vectors inside of the instance of FileHandler, which can be manipulated normally. However, from a separate file, I call the .push_back() method (again, dulled down):
// This vector is pre-made and pre-filled before this is called,
// and I have verified it _is_ filled
std::vector<FileHandler> fileHandlers;
FileHandler testInstance = fileHandlers.at(index);
testInstance.fullPath.push_back("a_string_here");
std::cout << testInstance.fullPath.at(0);
And I get the following error (at runtime):
std::out_of_range at memory location 0x00000...
What I've tried so far
I had a suspicion that it might not like trying to push_back from outside of the constructor, so I made a separate function inside of the constructor:
void addFullPath(std::string fullPath) {
this->fullPath.push_back(fullPath);
}
However, passing a string in a similar way as before:
std::vector<FileHandler> fileHandlers;
FileHandler testInstance = fileHandlers.at(index);
testInstance.addFullPath("a_string_here");
std::cout << testInstance.fullPath.at(0);
Still gives me the same out_of_range at memory location error.
This implies (at least to me) that the second line; being the .push_back() method, is not functioning as it should. My main hypothesis at this point is that I am not instantiating the vectors in the constructor in the correct format, or that the vectors cannot be edited, however I am as of yet unsure. Please advise.

Answer 1

After running the following similar dummy code, everything worked normally.
There may be an error elsewhere in the extraneous code you did not show.
#ifndef UNTITLED_MYCLASS_H
#define UNTITLED_MYCLASS_H
#include <string>
#include <vector>
class MyClass {
public:
std::vector<std::string> a,b;
MyClass();
};
#endif //UNTITLED_MYCLASS_H
#include "MyClass.h"
MyClass::MyClass() {} //the vectors are dynamic and don't require initialization here
#include <iostream>
#include "MyClass.h"
int main()
{
// should not be file() which is a function call for the default constructor
MyClass file; //declares default object instance
file.a.push_back("a string");
std::cout << file.a.at(0);
return 0;
}
untitled\cmake-build-debug\untitled.exe
a string
Process finished with exit code 0

Answer 2

Issue solved:
My problem was the following line of code:
FileHandler testInstance = fileHandlers.at(i);
I falsely assumed that the testInstance would inherit all properties of fileHandlers.at(i), making it completely usable, however this was not the case. Replacing:
FileHandler testInstance = fileHandlers.at(index);
testInstance.fullPath.push_back("a_string_here");
std::cout << testInstance.fullPath.at(0);
With:
fileHandlers.at(index).fullPath.push_back("a_string_here");
std::cout << fileHandlers.at(index).fullPath.at(0);
Yields expected results.