Rule Definition
While iterating over a container almost all modifications of said contained will invalidate the iterator and bring undefined behaviour. Moreover the encapsulation in a class can mask that we are modifying the container by the use of class method and consequently invalidate the iterator.

Remediation
Do not modify the container while iterating over, unless you modify the iterator itself.

Violation Code Sample

class A{
    std::vector<int>& getVector();
    void removeElement(int index);
private:
    std::vector<int> m_integerVector;
};

void func(){
    A a;
    ...
    for(int val : a.getVector()){
        ...
        a.removeElement(index);
        ...
    } 
}

Fixed Code Sample

class A{
    std::vector<int>& getVector();
    void removeElement(int index);
private:
    std::vector<int> m_integerVector;
};

void func(){
    A a;
    ...
    for(int val : a.getVector()){
        ...
    } 
    ...
    a.removeElement(index);
}