Question

This Code returns an error on the delete [] placard_; call

void Protestor::destroy() { //Free's caller Protestor's dynamic memory
    delete [] placard_;
}

This code does not.

void Protestor::destroy() { //Free's caller Protestor's dynamic memory
    delete placard_;
}

This goes against my class notes, which state to ALWAYS call

delete []

rather than

delete

What is the explanation for this behaviour? Under what conditions must ‘delete’ be called instead of ‘delete []’?

Here is the Definition for the Protester and Sign classes.

class Protester
{
  public:
    Protester(string name, string signSlogan, int signHeight, int signWidth,
              int rcmp_file = 0 );
    string getName() const;
    Sign getPlacard() const;
    void changePlacard( string newSlogan, int newHeight, int newWidth);
    void setRCMPfile(int RCMP_file);
    int getRCMPfile() const;

    //Big Three
    Protester(const Protester& other); //Copy Constructor
    ~Protester(); //Destructor
    Protester& operator= (const Protester& other); //Assignment Constructor


private:
    // name of the Protester
    string name_;

    // a sign the protester is wielding
    Sign* placard_;

    // the RCMP file number tracking this person (zero means no RCMP report)
    int rcmp_file_;

    //Big Three Helper Functions
    void copy(const Protester& other); //Performs Deep Copy of const Protester&
    void destroy(); //deletes [] placard_
                    //sounds better then cleanup, in my humblest of opinions.
};

class Sign
// a class representing information about signs/placards
{
public:
    // constructor to initialize sign text and dimensions
    Sign(string statement, int height, int width);

    // return sign text
    string getStatement() const;

    //return sign height
    int getHeight() const;

    //return sign width
    int getWidth() const;

    // change sign text
    void setStatement(string statement);

    // change sign dimensions
    void setSize(int height, int width);

private:
    // the text of the sign
    string statement_;

    // dimensions of the sign
    int height_;
    int width_;
};

Answer 1

This goes against my class notes, which state to ALWAYS call delete [] rather than delete

No that’s wrong. You have to pair calls to new and delete and calls to new[] and delete[].

Note:
But that’s not how you should to that in modern C++. Use std::shared_ptr or std::unique_ptr instead. This is usually a much safer choice. Calls to new/new[] should be nearly always enclosed in smart pointer and delete should not be needed at all. Very few exceptions.