Question

I enjoy using the operators new and delete in C++ a lot but often have a problem calling delete later on in the program’s code.

For example, in the following code:

class Foo {
public:
    string *ace;
    Foo();
    ~Foo();
};
Foo::Foo() {
    ace = new string;
}
Foo::~Foo() {
    delete ace;
}
void UI::ButtonPressed() { //Happens when an event is triggered
    Foo *foo = new Foo;
    ui->label = ace; //Set some text on the GUI
    delete foo; //Calls the destructor, deleting "ace" and removing it from the GUI window
}

I can declare a new string but when I delete it, it removes the value from the GUI form because that string has now been deleted.

Is there a way for me to delete this allocated string somehow later on?

I don’t want to declare it as a global variable and then delete it on the last line of the program’s source code. I could just never call delete but from what I have been taught that’s bad and results in memory leaks.

Answer 1

You should read about the RAII pattern. It is one of the most important concepts to know for a C++ programmer.

The basic idea is that the lifetime of a resource (a new’ed object, an HTTP connection, etc.) is tied to the lifetime of an object. This is necessary in order to write exception safe code.

In your case, the UI widget would make a copy of the object and free it in its own destructor. The calling code could then free its copy right away (in another destructor).