valgrind how to check a pointer without errors

[noahpereira20]

Hello fellow earthlings,

I need to check a char array to see if it is valid or not, it could be a nullptr, but not a unsigned pointer, and the safe state is has is first element = '\0'

here is my code that i tried to make work for valgrind but did not succeed

	bool Menu::isEmpty(void) const {
		//Return true if title is in safestate
		bool rtn = false;
		if (m_title != nullptr) {
			rtn = m_title[0] == '\0';     <<<<Valgrind doesn't like this
		}
		return rtn;
	} 

here is the error

==106406== Invalid read of size 1
==106406==    at 0x401ACD: sdds::Menu::isEmpty() const (Menu.cpp:230)
==106406==    by 0x401AA7: sdds::Menu::operator bool() const (Menu.cpp:223)
==106406==    by 0x40114E: main (ms1_MenuTester.cpp:68)
==106406==  Address 0x5a24040 is 0 bytes inside a block of size 16 free'd
==106406==    at 0x4C2BB8F: operator delete[](void*) (vg_replace_malloc.c:651)
==106406==    by 0x4017BF: sdds::Menu::~Menu() (Menu.cpp:114)
==106406==    by 0x401126: main (ms1_MenuTester.cpp:65)
==106406==  Block was alloc'd at
==106406==    at 0x4C2AC38: operator new[](unsigned long) (vg_replace_malloc.c:433)
==106406==    by 0x40169D: sdds::Menu::setTitle(char const*) (Menu.cpp:72)
==106406==    by 0x4016F5: sdds::Menu::Menu(char const*, int) (Menu.cpp:82)
==106406==    by 0x400D0B: main (ms1_MenuTester.cpp:25)
==106406==

[helios]

You can't. There's no test you can perform on a non-null pointer that will tell you if it's valid or invalid. All you can is always correctly initialize all pointers you use.

[ne555]

by the log, it seems that you did something like

Menu *p = new Menu("foo", 42);
delete p;
p->isEmpty();

so perhaps you should try to not shoot at your foot.

[noahpereira20]

its a design error i've come to accept, the pointer goes into a scope by value and gets deleted by destructor when exiting scope, then the next call from client code (which i cant change) checks the isEmpty() funciton. Trying to figure out a way i can implement a variable that stays alive through the destructor of sorts.

[helios]

You need to implement a copy constructor that performs a full copy of the object, that way when the copy is destructed only the copy's pointers are released, not the original's.

Example:

class A{
    int *p;
public:
    A(int n): p(new int(n)){}
    A(const A &other): p(new int(*other.p)){}
    A &operator=(const A &other){
        *this->p = *other.p;
        return *this;
    }
    ~A(){
        delete this->p;
    }
}

[noahpereira20]

I have a copy constructor, sorry im new to object oriented and c++ but i'm easy to talk too,
heres the scope it goes into

testMenus(mainMenu, subMenu1, subMenu2);

void testMenus(Menu m, Menu sub1, const Menu& sub2)

is their something i can implement to ensure that the object doesn't get lost in the scope?

here is my copy constructor

	Menu& Menu::operator=(const Menu& menu) {
		//Check if menu has title
		if (menu) {
			//Check if menu has items
			if (menu.m_menuCount > 0) {
				//Check if we have items
				if (m_menuCount > 0) {
					//Delete our items
					for (int i = 0; i < m_menuCount; i++) {
						delete m_menuItem[i];
					}
				}

				//and allocate for new items
				for (int i = 0; i < menu.m_menuCount; i++) {
					//Add item updates count aswell
					add(menu.m_menuItem[i]->m_item);
				}
			}
			else {
				//Delete our items
				for (int i = 0; i < m_menuCount; i++) {
					delete m_menuItem[i];
					m_menuItem[i] = nullptr;
				}

				//Reset count
				m_menuCount = 0;
			}

			//Copy indentation
			m_indentation = menu.m_indentation;

			//Clear our title
			delete[] m_title;

			//Set new title
			setTitle(menu.m_title);
		}
		else {
			//Set to safe state
			setSafe();
			m_menuCount = 0;
			m_indentation = 0;
		}

		//Return object
		return *this;
	}

[doug4]

This is an assignment operator, not a copy constructor. It will be very similar, but it is not the same.

The variable menu is a reference, not a pointer, so line 3 is meaningless. Generally you cannot have a null reference.

You never set m_menuCount in the case when menu.m_menyCount > 0 (unless that's done inside "add".)

I'm not sure how m_menuItem is defined. I don't think you are properly cleaning up the table itself.

If you used std::string, std::vector and std::unique_ptr, all of your news and deletes would go away and you would have a much more robust, safe project.