C++ :: check Pointer To Class Object For Null Value?
Feb 14, 2013
lets say I have a pointer p_unit of type c_unit* (c_unit is an a.b.c.)
I have a function that returns a pointer to a new c_unit object:
c_unit * man_add_unit() {
c_unit * local_p_unit;
unsigned short int local_run_code;
print_man_add_menu();
local_run_code = get_a_run_code(); // this bit just gets user input
switch (local_run_code)
[code]....
I assign that to p_unit, then add it to a vector v_units:
p_unit = man_add_unit();
v_units.push_back(p_unit);
cout << "New unit added.
";
The whole program runs on a loop, and another thing the user can do is to print out data on c_unit objects pointed to by v_units. The problem is, in that function up there ^ I give the user the option to go back to main menu without creating a unit.
Since "local_p_unit" is declared but not assigned an initial value, I'm guessing the function would return a "null" pointer (which is what's hanging me up). If I just let this run with the above code, and go to print out the unit data, the program crashes.
I tried to make an if thing with p_unit == 0 but this always returns false and doesn't catch the "bad" unit that will subsequently cause a crash.
Btw, I have considered assigning a reference to a generic c_unit object to that there local_p_unit so it won't return null, then remove pointers to that object from v_units at the end of the loop.. But I know there's got to be a better way.
#include <iostream> #include <vector> #include <string> #include <map> using namespace std; struct bop { string realname; //real name
[Code] ....
Okay, so first thing's first. The program will not compile due to lines 39-45. If I were to change those pointers into regular objects, it will not change the values of my class object. So what is the right way to do this?
I want the user to be able to input the # of employers/programmers into the system. But I cannot do that with an array of classes because when declaring an array; the array size must be constant.
Lets say we have a class that holds a pointer member to another object. If I delete that pointer in the destructor I get an error (and I understand why). My question is : is it possible to overcome that without memory leaks ?
1 #include<iostream> 2 using namespace std; 3 4 class A { 5 public: 6 ~A() { 7 cout<< "~A()" <<endl;
Is there a way to copy a derived class object thru a pointer to base?
For example:
class Base { public: Base( int x ) : x( x ) {} private: int x; }; class Derived1 : public Base { public: Derived( int z, float f ) : Base( z ), f( f ) {} private: float f;}; class Derived2 : public Base { public: Derived( int z, string f ) : Base( z ), f( f ) {}
[Code] ....
The question is whether *B[0] would be a Derived1 object and *B[1] a Derived2 object?If not, how could I copy a derived class thru a pointer to the base class?
class A (abstract) class B : A class C { void add ( A(&*?) a ) std::vector<std::unique_ptr<A>> data; //unique_ptr<A> because A is abstract and therefore vector<A> isn't possible }
upper situation. What is the best way to pass add an object of class B to C?
with C::add(A* a){ vector.push_back( unique_ptr<A>(a) ); } and int main() { C c; c.add( new B() ); }
This works, but i don't think it's very nice, because you could delete the pointer in main. What happens then with the unique_ptr? I could probably make C::add( std::unique_ptr<A> u_p ); but maybe it can be avoided that the "user" (in main() ) has to create the unique_ptr itself.
I know if i will not use the pointer base class function "virtual double grossPay" will be called for both base class object and derived class object and when i will use pointer with reference to the object because base class function is virtual it will look for same function in derived class and if available it will execute it.
public class ColorGenetics { public static hk1Class jw; public static linkedClass link; }
[code]...
Code builds fine but compiler says I tried to use bject link w/o providing an instance. Class linkedClass has an instance of Class hk1Class set to var jw1.I had linkedClass useing the same var jw for the object instance. I changed it to jw1 thinking that would clear it up and it didn't.
Say I have an object and 10 pointers to it in several other objects of varying class types. if the object gets deleted, those pointers have to be set to null. normally I would interconnect the object's class with the classes which have pointers to it so that it can notify them it is being deleted, and they can set their pointers to null. but this also has the burden that the classes must also notify the object when THEY are deleted since the object will need a pointer to them as well. That way the object doesn't call dereference a dangling pointer when it destructs and attempts to notify the others.
Auto pointers and shared pointers are not what I'm looking for - auto pointers delete their object when they destruct, and shared pointers do the same when no more shared pointers are pointing to it. What I'm looking for is a slick method for setting all pointers to an object to null when the object destructs.
When I try to run this, I only get the message "Material pointer is null". So, somehow the pointer is null after the hitObjects-function is called, even though it isn't inside that function.
So I'm writing a small program for class, and for some reason I keep getting an error when trying to initialize head to NULL. Even threw in the namespace just to see, nothin'.
#ifndef NUMBERLIST_H #define NUMBERLIST_H using namespace std;
[Code].....
There's my header file. Not sure what I'm doing wrong with the constructor.
EDIT: Got it to work with nullptr, but still curious why that isn't working
In jumping into C++ it says something like this: It's not necessary but when you delete a pointer it's a good idea to reset it as a null pointer. That if your code try's to dereference the pointer after being freed, your program will crash. This happens to a lot of experienced programmers.
This could corrupt users data. delete p_int; p_int = NULL;
1. If you can deference a pointer after the memory is freed, why can't you just delete the pointer?
2. If you can do 1, how do you delete the pointer using code?
3. Every thing I've read says that free memory is handed out in a sequenced order. I don't believe that is true at all. I may be wrong. Why can't you put the data in any number of places if it will fit. Isn't the compiler smart enough to know where bytes (bits)and pieces are stored?
4. If you storing anything in free memory must use a pointer to it?
5. Can a pointer or something similar be used with stack memory?
in the below program for both class pointers pointing to null. Class contains normal function executing but class contains virtual function getting segmentation fault.
m_gradient() constructor is called to construct m_gradient, which in turns sets a variable called m_S to NULL(0), then in the assignment operator, because the m_S is NULL, Visual Studio throws an exception saying that destination pointer is null, but it's not fatal. I don't like that to happen because I have to press ignore many many times when the program is debugged, how can I fix this bug (not mine) to avoid the null destination pointer exception?
As an intermediate layer, the function calls boost unbounded array operator = which in turns calls std::copy which results in this error
struct mystruct{ int n; }; class mscope{ public: std::vector<mystruct> mv;
[Code] .....
So I have a vector of structs and I want to traverse it, find a struct that matches a constraint and obtain a pointer to that struct. I made a function for this purpose which takes a number and an empty pointer that will store the reference. However, after function returns the pointer becomes null. What could be causing this?
getting an invalid null pointer error message after a successful build. This program is supposed to ask for firstName, lastName, age and maJor and keep doing so until the age that is input is 0 and then the program closes. Here is what I have.
#include <iostream> #include <string> using namespace std;
Writing this code that handles clicks with the left mouse button. But I want to be able to check for clicks within a 200x200 pixel area. Here's what I have:
void EGame::OnLButtonDown(int mX, int mY) { int ID = mX / 200; ID = ID + ((mY / 200) * 3);
if(grid[ID] != GRID_TYPE_NONE) {
[Code] .....
Using the width and height of the object, e.g; pX->w is width and pX->h is height ....
How would I check within the 200x200 area with mX and mY?
I create an instance of a base class (not derived class) and assign it to base class pointer. Then, I convert it to a pointer to a derived class and call methods on it.
why does it work, if there is a virtual table?
when will it fail?
// TestCastWin.cpp : Defines the entry point for the console application.//
I'm having an issue coming up with an if() statement to check if a word match the one in the value of a pointer's address. So far the best I've come up with only matches the first letter of the words, you'll find it in the code below.
#include"Header.h" int Colour(struct MyStruct *ArrayPointer, int ArraySize) //ArraySize = 3 for this run. { int ColourCount = 0; for (int i = 0; i < ArraySize; i++) {
[Code] ....
An example run you can see in attached pic.
I want to have an if statement that only accepts "Red" and not the occasional "Ravaged_Anus".
I'm using MVS Express 2013, .c source files, and the C++ compiler.