C++ :: Returning A Pointer To Encapsulated Data Member
Jun 22, 2014
So, I've got this class in SDL Player that has, among other things, an SDL_Texture* to hold an image that represents the player on the screen. I'd assume it's good practice to do get() and set() functions for the class; but because textures are handled via pointers, when I write a get() function I end up returning a pointer to an internal resource; which isn't good practice I hear as it "breaks" encapsulation.
Find my code below:
#ifndef PLAYER_H
#define PLAYER_H
#include "SDL.h"
#include "SDL_image.h"
#include "CTexture.h"
class Player {
[Code] .....
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Apr 30, 2012
When I do this:
// header file:
#include <list>
using namespace std;
class myClass {
list<int> myMethod();
};
// cpp file:
list<int> myClass::myMethod() {
}
In the cpp file, 'myMethod' is underlined in red and when I hover over it, it says:
"std::list<int, std::allocator<int>> myClass::myMethod()
Error: declaration is incompatible with "<error-type> myClass::myMethod()""
But when I make it as a standalone function, outside a class, with no pre-declaration, there is no problem.
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Mar 6, 2014
I have this method called "walk" and what it's supposed to do is, walk through a bunch of triangles one step at a time (the're all connected [not to each other, but in a mesh]).
To do so, I need to perform a containment test.
Normally, I guess I'd just write a separate function but what if I wanted to be weird and write my inclusion test as a lambda in my walk method? Literally the only function that needs this code is my walk() procedure and I need to call the test an arbitrary amount of times.
Is this frowned upon? Would this be the jarring type of code I've been warned about? Or should I just say yolo and do what I want?
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Nov 21, 2014
As the title says, i'm using a function which returns a pointer to a struct:
the struct is the following:
Code:
typedef struct POINT
{
uint16_t x;
uint16_t y;
}
Coordinate; the function i'm using:
Code:
Coordinate * Read_XTP2046(void)
{static Coordinate screen;
//calculations to determine the coordinates
screen.x=(temp[1]+temp[2])/2;
screen.y=(temp[0]+temp[2])/2;
// and so on...
return &screen;}
The question is: how do i catch this pointer and make it into a Coordinate struct in which i can read the x and y.
In my main program i would do the following:
Code:
Coordinate cor;
cor = Read_XTP2046();
This does not work, as the function returns a pointer, but how to transform this pointer into a Coordinate struct.
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Apr 24, 2013
I hear all the time about the rule of not returning a local pointer, but I'm not exactly sure what that insinuates. Would this example be classified as violating that rule?
class Foo {
public:
Bar* ReturnBar() {
Bar* temp = &bars.at(0);
return temp;
}
private:
vector<Bar> bars;
};
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Mar 24, 2015
I am trying to return a char pointer so that i can re use it again. I am writing a vigenere function that takes a message, a key and an initialization vector where it performs the encryption, prints out the encrypted message and returns the encrypted message. I print out the process step by step and everything works, however i pass the answer and print it out again and only the first letter gets changed. I put my code below and my output right after that.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <time.h>
void decrypt(char *to_encrypt, char *key, char* pct);
enum flag{encryption = 1, decryption = 0};
[code].....
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May 29, 2014
I have a class which looks like the following:
Code:
class Gadgets {
public:
Data* GetData() const {
return _data.data();
}
const Data * GetDataConst() const {
return _data.data();
}
private:
QSharedPointer<Data> _data;
}
You may ask why i have the second GetData() method that return a pointer to my private member, which will then expose my private member externally. I provide this function just for the case where user can do the following
Code:
Gadgets * myGadgets = new Gadgets();
myGadgets->GetData()->ModifyData(.....);
My question pertains to the use of QSharedPointer of data(), where in the doc, it says
T * QSharedPointer::data () const
Returns the value of the pointer referenced by this object.
Note: do not delete the pointer returned by this function or pass it to another function that could delete it, including creating QSharedPointer or QWeakPointer objects.
Whereas, if i were to use
Code:
Data* data as my private member class instead of QSharedPointer;
I will not need to worry if the caller do as follow
Code:
Data * data = new Data();
Data *data2 = myGadgets.GetData();
data2 = data;
delete data;
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Aug 18, 2014
Should i never return a pointer to a local variable. i have seen the following code in the standard text book.
Code:
struct node *add_to_list(struct node *list, int n)
{
struct node *new_node;
new_node = malloc(sizeof(struct node));
// some code here
return new_node;
}
new_node is a local pointer then why is he returning the new_node? Is it wrong or am i failing to understand something?
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Mar 5, 2013
I have a program that is trying to find all factors of an integer given. It needs to be done in a recursion function. Right now i have code similar to just getting the prime factors of a integer.
unsigned int * find_factors_using_recursion(unsigned int x ) {
unsigned int * factor = new unsigned int[];//do i put x in here ?
for(unsigned int i = 2; i < x; ++i) {
if(x % i == 0) {
find_factors_using_recursion(x / i);
*factor = (factor[i] = i);
} }
return factor;
delete [] factor;
}
When i cout the *factor = (factor[i] = i) it gives me the prime numbers of the integer passed into the function but when I return the pointer it only returns one of the prime numbers. I'm new to c++, how to return pointers from functions that would be great with an example to go with it.
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Dec 10, 2014
I know how to pass a 2-D array to a function. The prototype for that is void f(int (*p)[2]) assuming the array is of integers and there are 2 columns in it.
However, if I wanted the same function to return a pointer to a 2-D array, what would be the prototype?
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Apr 25, 2014
I am writing a class Player which has several char arrays as private fields. I am trying to write a method which returns an array as a pointer, but doesn't alter the array in any way, thus the const.
Here is a snippet:
Code: class Player
{
private:
char state[MAX_STATE_CHAR + ONE_VALUE];
int rating;
char last[MAX_NAME_CHAR + ONE_VALUE];
char first[MAX_NAME_CHAR + ONE_VALUE];
int groupNumber = NEG_ONE;
public:
char * GetFirst() const
{
return first;
}
Visual studio is saying that the return type doesn't match.
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Sep 27, 2013
I have a function, which has to return some int data. In that function there are multiple return statements. There is also possibility that function may return in some cases. Will this result in undefined behavior???
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May 2, 2013
Okay, I've been working on this Texture class that's going to be compatible with both SDL and OpenGL with the capability to create an array of textures within one class as opposed to multiple classes and such.
Now I've run into a slight issue if I want to return the value of a texture. There's two different types of textures: SDL_Texture, and a standard GLuint Texture. The problem I have is that one or the other is used, not both which is depending on whether or not the person is using OpenGL.
So when the user wants to get the texture, I need the ability to return either an SDL_Texture, or GLuint depending on whether or not OpenGL is being used.
I tried this with a template class, but it didn't work, but I'll post the code so you can see what I'm trying to do.
template <class Tex> Tex Texture::GetTexture(int TextureNumber)
{
if (TextureNumber > NumTextures || TextureNumber < 0)
return;
[Code]....
It basically just comes down to the last four lines of code. If the person is Using OpenGL return a GLuint, if the person is using SDL, return an SDL_Texture.
I would prefer to have the GetTexture function to be one function instead of two separate ones so I don't have to call a different function every time to check if I'm using SDL or OpenGL.
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Jun 6, 2012
I am trying to use "remove_if" with a predicate function inside a class. The code intends to remove the grid cells which an agent cannot move into (from among all possible cells).
Code:
void classname::function1()
{
vector<MoorePoint> neighbors;
....
[Code]....
That code would work if it was not in a class and the predicate was not a member function. However, now I receive long error messages which I guess refer to incompatibility of remove_if template with the predicate parameter (one error includes : error C2064: term does not evaluate to a function taking 1 arguments).
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Jan 10, 2015
I have the following scenario :
Code:
class test {
public :
int num;
};
int main() {
test t1;
test *ptr = &t1;
int test :: *mem_ptr = &test::num;
}
I need to access mem_ptr (pointer to a class member) through :
a. object (t1)
b. pointer to the object (ptr).
How can i do that ?
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May 15, 2013
I have a class with member functions and a pointer like this:
Code:
class MyClass{
private:
typedef void(MyClass::*memFnPtr_t) ();
public:
memFnPtr_t fnptr;
void fn1();
void fn2();
};
Now in a regular function I create an instance of the class and try to assign the pointer:
Code:
MyClass mc;
mc.fnptr = &mc.fn1;
The compiler does not like it and suggests this instead:
Code:
mc.fnptr = &MyClass::fn1;
This seems to work but what if I have two instances of the class:
Code:
MyClass mc1, mc2;
How does the compiler know to distinguish between
Code:
mc1.fn1
and
Code:
mc2.fn1
when the assignment now looks identical:
Code:
mc1.fnptr = &MyClass::fn1;
mc2.fnptr = &MyClass::fn1;
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Aug 19, 2014
I have the following problem: I am using NLOpt for optimization. The API provides functions to set the objective. This is done as follows:
double objective(const vector<double> &x, vector<double> &grad, void *data)
{
return x[1]*x[0];
}
int main(){
nlopt::opt opti(nlopt::LD_MMA,2);
opti.set_min_objective(objective,NULL);
vector<double> x(2);
[Code]....
Now I want to make the function objective a member of a class:
class Foo {
public:
double objective(...){..}
};
How can I give this method to opti.optimize? If I make objective static I can use
opti.optimize(Foo::objective,NULL);
but I do not want to have a static member. Is it possible to create an object of type Foo and give it to opti.optimize?
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Apr 17, 2013
I have following:
struct Point {int* a; int b;};
vector<vector<Point> > numbers;
vector<int> example;
The numbers vector has a matrix of a sort and each of the members are pointing to one member in the example vector. A member numbers.at(2).at(3).a is pointing at example.at(3). Now, can I remotely delete a member in the example vector using the pointers? Like so:
delete (*(numbers.at(2).at(3).a));
I know there is a more convenient way to delete members, but this is a very specific case I'm working on.
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Jan 30, 2013
Here is the assignment: (3pts) Given the following class header file, write the class’ source code for each of the accessor and mutator functions listed. (How the functions have listed their parameters, varying between passing by reference and by value.) Don’t forget to comment your code – it counts!
class Album {
private:
char * artist; // band or singer’s name
char * title; // title of the album
[code]....
The input will be an array. My questions: First, am I on the right track?
When using (char * a) for a function, for example, this is passing the address of a, correct? so then *artist=a; changes what the address of a points to?
also, the functions are bool when I would expect void. Why? for all of the set_" " functions, the parameter is *... but for set_record_label it is *&. That appears to be a mistake to me. Is that right?
what is the difference between *& and * as parameters?
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Nov 26, 2013
I have the following piece of code.
Code:
#include<iostream>
using namespace std;
class Test {
public:
Test(){cout<<"Test"<<endl;}
void fun() {
int i=5;
[Code] ...
Compiled with g++.
Executing this give output fun5.
It is correct? I have not allocated any object and so this pointer is not created. Then how it is able to run and call the function.
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Jan 6, 2013
I'm currently programming a server which uses multiple threads- I have a class for one map in the game. Each map has a thread for timed events(tile regeneration, NPC regeneration, etc.), and a thread for handling NPCs(movement, combat, etc.). A basic structure of the class looks like this:
class Region {
public:
/* game values are here, they are public so
they can be accessed from outside of the class
inside of packet-handling functions and such */
int value;
void *Function();
[Code] ....
The program crashes when I use a member of the same class the function is located in- in the context I have shown about it would crash on "value++".
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Dec 3, 2013
I have little problem which causing memory leaks.
Parent > Multiple Child(Parent parent) > Child destructor deleting parent => next Child destructor crash
Example code: without using:
class Parent {
public:
Parent() {
for(int i = 0; i < x; ++i) {
for(int j = 0; j < y; ++j)
childs[i][j] = new Child(this);
[Code] ....
If you read code, on Parent destructor i = 0 & j = 1 its going crash.
Parent will be deleted aswell, but it give me assert: _block_type_is_valid(phead- nblockuse)
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Jan 3, 2015
I am trying to create a callback system for input events in my game engine.
Here is a cut down version of the EventManager.h file
#include "Controls.h"
#include "Object.h"
enum MouseEventType{PRESSED, POINTER_AT_POSITION, PRESSED_AT_POSITION };
[Code].....
This works if the function pointer being passed to the event manager is not a member function.
I have two other classes Scene and Object that could potentially use this EventManager to create callback events. Scene and Object are both pure virtual objects. How can I pass a pointer to a member function of the child classes of both Scene and Object? I am fine with just having two separate watchEvent functions for Scene and Object but how do I pass the type of the Object or the type of the Scene? The child classes are unknown as they are being created by someone using this game engine.
For example, if I want to make a player object it would be something like
class PlayerObject : public Object{...};
Now that PlayerObject type has to find its way to PlayerObject::functionToCall(). I think I need templates to do this but, since I never used them before
This is how I intend to use this
class OtherScene : public Scene{
void p_pressed(void){
//pause
}
[Code].....
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Sep 19, 2014
This project is for an embedded micro controller. In the project i wrote a class that generically services uarts. then i declare 6 objects of that class and hand them configurations for each specific uart.
internally all the objects have a send buffer of data that is still to be sent that gets populated by the object member function.
how can i make an array of function pointers that can point to the same member but of six different objects.
for example (not a working one)
class uart {
private:
struct myData {
unsigned char data[20]
int head
int tail
int count;
[Code] ....
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Mar 6, 2013
Below is a working program. Please note that the vector <int*> pointer is declared as a public member of the class A.
The problem arises when I try to make it a private (instead of public). First, let's look at the File 1 below. It compiles and works fine.
File 1: main.cpp (working fine)
#include <vector>
#include <iostream>
using namespace std;
[Code].....
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Sep 30, 2014
I've got the following code which demonstrates a problem :
Code:
struct A {
double x[2];
double i;
};
struct B : virtual public A {
double y[2];
[code]....
I'm wondering if this is a compiler bug. Why doesn't the pointer-to-(derived)-member work for the array if it works for the non-array?
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