Is it possible to initialize class member variables in their definition statement instead of using a constructor?
Is the code bellow correct?
class rectangle
{
float a=0;
float b=0;
public:
string color="Red";
...
};
Which C++ Standard allows it?
I recently discovered the new - new to me anyway! - feature of modern C++ that allows you to set the initial value of a data member when you declare it:
class CINTWrapper{
private:
int m_iData=0;
};
This even extends to calling member functions that work with initialization I believe:
class CStringWrapper{
private:
wchar_t* Allocate_Array(const int iBufferSize);
wchar_t* m_pString=Allocate_Array(1);
};
At first, this seemed an extremely useful piece of functionality that C++ had been lacking all along. However, the more I thought about it the more it struck me this feature actually undermines one of the principle design elements of the language - that being the Constructor.
As I understand it the primary purpose of the Constructor is specifically to give the programmer a place where it is guaranteed he can always initialize his data members before anything else is done with the class. However, given the new initialization rules this is no longer necessary. So it largely seems to me that Constructors as a whole are no longer necessary either! Copy-Constructors are a special and vital case. Admittedly when I was using them for their intended purpose I hated either the redundancy you had to introduce across multiple Constructors; those with and without arguments and so on, or alternately the fine tuning of helper-functions to do common initialization between these variants. Now however I sort of regret this cast-iron rule has been taken away.
As a last point, I am trying to change the way I think about programming. I am trying to employ more objects than pure C-style ('int' or 'double', etc) data types and especially to move into templates (although absolutely NOT the Hewlett Packard template library!). Given my current understanding of inheritance in particular it seems to me that using pre-initialized data members rather than Constructor-initialization makes object derivation even more complicated, not less so.
In C++, how do i call a method member of class A from a class B, using a pointer. By the way Class A and B are of different types.
I read that when a pointer is pointing to member function it can only point member functions within the class. But how can i point to a member function outside the class.?????
for example
class A {
public:
int add(int x) {
return x+x;
[Code] .....
The question is: Define the class Counter. An instance of this class is used to count things, but the counter should never be less than 0 (non negative number). The member variable should be private. I realize what I'm suppose to be using but can't implement the member functions needed..
int main(){
int value;
cin >> value;
Counter myCounter(value);
for (int i = 1; i <= MAXLOOP; i++) {
myCounter.increment();
[Code] ....
i want to store reference to a const object in my class as a member variable, as follow:
I basically want a readonly reference to |Data| in Device object.
Code:
class Device {
Device(const QList<QSharedPointer<Data>> & dataList) : _listRef(dataList) {
} protected:
const QList<QSharedPointer<Data>> & _listRef;
}
This does not allow me to initialize _listRef as something like NULL when it is not applicable.Also, i must change all my constructors and its child class to include an initialization of _listRef!!
What is the alternative? Is pointer the nearest? which of the following should be used?
Code:
const QList<QSharedPointer<Data>> * _listRef;
or
const QList<QSharedPointer<Data>> *const _listRef;
or
const QSharedPointer<QList<QSharedPointer<Data>>> _listRef; ????
I am having a problem concerning a static const member variable I want to use to set a certain property of my class during development time. The question actually concerns proper implementation as I do have a solution that "works" at least. The variable should denote the size of a member array which I don't want to allocate on the heap due to serious performance issues. So here is my code:
//MyClass.h
class MyClass{
public:
static const int MyArraySize = 256;
private:
int MyArray[MyArraySize];
};
This works but it's not nice for two reasons:
1) It doesn't separate interface from implementation. I would prefer to define the variable in the corresponding .cpp file but it doesn't work:
//MyClass.h
class MyClass{
public:
static const int MyArraySize;
[Code] .....
If I delete the line int MyArray[MyArraySize]; the above code works but when I use it to define the size of the array I get a "constant expression expected" error for the line int MyArray[MyArraySize]; which makes sense as the compiler does not know the value of MyArraySize when he reaches int MyArray[MyArraySize]; and therefore can not allocate the memory. Of course I can move MyArray to the heap like that:
//MyClass.h
class MyClass{
public:
static const int MyArraySize;
static const int MyValue;
[Code] .....
But as I mentioned before this causes a remarkable loss of performance.
Something like the following does not work:
//MyClass.h
class MyClass{
public:
static const int MyArraySize = (int) pow(2, 8);
private:
int MyArray[MyArraySize];
};
This gives a "constant expression expected" error for the line static const int MyArraySize = (int) pow(2, 8);
Interestingly the following code works:
//MyClass.h
class MyClass{
public:
static const int MyValue;
};
//MyClass.cpp
#include "MyClass.h"
const int MyClass::MyValue = (int) pow(2, 8);
So if I use pow outside of the class definition I get no errors. Is there any solution to those problems? So what I want is:
1) Don't allocate the array on the heap
2) Separate interface from implementation
3) Being able to use functions like pow to define MyArraySize
4) Not use global variables
Here's a part of my program. What I need to know is how I can pass an argument to the Book constructor so I can change the const data member Category (with cascading capacity if possible. I also posted some of my set functions for further comprehension.
class Book {
friend void CompPrice(Book &,Book&);
//friend function that has access to the member functions of this class
//The arguments sent to it are by address, and of type the class Book, so that it can have access to its member functions
private:
//private data members
[Code]...
So I have a class object that contains the private member variable spot and the public member function MoveLock. Within MoveLock, is a member variable called numbers that holds the place where a user is on a "lock knob". Now, what I'm trying to accomplish is that whenever the user turns the "knob" in the wrong direction, the position is updated with that current numbers so that the clicks needed to unlock the first state is also updated. But I get these errors:
Error E2096 C:Users...switchtest.cpp 34: Illegal structure operation in function main()
Error E2294 C:Users...switchtest.cpp 39: Structure required on left side of . or .* in function main()
Ultimately, what I have in main() is a piece of what I'm going to implement in a class member function. I'm also thinking about moving the if else statements out of the for and creating a second one for the else portion.
#include <iostream>
#include <windows.h>
#include <iomanip>
using namespace std;
HANDLE inKeys = GetStdHandle(STD_INPUT_HANDLE);
HANDLE screen = GetStdHandle(STD_OUTPUT_HANDLE);
[code]....
I tried to initialize the frames variable but when I go to debug it, it just gives me 30 errors compared to the one error when I don't initialize the frames variable.
#include "Gfx.h" // general gfx lib I made for SDL
#include "Input.h"
#include "General.h"
#include "Sprite.h"
#define screenw 620
#define screenh 480
/** CHARS ARE DONE */
//int mousex; int mousey; int mouseon = 0;
//int red = 0; int green = 0; int blue = 0;
[Code] ....
#include <iostream.h>
#include <conio.h>
int main() {
int a;
cout<< "ingrese un numero entre 1 y 4 gracias
";
cin>> a;
switch(a)
[code].....
i always get error case bypasses initialization of a local variable
here's my coding
my header files
Code: #include <stdio.h>
#include <conio.h>
#include <iostream.h>
#include <fstream.h>
[Code].....
i'm using borland c++ v5.02
What does the order of console output from your program tell you about when the static object is initialized?
#include <iostream>
using namespace std;
//class
class Firstclass {
private:
Firstclass(); //constructor
~Firstclass(); //destructor
[Code] ....
Doesn't it allocate the class static variable to the heap, thus executing its algorithm then destroying it when the program ends - or. What exactly does it tell me? When the static variable is initialized, it takes place first before any of my other functions?
i have highlighted the errors in block letters.
#include<fstream.h>
#include<conio.h>
#include<string.h>
#include<stdio.h>
#include<stdlib.h>
void login();
struct date {
int dd,mm,yy;
[code].....
I am having trouble compiling my interface. I am trying to store a reference variable as a member variable of the interface object. Compiler says that the variable has not be initiated correctly.
LCD inherits from VisualInterface which is expecting a DisplayDriver object to be passed in (DisplayDriver is another interface, but thats not important).
I pass the displayDriver object in when LCD is instantiated in maininterfaces.zip
I was pasing it before as a pointer but was told that this could cause me problems with memory leaks and a reference was better, but now I cant seem to get it to compile.
I created a class (let call it X) which contains the structure to store the data from my data base. Them I have a class (call Y) which will contain a list for each row in my data base. Third, I have a class with thousands variables (Z). What I am trying to do is to take the list of objects (Y) that contains the data to initialize Z. What I want to now if I can do something like that.
Imaging that one of my rows contain the following data:
Type Nameofvariable etc...
"static const double; MNFAIL ; 0; 0; 0,25"
In my list I have a node with contain this data
I want to use the field Nameofvariable to initialize the variable called MNFAIL contained in my class Z.
Is it possible in C++
I have a class containing a map member that I want to initialize at declaration time. I know I can do it in the cpp file but I'm having a problem with the order of initialization (static initialization order fiasco).
My questions are:
Is it possible that the scenario in which, the Test's constructor's implementation and the map initialization instruction are in the same cpp file and constructor is called when the map is not initialized yet, could happen?
Is it possible to initialize the map in class like I did? I get these errors:
in-class initialization of static data member 'std::map<std::basic_string<char>, Test*> Test::a' of incomplete type
temporary of non-literal type 'std::map<std::basic_string<char>, Test*>' in a constant expression
If yes, does this initialization resolve the static initialization order fiasco?
class Test {
public:
static std::map<std::string, Test*> a = {};//this is an error
Test(std::string ID) {
[Code] ....
I've defined a class with a copy constructor. Some sample code:
Class* c = new Class(*this) // My copy constructor must take in a const Class&
And the compiler gives:
error: cannot convert 'Class' to 'Class*' in initialization
Why does this syntax work with other data types? For example, int* MyInt = new int; compiles fine. What about a copy constructor makes it fail?
i'm currently working on a research project and i've been given some specifications
Is there a way i can access/use the array initialisation list i.e
{value,value,value}; .
For my own class? Like this
myclass foo={value,value,value};
Code:
class Base {
public:
int base;
Base(int init=0):base(init){}
virtual ~Base(){}
[Code] .....
Invalid initialization of non-const reference of type 'Base&' from an rvalue of type 'Derived'
What does it mean, and why can't I return the Derived class by value (I'm trying to create an exact copy of Derived).
How to initialize a static member of a class with template, which type is related to a nested class?
This code works (without nested class):
#include<iostream>
using namespace std;
struct B{
B(){cout<<"here"<<endl;}
};
template<typename Z>
[Code] ,....
I am writing a program which is using SDL library. I have two different classes which one of them is Timer Class and the other is EventHandling Class.
I need to use some member functions and variables of Timer in some Eventhandling Class member functions, Although I want to define an object of Timer in int main {} and relate it to its member function that has been used in Eventhandling member function in order that it becomes easier to handle it, I mean that I want to have for example two objects of timer and two objects of Eventhandling class for two different users.
I do not know how to relate an object of a class from int main{} to its member function which is being used in another class member function.
Lets have it as a sample code:
class Timer {
private:
int x;
public:
Timer();
get_X();
start_X();
[Code] ....
So I have an ImageManager class, Board class, and Box class. In Board.h I can declare ImageManager imgr; and in Board's constructor I can use imgr and its functions and such. However, in Box.h when I try and declare ImageManager imgr; I get the error "cannot access member declared in class ImageManager". Both declarations are under private, and exactly the same, but one doesn't work. Also, is there a way to only have one instance of ImageManager?
View 19 Replies View Relatedthis program is not giving to chance to enter the ooplevel value.
Code:
#include <iostream>
using namespace std;
const int SLEN = 30;
struct student {
char fullname[SLEN];
char hobby[SLEN];
int ooplevel;
};
// getinfo() has two arguments: a pointer to the first element of
// an array of student structures and an int representing the
[Code]...
I need to keep a static variable in a member function of a class that I have many objects of. I've had some trouble with it, and when I read up I found that such variables are static across all instances. Is there any way around this?
View 3 Replies View RelatedI've created a class called Voter with a private member variable ID, also I have a variable in my main function to be ID as well. I'm trying to compare the two ID's but when I do so:
if (ID == V.ID)
I get the error - 'std::string Voter::ID' is private within this context.
I know that because it's private I can't access it, but how do I?
So, one can do stuff like this using #defines:
#include <iostream>
#include <array>
#define x arr[0]
#define y arr[1]
#define z arr[2]
class Point {
[code]....
... that is, to be able to reference the same data by "member variables" as by referencing a stl container. But defines are the devil's work - adding in a "#define x arr[0]" is a dangerous statement. I'd really like some nice clean C++ method (C++11 or C++14 are just fine) to do this without defines, but so far I'm drawing a blank. If arr wasn't an STL container, if we just wanted a pointer-based array, I could do it this way:
class Point
{
...
float x __attribute__ ((aligned (sizeof(float))));
float y __attribute__ ((aligned (sizeof(float))));
float z __attribute__ ((aligned (sizeof(float))));
float*const arr = &x;
};
... but you obviously can't do that if arr is an STL container.
The best I've come up with is to make x, y, and z function pointers, but then you can't call them like p.x, you have to call them like *p.x(), it's not very clean and I'd expect some added overhead. One could go even uglier and make x, y, and z be instances of some custom class with overridden operators that reference arr[], but that seems like it'd be just getting ridiculous in terms of overhead (both coding and performance)