C++ :: Tag Dispatching Constructors
Dec 24, 2014
#include <iostream>
#include <string>
struct A {
struct Tag{};
std::string name;
int value;
A (const std::string& n, int v) : name(n), value(v) {}
A (const std::string& n, int v, Tag) : name(n), value(v) {std::cout << "Constructor with Tag called.
[Code] ....
How to avoid having to type out a second near-identical constructor for any class like D and E which have specialized constructors different from A? You can imagine the nuissance this causes if there are many classes like D and E (and with many parameters) that need the Tag parameter. The nuisance will be there to when making changes to the constructors. Delegated constructors I don't think will work because of passing `tag` into the parent's constructor. Is there some sort of inheritance trick I can apply simultaneously to all such classes to get them all to behave like B and C's constructors?
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Apr 6, 2013
I need to implement a singleton, so I've been reading about it online and I'm still not quite sure about all the types of constructors I need to declare:
Code:
class my_singleton {
private:
my_singleton();
my_singleton(my_singleton & X);
my_singleton(const my_singleton & X);
[Code] ....
Is this OK?
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Dec 16, 2014
I have an inherited class that essentially manages a Qt Window.
It is as follows (prototype below):
class QTMyOpenGLWindow : public QWindow, protected QOpenGLFunctions {
Q_OBJECT
[Code] ....
Now, I can understand the confusion of the compiler, but the functionality as I laid it out works for me (I can create the class with just specifying the parent and also have the option of preventing auto-initialization when creating). But, is there a better approach?
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Apr 7, 2014
As long as no base class constructor takes any arguments, the derived class need not have any constructor, if one or more arguments are used then it is mandatory for the derived class to have a constructor and pass the arguments to base class constructors. While applying inheritance, we usually create objects using derived class. Then it makes sense for the derived class to pass arguments to the base class constructor. When both the base and derived class contain constructors ,the base class constructor is execute first.
In case of multiple inheritance, the base classes are constructed ,in the order in which they appear in the declaration of the derived class. Similarly in a multiple inheritance the constructors will be executed in order of inheritance. Since the derived class takes the responsibility to supply initial values to the base class,we supply the initial values that are required by all the classes together where the derived class object is declared.
The constructor of the derived class receives the entire list of values of arguments and pass them on to the base constructors int the order in which they are declared in the derived class
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May 16, 2013
#include <iostream>
using std::cout;
using std::endl;
class CBox // Base class definition
{
public:
// Base class constructor
explicit CBox(double lv = 1.0, double wv = 1.0, double hv = 1.0) : m_Length(lv), m_Width(wv), m_Height(hv)
[Code] .....
This example produces the following output:
// Derived class copy constructor
CCandyBox(const CCandyBox& initCB): CBox(initCB) {
std::cout << std::endl << "CCandyBox copy constructor called";
// Get new memory
m_Contents = new char[ strlen(initCB.m_Contents) + 1 ];
// Copy string
strcpy_s(m_Contents, strlen(initCB.m_Contents) + 1, initCB.m_Contents);
}
It will work right? Cause when I do "CBox(initCB)" it only sends the BASE part of the derived object to the base class copy constructor, is it copy or default?
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Sep 13, 2014
The code below references to a header file and implementation .cpp file, which are not important. My question is what is the proper way to use a constructor in a main file. I have been getting "invalid use of" errors when using letters.Pair(a,b), where Pair(T a, T b) is a constructor that accepts arbitrary type T of variables 'a' and 'b'. So I played around a bit and suddenly found a syntax that works. I need verification for the syntax below:
#include <iostream>
#include "pair.h"
#include "pair.cpp"
[Code].....
Are the comments with the asterisks correct? As in this is always the way you initialize and assign? So letters.Pair(a, b) is not the right way to use constructors?
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May 28, 2014
I created 3 Rectangle pointers. And later in the program, I would like to modify these existing Rectangles by calling constructors with parameters. Is this possible? I have a sense that it involves creating overload operators, but I am not sure how to do it, or if that's the correct path.
#include <iostream>
#include <string>
using namespace std;
// Base class
class Shape {
protected:
int width;
int height;
[code]....
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Feb 16, 2015
I'm getting a massive 1300 char link error with VC10. It appears to complain that it can't see the constructor although the constructor is definitely there.
Error:
test_GatewayNS.obj : error LNK2019: unresolved external symbol "public: void __thiscall std::allocator<class BinarySearchVector::ElementTemplate<class Gateway,unsigned __int64> *>::construct(class BinarySearchVector::ElementTemplate<class Gateway,unsigned __int64> * *,class BinarySearchVector::ElementTemplate<class Gateway,unsigned __int64> * const &)"
[Code] ....
However, the constructors seem to be there and if I copy them into my program just to make sure - the compiler complains that they are already defined:
namespace BinarySearchVector {
template <class ElementType, class IdType> class ElementTemplate //allows comparison functions to be redefined {
public:
ElementTemplate(IdType myId) : id(myId), tickCount(0), requestingDeletion(false) {};
[Code] ....
Any clues as to what I'm missing ?
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Dec 13, 2013
I am trying to figure out copy constructors for a dynamic array and I am definitely missing something. If I go into the copy constructor routine during debug, the values appear to be correct but they don't percolate up to the newly created object. I'll post a portion of the code below:
Code:
// include header files for the classes that are being used
#include "stdafx.h" //
NOTE: THis reference must be added to all cpp files in Visual Studio Express 2013
#include <iostream>
#include <string>
#include <cstdlib>
#include <map>
using namespace std;
const int ARRAY_SIZE_DEFAULT = 32;
class vectorOfInt {
public:
[code]....
The size of c is 0. Values of a were not copied to c, although they appear to do so within the copy constructor routine.
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Jun 8, 2013
I'm working on trying to figure out constructors and header files. Can ya'll help me out with this? I'm sure my code looks like a mess as I tried to piece together different solutions I've found. There's also an attempted copy constructor and operator function. Basically my problem is my source file says there is no default constructor for my class type. Here's my header code:
#include <iostream>
#ifndef _car
#define _car
[Code].....
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Jan 30, 2014
I am currently trying to write a test program involving constructors. I know what I am about to ask is pretty basic stuff but Visual Studio is not recognizing when I declare strings for some reason. My code isn't completed yet but as I am typing string in to declare the variable in the class Visual Studio is not recognizing string as a usable value.
Code below:
#include <cassert>
#include <iostream>
#include <stdio.h>
#include <string>
using namespace std;
class college {
[Code] .....
Like I said... this is completely unfinished I just need to understand why my strings aren't being recognized.
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Oct 29, 2014
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.
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Dec 6, 2013
I have a class that extends another class, and I want multiple constructors in the child class, but the child constructor needs to call the parent constructor. This is what I have
In the child class:
ChildClass::ChildClass() {
ChildClass(1);
}
ChildClass::ChildClass(int i)
: ParentClass(i) {
// do stuff
}
In the parent class:
ParentClass::ParentClass(int i) {
// do stuff
}
In my main program:
ChildClass child1;
// do stuff with child1
// breaks
ChildClass child2(1);
// do stuff with child2
// works fine
Using the default constructor breaks my program at runtime, but using the one with a parameter works fine. The default constructor calls the other with the same thing as the main part in the program, so I would think this should make no difference, but obviously that isn't the case.
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Jan 7, 2013
I have a main .cpp file which contains int main(int argc, char** argv) and 2 included personal headers which correspond to their linked implementation files.
What I am trying to do is use ifstream to pass integer values from a text file into my program, have the program execute, and then output those modified integers back into the same text file. I have tested the "program" portion of my project using declared values. I know that my program does what I have intended for it to do, but I can't seem to properly inject any outside data using files.
Here's what I have (not including all of the unrelated objective code):
Main.cpp
int main(int argc, char** argv) {
file File;
std::string gameFile = File.findFile(); // prompts user for existing file
if(gameFile.size() > 0) { // load saved game
std::ifstream in;
in.open(gameFile.c_str());
[Code] .....
In game.cpp, I am unsure of Player = new player; and Player = new player(data);. I have never tried doing this. This is simply my best guess of how to make this mess work properly. Hopefully I am close?
Game.h:
#ifndef _GAME_H_
#define _GAME_H_
#include "player.h"
class game {
std::string loadedGame;
[Code] .....
In game.h, I haven't yet tried anything like player* Player;. As stated in my comments above for game.cpp, I have no clue how to go about this.
player.cpp:
#include <iostream>
#include <fstream>
#include <string>
#include <time.h>
#include "player.h"
player::player() {
srand(time(0));
[Code] .....
In player.cpp, I really have no clue where to start with the issue. I'm not even exactly sure if any values are being passed to these variables. Though, I honestly haven't taken much time to problem solve. I don't want to waste a lot of time just to find out that my attempt is incorrect and/or unconventional.
player.h
#ifndef _PLAYER_H_
#define _PLAYER_H_
class player {
public:
player(); // default
[Code] ....
So really, my concerns are: Is my attempt mostly correct? If not, why?
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Dec 26, 2014
I want to overload prefix and postfix increment(++) operators with friend function. I also have to use the constructors for this. How can I do this? in C++
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May 7, 2012
Previously I made a infix postfix calculator with integers only using structures, which had only three files
1.main.cpp,
2.arithmetic_expression.cpp
3.arithmetic expression.h,
I manged eventually to bring it together. now there are 5 files which are using classes, constructors and destructors and there can be letters in the expression that can have assigned values. Newnode, struct node and all that fine and dandy stuff made sense, this doesn't. What needs to be done in those empty functions in arethmetic_expression.cpp? It was going well until I reached createxpressiontree{ in th arithemetic_expression.cpp and those constructors in there. At that point I missed my structures and was totally confused by the constructors. I know constructors are supposed to create objects, but then I dont know what i am creating the object for and what that object is supposed to do.
1.main.cpp
2.arithmetic_expression.cpp
3.tree.cpp
4.arithmetic_expression.h
5.tree.h
Main.cpp
Code:
#include <iostream>
#include <string>
#include <vector>
#include "arithmetic_expression.h"
#include <map>
int main() {
// arithmetic_expression expression1;
// Testing the RPN input
[Code] .....
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Mar 30, 2013
I'm having some difficulties in understanding the topic which I stated above.
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