C++ :: What Are Rvalue References And Temporary Objects
Dec 30, 2014What rvalue references are? How are they useful? What are temporary objects?
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C++ :: RValue - References As Return Values Of Functions
Aug 13, 2014I am trying to understand RValue-references as return values of functions. First let's consider a simple function, that transforms a string into upper case letters.
const std::string
toUpper(std::string orig) {
std::transform(orig.begin(), orig.end(), orig.begin(), ::toupper);
return orig;
[Code] .....
It compiles, but I get the output 0 . Here I am wondering why the code above does not move the substr correctly while the code below does (prints out 1):
const std::string&&
no_sense(std::string abc) {
abc = abc.substr(1, 1);
return std::move(abc);
[Code] .....
In both cases abc is a temporary object inside of the function and gets deleted after the function is left. But why does the second version work and the first one does not?
cat.substr(1, 1)
And as my last question. Why doesn't
return std::move(abc.substr(1, 1));
work?
C++ :: When Are Rvalue References To Primitive Integers Short-lived Or Long-lived
Sep 4, 2012I have done some experimentation on rvalue references with the TDM-GCC 4.6.1 compiler and made some interesting observations that I cannot explain away with theories. I have a very simple program that does not deal with objects but int primitives and that has defined 2 functions:
foo1 (returning a local variable by rvalue reference) and
foo2 (returning a local variable by value)
#include <iostream>
using namespace std;
int &&foo1();
int foo2();
int main() {
int&& variable1 = foo1();
[Code] .....
It seems the value returned by foo1 and received by variable1 dies out after some time - perhaps, a brief period of some milliseconds. Notice that I have prevented cout from printing "My name is softwarelover" by commenting it out. If I allow that statement to run, the result is different. Instead of printing 5, 0 it prints 0, 0. Seems like it is because of the time-delay introduced by "cout << "My name is softwarelover." that 5 turns into 0.
Is the above how an rvalue reference is supposed to behave when referring to a primitive integer which a function returned by reference as opposed to return-by-value? By the way, why is it 0, why not garbage?
Notice also that variable2 never seems to die out, no matter how many times I print it with cout! variable2 refers to a primitive integer which a function returned by value, not return-by-reference.
C/C++ :: Objects Hold References To Other Objects?
Nov 12, 2014This has been bothering me for a while now, and I finally put together an example:
#include <iostream>
#include <string>
using namespace::std;
[Code]....
In the code above, the two classes hold pointers to each other, and that's fine but it doesn't seem right since C++ prefers to pass by reference. Yes, it can still do that (see testbox and testball) but even that seems odd to me because still you need to use pointer notation for the enclosed object. Am I the only one who feels this way, and should I just get over it? Or am I missing something that would allow an object to hold a reference?
C++ :: Testing For References To Identical Objects
Jun 18, 2014I am checking to see if two references are bound to the same object. My instincts tell Me, "Check their addresses. If they match, they are bound to the same." At the same time, I have not found anything in the C++ standard which would support this approach. Am I missing something? Is there wording which backs up My instincts? Is there a standard function to do this?
View 4 Replies View RelatedC/C++ :: Creating A Temporary Vector Of Pointers?
Sep 10, 2014Is it possible to create a temporary
std::list of pointers
I would like to pass a temporary
std::list
to the constructor of a class to initialize its own one.
For example, using a
std::vector
:
#include <iostream>
#include <vector>
void func(const std::vector<int*>& myVec) {
for(int i=0; i<myVec.size(); ++i){
[code]....
Can we do this? What are other possible problems in addition the ones I have just mentioned above?
C++ :: Lvalue Reference Bind To A Rvalue Object?
Sep 10, 2014Trying to understand the lvalue and rvalue references, and come up with some strange codes, which compile and run, but is confusing to understand.
Define this class Thing:
class Thing {
public:
Thing(int k = 0): i(k) {};
~Thing() { std::cout << "destroying Thing (i=" << i << ")" << std::endl; }
int getValue() const { return i; };
Thing &getMe() { return *this; };
private:
int i;
};
and a non-member function:
Thing construct_Thing10() {
Thing t(10);
return t;
}
Then these two lines in main():
Thing &thg=construct_Thing10().getMe();
std::cout << "member=" << thg.getValue() << std::endl;
The output is:
destroying Thing (i=10)
member=10
My understanding is that the rhs of line 1 construct only a temporary object. getMe() then return the reference of this temp object and bind it to thg (as a lvalue reference). After line one, the temp object is really destroyed (hence the first output line). At this point thg is really binding to a destroyed, invalid object. But somehow the 2nd line still prints the correct value of 10 is because the memory storage is not yet corrupted (still holding the previous value). Is this correct?
C++ :: What Can't Non-const Object Receive Temporary Object
Sep 11, 2014a function returns a temporary object like
int myfun(){
int x = 0;
return x;
}
this function will return a temporary integer now void fun1(const int & num); this function can receive from myfun().BUT void fun2(int & num); this function cannot receive from myfun() Why is that, Moreover what is lifetime of a temporary object like one returned in myfun() ???
C++ :: Scope Of References?
Apr 22, 2013Did a little Googling on this but couldn't find anything definitive. Is it safe to do something like
Code:
void MyClass::myFunc(){
my_type_t &foo = some_obj->get_member_reference();
store_for_later(&foo);
}
Then at some pointer later in execution, another function uses the pointer passed to store_for_later.
C++ :: Swap Between Two References?
May 28, 2013I have the following code segment:
Code:
void Swap(Number& num1, Number& num2)
{
cout<<"Before swap:"<<num1<<" "<<num2<<endl;
Number& temp=num1;
num1=num2;
num2=temp;
cout<<"After swap:"<<num1<<" "<<num2<<endl;
}
[code]...
to which the output is:
Code:
Before swap:13 11
After swap:13 11
13 11 that seems confusing.
why doesn't Swap() swap the two Numbers?
C# :: Programmatically Add Dll References?
Jan 11, 2014I am trying to make a utility program for work that will update multiple projects with local dll references. Basically I work with two solutions (for talk sake solutIon1 and solutIon2). Generally solutIon1 will reference the dll's built In solutIon2 which reside on a server. However for debugging proposes I sometimes need to D/L the solutIon2 projects and build them local-ally, so that I can reference the solutIon2 dll's local-ally (this Is so that I can easily attach the dll and step Into the code). However this require changing the reference paths, so that I am pointing to the local-ally built dll's, which Is quite a laborious task.
So the question is how would I update references in solution1 from the program that I am making. I don't really know what to start reading about as I have never done anything like this before.
C++ :: Vector Of Pairs Of References?
Jun 19, 2014I am attempting to combine two vectors into a vector of pairs. I want to be able to alter the first and second of each pair and have those alterations reflected in the original vectors. I thought the following code might work but get compilation errors about a lack of viable overload for "=" for the line with the call to std::transform:
void f()
{
std::vector<int> a = {1,2,3,4,5};
std::vector<int> b = {6,7,8,9,0};
[Code].....
C++ :: Declaring A Valarray Of References?
Sep 26, 2014Is it permissible to declare, for example, `std::valarray<int&>`? If so, how do I initialize such if the `valarray` is a class member?
View 3 Replies View RelatedC++ :: Parameter Pack Of Constant References
Aug 20, 2013I have this basic prototype:
struct int_wrapper{int i;};
template <const int_wrapper&... IPack>
void display_all(const int_wrapper&, IPack...);
But when I try to compile it, the compiler says IPack is not a type on the last line. Are packs of references not allowed?
C# :: Move Class To Its Own Project And Keep Its References?
Apr 3, 2014I was wondering if there is way to convert a C# class to its own project and it automatically keeps its references.
View 3 Replies View RelatedC++ :: Range-based Looping Over Container Of References?
Mar 6, 2015Is it possible to create a class that stores (non-const) references to some objects and enables users direct access by using range-based for loops on them?
Code: class container {
public:
void add(int& value);
void remove(int& value);
...
};
int main()
{
container c;
for (auto& value:c) {
// `value' should be accessible as type `int&' instead of being a pointer, `std::reference_wrapper<int>' or something like that
}
}
C++ :: Passing Variadic References To Template Function?
Dec 14, 2014I'm having some problems in understanding how the code below works and why it produces the output it produces.. What I'd expect is that both functions, namely `add_1' and `add_2', would print the same output; but I've been proven wrong :/ So why does the second one get different memory addresses for the same variable?
Code should be self-explaining:
Code: template<typename... Types>
void add_1(Types&&... values)
{
// by the way: why do i have to use `const int' instead of `int'?
std::vector<std::reference_wrapper<const int>> vector{
std::forward<Types>(values)...};
std::cout << "add_1:" << std::endl;
for (const auto& value:vector) {
std::cout << &value.get() << std::endl;
[code].....
C++ :: How To Hold Pointers / References To Abstract Class
Nov 15, 2014I have an abstract class named Terrain, and a class named RoadMap, which supposed to hold an N*N array of Terrains. But I'm not sure what type should the RoadMap class hold:
Code:
#ifndef TERRAIN_H
#define TERRAIN_H
class Terrain {
[Code] ....
I can't use an array of refernces here, so I tried this:
Code: Terrain** terrain; and then I thought this was the way to go:
Code: Terrain (*terrain)[]; But now I'm not sure.
The N*N matrix size supposed to be determined according to a given input... What type should I use there?
C++ :: 2 Errors In Passing Values / References To Function
Jan 28, 2014#include "stdafx.h"
#include <iostream>
#include <math.h>
#include <time.h>
#include<iomanip>
#include<array>
#include <algorithm>
using namespace std;
const int AS = 6;
void FillingRandomly(int (*)[AS]);
void printing(int (*)[AS]);
[Code] ....
Basically I have to create an array, fill it, and then print it on screen. The tricky thing is that need to use pointers to fill it and print and later on sort it. My problem is that with this code is that i get
Error2error C2109: subscript requires array or pointer typec:userspcdesktopusbanthonydocumentsvisual studio 2012projectsessaieessaieessaie.cpp55
and
5IntelliSense: expression must have pointer-to-object typec:UserspcDesktopUSBAnthonyDocumentsVisual Studio 2012ProjectsEssaieEssaieEssaie.cpp55
Whenever I try to run it.
C++ :: Why Cannot Dynamic Memory Allocation Work With References
May 5, 2013Why cant a dynamic memory allocation work with references? I was told that references work with const pointers deep down so shouldn't this be legal code?
int &&a=new int;
My compiler says that a entity of int* cannot be used to initialize a entity of int&&?
Does that mean that the compiler thinks of them as different types except deep down a reference is implemented with a pointer? Is this right?
C# :: Cyclic References In A Database Access Layer
Jan 21, 2015I'm looking to implement a Database Access Layer for the project I'm working on, it's a mature project and I'm trying to simplify the database access and as far as possible and remove the Database logic from the Business logic.
Bringing in an ORM solution isn't an option at the moment so I'm looking at bringing in DAO objects to break the coupling. The problem I can't get around in my head is how to avoid Cyclic references
We currently have 2 projects
BL contains types such as Customer, Component and Product which need saving to the Database, the Database project can't know about these items or it would create the cyclic dependency.
I tried adding Dao items to the DB project to mirror these items and to also mirror the DB structure but that requires that the BL project knows how to convert between it's own types and the DAO types which is something I'd like to avoid.
I also tried inserting a third intermediate project that would control the conversion and saving, I called it my DAL project and tried adding functions that would take the BL item and perform CRUD operations but again I ran into the cyclic dependency issue.
My ideal solution would be that the BL project would just have to call a function along the lines of "SaveCustomer(Customer inCustomer)" and not have to worry about doing any conversion.
Is there a project structure that would allow for this?
C++ :: Cloning Object - Store Pointers Instead Of References?
Apr 8, 2013Is there a point in dynamically creating a pointer during runtime only to dereference it? (if that is the right term, as in *pointer, depoint it?)
In this case should I perhaps store pointers instead of references?
Inventory.cpp
Code:
bool Inventory::addItem(InventoryItem& item) {
addItemAmount(item);
if (item.getAmount() > 0) {
if (hasEmptySlot()) {
addNewItem(*item.clone());
return true;
[Code] ....
Also I was wondering, is there some sort of built-in cloning functionality or do I have to write the clone functions myself? When creating new instances I see that I can either pass the constructor properties or a reference to an object of the same type.
For instance:
Code:
new InventoryItem(index, name....);
new InventoryItem(const InventoryItem&);
Is the second one casting?
Visual C++ :: No Circular References But Undeclared Identifier
Jun 11, 2014Each of my header includes is protected by directives. I think I don't have to include Boolean in my work space because it is already included in the external dependencies section. and the Boolean.h is in the include path.
MachineShop, Boolean etc got undeclared identifier error
Tried to comment out the directives, to no avail.
Code:
#include <iostream>
#include <string.h>
#ifndef BOOLEAN_H_
# include "Boolean.h"
#endif
#ifndef PROCESS_H_
# include "Process.h"
#endif
#ifndef MACHINESHOP_H_
[Code] ....
C++ :: Data Type Size - Performance With References
Aug 16, 2012Here's what I'm trying to do : A simple readout that shows the input/feedback values for 10 different sensors (i.e. a motor, a thermocouple, light sensor, etc).
What I got so far:
The data is stored in 2 different arrays:
One array is a 2D string array that stores descriptions, and won't be changed:
Sensor ID, Sensor Type, Input Signal, Feedback Signal
["A"]["Motor"]["PWM Signal"]["RPM"]
["B"]["Thermocouple"]["N/A"]["TempC"]
etc
The second array is another 2D int array that stores all the data values:
Input Signal, Feedback Signal
[0][0] // for Sensor A, Input is 0 PWM, 0 RPM read from sensor
[0][25] // for Sensor B, Input is 0, 25C read from sensor
etc
My question: I'd like to re-write the code to incorporate the new things I learned in c++. Right now, the descriptions for all 10 sensors are in 1 array and the sensor values are in another array. If I use pointers to access the values, is there a performance difference between:
1. Keeping it as is, with 2 2d arrays
2. 1 big structure that has descriptions and sensor values for all 10 sensors (i.e. combining everything into 1)
3. 1 parent class, and 10 different objects for each sensor (i.e. splitting into 10)
C++ :: attraction Between Objects
Apr 2, 2013i am relatively new to c++, and am trying to make it so particles will gravitationally attract to the mouse pointer. i have gotten all the bugs that the compiler found, but there must be another because the program crashes the second i open it. my code could be way off, so just tell me if it is and ill do more research and learning before trying something like this again. here is the code:
#include <SDL/SDL.h>
#include <iostream>
#include <vector>
#include <math.h>
#include <stdio.h>
using namespace std;
float mouseX;
float mouseY;
[Code] .....
C++ :: The Proper Way To Do A Vector Of Objects
Oct 20, 2014I've been really busy but managed to get in enough down time to learn somewhat decent info about vectors. Anyways originally my program created a dynamic array of pointers to class objects and I came across a few problems because of this. Apparently an array of pointers is now outta of the question and I will now be switching to a vector of objects instead.
Why I want a list of objects instead of pointers this little comment should clear things up.
tiles[i]->show() dereferences tiles[i] (i.e. accesses whatever it points at) before calling the show() method.
That is undefined behaviour. Once undefined behaviour occurs, there is no recovery, and there is nothing the show() method (or any subsequently called function for that matter) can do to recover (short of invoking their own forms of undefined behaviour - compiler specific hacks, etc).
Even if the show() method initialises the object correctly, it cannot change the pointer tiles[i] which is in a different scope (within main()).
What I'm trying to do is create a vector of already intialized objects so that I can use a conditional statement of every single element to properly layer my games art resources. This should also automatically fix a mild unrelated collision dectection problem too but first thing first layering.