C++ :: Difference Between Iterator And Pointer
Jan 21, 2013
I just figured out that some std functions (for example: copy) need the resource and target objects have iterators to work. Otherwise, the compiler rejects. In case of two arrays, declared as:
double myA[100] = {};
array<double, 100> myB = {};
myA[0] is like a pointer, myB.begin() an iterator, if I do not make any mistake. So, what is exactly the difference between the pointer and the iterator here? They all give the access to elements.
If I need the target of copy to be an array like myA which cannot give an iterator, is there a way to make the command "copy" work for it?
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Apr 19, 2014
I'm making a system like twitter for class called ShoutOut.com I want to be able to get the PublicShoutOut pointer pointed to by the start iterator and assign it to firstShoutOutToDisplay and secondShoutOutToDisplay because I need that in order to pass the pointers to one of my functions. When I step through the debugger the values in start are all default values like "" and so are the values in this->firstShoutOutToDisplay but the message that start points to is being output just fine.
EDIT: got rid of irrelevant code. Am I using the correct syntax to do this?
if (start != finish) {
//getting these because a shoutout needs to be passed to the function that displays
//options for a shoutout
this->firstShoutoutToDisplay = (*start);
[Code] ....
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Nov 27, 2014
im trying to port a code from vc6 to vs2013 and im having this error
Code:
Error11error C2440: 'initializing' : cannot convert from 'char *' to 'std::_Vector_iterator<std::_Vector_val<std::_Simple_types<char>>>' on this line
Code:
vector<char>::iterator BufIt = (char*)lpBuffer;
what i do with this is to stack fragments of data of type char* coming from a socket in buffer to a vector that acts as buffer, I do this since I transfer big chunks of data and the data gets fragmented by the nature of the sockets, I stack the data once its complete I retrieve the final result from the vector.
this code worked flawlessly for long time but now Im trying to port and compiler throws this error, whats the new way to assign a char array pointer to a iterator so i can stack it in the vector.
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Jul 12, 2014
I've read about the difference between the two, what one can do that the other can't syntactially, but I'm still very confused by the concept of references in C++. I'm trying to understand what is the difference in terms of undelying implementation.
Take, for example, the following code:
void foo(int *bar) {
++*bar;
}
int main(void) {
int n = 0;
foo(&n);
return 0;
} Translated into: Code: __Z3fooPi:
[Code] ....
Considering the above assembly code, and the memory that was used in both cases, I'm tempted to say that references was added to C++ to make the synax looks prettier... Where would I be wrong?
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Jan 24, 2013
What is the difference in pass by pointer and pass by reference? As per my understanding, there is no difference much.If a function accepts pointer, then NULL check can be performed.other than this i'm not able to see any big difference..
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Aug 6, 2013
void func(float arg); // arg is supposed to be in type, with no referencing, just a copy of it
void func(float &arg); // arg is supposed to be in, out type, with something contained in it already
void func(float *arg); // arg is supposed to be in-out type, with nothing contained in it already and can be NULL
void func(const float& arg) // arg is supposed to be in type, with nothing contained in it already and cannot be NULL
Am I all correct?
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Mar 9, 2013
i'm still unclear between the difference between using pointer and a reference
I understood the concept of pointers in c in the class i took last year
and that was to change the actual value stored in the memory address Code:
void change_a(int a*){
a=6;
}
int main(){
int a=5;
change_a(&a);
}
but in c++ I've been using references in all my assignments because I don't know how to correctly use pointers in c++ I may have missed a class but I'm on spring break and would like to clear things up
so in c++
in my assignments I would call it like this Code:
void change_a(int &a){
a=6;
}
int main(){
int a=5;
change_a(a);
}
so does this change the value in the address or does it make another copy of a in my c++ code and stores 6 in that copy
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Aug 30, 2014
whats the difference between functions as pointer and normal function, eg:
void function1(void)
void *function1(void)
What is the difference between the two?I'm doing parallel programming and we use pointer functions (void *function1(void)) when calling threads. I want to why it is done.
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Jun 17, 2013
I'm making my first steps in STL, and I have a few question:
Is there a way to get an iterator to the i'th element in the collection (set or list), instead of just to the end or the begin?
And another question: Let's say I have an iterator, pointing to some element in my collection, and I use erase() (which takes as parameter an iterator that points to the soon-to-be erased element), what happens to that iterator? will it now point to NULL?
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Aug 20, 2014
I have this class with template:
template<template<class, class> class ContainerType>
class Movie {
public:
typedef ContainerType<Actor*, std::allocator<Actor*> > Container;
and i have a member func to find actor in movie according to actor id:
const Actor* findActor(int id) const {
typename Container::iterator it;
Actor* p(NULL);
it=actors.begin();
std::for_each(it,actors.end(),(*it)->getId()==id?p=*it:0);
if (p==*actors.end()) return NULL;
return p;
}
I receive error that no match for operator= in it.
How to declare correct iterator ?the type of actors is Container..
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Apr 8, 2014
Why this code works
Elenco e1;
e1.add(Persona("a","b"));
e1.add(Persona("c","d"));
e1.add(Persona("e","f"));
e1.add(Persona("e","f"));
e1.remove(2); //list of 4 elements
but this not work?
Elenco e1;
e1.add(Persona("a","b"));
e1.add(Persona("c","d"));
e1.add(Persona("e","f"));
e1.remove(2); //list of 3 elements
This is remove method:
Persona Elenco:: remove(int pos){
list<Persona> ::iterator iter=l.begin();
for(int i=0 ;i<pos;i++){
iter++;
}
return *(l.erase(iter)); //erase ritorna un iterator
}
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Nov 18, 2013
Here's a few parts of a program I'm working at. It does compile, and it does work as expected. Anyway Eclipse Kepler marks one line as a bug with the remark Field 'befehl' could not be resolved. The bug sign didn't show up when both classes were in one file.
ScriptInterpreter maintains and processes a vector of Objects, initialised with example data. An iterator of the vector keeps track of the current position while various methods process the data. I've copied the relevant lines only.
I can live with a few wrongly bug-marked lines in Eclipse. What I don't want is any hidden errors that express at some time later.
Is there anything wrong with the code? Anything that's not recommended and compiles anyway? Is anything c++11-specific about the questionable line?
AtomicCommand.h
class AtomicCommand {
public:
int befehl;
[Code] .....
Note that line 9 has a bug sign, too. Eclipse doesn't recognise all my c++11 code.
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Feb 22, 2013
I am running some simulation, in which i need to control the iterator of a multimap, like
multimap<double, int> mapp;
......
int n=6;
for(int i=0; i<5; i++) {
for(auto it = mapp.begin()+i*n; it!=mapp.begin()+(i+1)*n; it++) {
......
However this way of controlling is not possible for multimap. Why and how the make it work? Do I need to overload the operator +?
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May 24, 2013
I can't seem to make the STL iterator class work how I need it to.I am implementing a multi list graph and I need to iterate through my STL list of Vertex pointer objects. I keep getting the error:
Error 1 error C2679: binary '=' : no operator found which takes a right-hand operand of type 'std::_List_iterator<_Mylist>' (or there is no acceptable conversion) and the same for != for my terminating condition.
template <typename V, typename E>
void Graph<V, E>::InsertEdge(V from, V to, E edge) {
list<Vertex<V>*>::iterator iter;
for(iter = m_Vertices.begin(); iter != m_Vertices.end(); ++iter)
[code].....
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Jan 25, 2013
I want to get the iterator position after to use find if:
std::list<Texture*>::iterator result = find_if(
texturelist.begin(),
texturelist.end(),
std::bind2nd<CompareTEX>(CompareTEX(),n_tex));
if (result != texturelist.end()) {
return // position result
}
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May 25, 2013
I am receiving the error: Vector Iterator not incrementable. However, when erasing I'm already re-setting 'it' and pre-incrementing at the end of the while-clause.what's wrong?
vector<st>::iterator it = map[0][0].begin();
while(it != map[0][0].end()) {
if((*it).val == 5) {
[Code] .....
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Apr 10, 2014
We were given a task to use lists and iterators. We were supposed to make them from scratch. I'm done with that. The problems that I'm having are as following:
1. I'm not able to access the list made of Course datatype which is present in each Student instance. Does this mean I need to make an iterator for that course list inside the student class?
2. Similarly since I don't have direct access to The course list so I added the course into the Student list through the student objects not through the iterator. How can I do it through the iterator?
3. Printing of a particular student and his courses is not happening as my iterator made for student only prints out the students, not the courses present in their courselist. How to do that?
Here's the code
#include <iostream>
#include <string>
using namespace std;
const int ILLEGAL_SIZE = 1;
const int OUT_OF_MEMORY = 2;
const int NO_SPACE = 3;
const int ILLEGAL_INDEX = 4;
[Code] .....
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Jan 10, 2013
I'm trying out the code below; it runs and produces the output I'd expect, but could it lead to undefined behavior?
To test with, I show the key and wait for user input; if the input doesn't match the value, the iterator is stored in the vector.
map<string, string> test;
map<string, string>::iterator it;
vector<map<string, string>::iterator> bad_input;
string input;
test["key 1"] = "value 1";
[Code] ....
I am also interested in knowing if I can use an iterator to walk through the vector? I tried
vector<map<string, string>::iterator>::iterator v_it;
for(v_it = bad_input.begin(); v_it != bad_input.end(); ++v_it)
cout << v_it-> ??
However, I couldn't figure out how to access "first" and "second" using this method. Is it possible to do so?
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Jan 5, 2015
I have created a multimap for my road points. The key refers to the road number and the values are vec3 points that make up the road.
I am trying to iterate through the values of each key point and create a road segment at each point on the road (except the last), adjust the values to be on the road points and then store them in a std::vector.
The RoadSegment constructor creates 6 vec3 points and pushes them onto a std::vector.
I have a segmentation fault in the line marked in bold
[for(mapIt = it.first; mapIt != it.second; ++mapIt)]
When i take out the lines creating the new objects and pushing them onto the std::vector it works fine.
std::vector<glm::vec3>::iterator SegIt;
for(int i = 0; i < m_genRoads->getKeyValueData().size(); i++)
{
int numberDesired = m_genRoads->getMultimapData().count(i) - 1;
[Code]...
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Mar 6, 2013
I have a list and a vector, for example like
list<int> mylist = {1, 1, 1, 0, 0, 1, 0, 0}
vector<int> myvec;
I want to create an efficient algorithm to erase all element of value "1", and give these elements to a vector. My code (not efficient) is as follows
for(auto it = mylist.begin(); it != mylist.end(); it++) {
if(*it == 1) {
myvec.push_back(*it);
it = mylist.erase(it);
if(it != mylist.begin())
it--;
}
}
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Sep 4, 2013
I defined the following function to find out the iterator of a certain value in the vector. I defined it as such so if the value exist in the vector then return a iterator of it, if not then return a pointer pointing to nonsense.:
vector<tIndex_t>::iterator unlabelTit(const tIndex_t index) {
for(vector<tIndex_t>::iterator it=unlabelT.begin(); it<unlabelT.end(); it++) {
if(index==*it) return it;
} return NULL;
}
But looks this one is not accepted by compiler, saying I cannot do this kind of comparison like:
unlabelTit(i)!=NULL;
so I just delete the return NULL; and then the compiler giving me warning about no return statement before }.
a pointer pointing to nonsense? how to do that?
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Nov 18, 2013
I'm working on a program where I have a vector full of <myClassType> structs.
I'm trying to insert items into a vector, searching first through the vector to make sure the value isn't already in the vector before inserting it. The "find" function isn't working properly.
I keep getting C2678 "binary '==': no operator found which takes a left-hand operand of type "myClassType" or there is no conversion errors in Visual Studio 2010.
I know it's something having to do with the find function and my iterators, but I can't, for the life of me, figure out what it is.
I've tried switching to const_iterators, but I get the same error message.
void foo::insert(const myClassType& en) {
vector<myClassType>::iterator it;
vector<myClassType>::iterator it1;
it = find(Table.begin(), Table.end(), en.memberOne);
[Code] .....
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Jun 24, 2014
I'm trying to make a function to show the content of maps as follows:
template<typename A, typename B>
void show_map(const std::map<A,B> &src) {
for( std::map<A,B>::iterator it=src.begin(); it!=src.end(); ++it ) {
cout << it->first << " ____ " << it->second << endl;
}
};
However, I got this error in the 'for' line: "error: expected ‘;’ before ‘it’" (and then errors that it is not declared, etc). I'm sure that there is no missing ; in other functions. If you try to write a random line before that you get no error there for example, always in the for.
It is there something that you need to do with iterators when you declare generic datatypes?
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Oct 15, 2014
I got the core dump file so I know exactly which line it crashes. Below are some of the information:
OS: Red Hat Enterprise 5.8
Example snippets of when the crash happen:
Code:
typedef map<int, MyClassObj> MyMapT;
.
.
void someFunction( MyMapT& inMap )
{
for ( MyMapT::iterator iter=inMap.begin(); iter!=inMap.end(); ++iter )
{
.
.
}
}
Using GDB, I am able to find out that it crashes at the "++iter" as the .h file indicate it was a "++" operation for the iterator. Tracing up the stack frame it indicate it crash during the copy constructor of some "__rb_tree_node". I did some Googling and it seems that is some Red-Black tree implementation for the map. Honestly I do not quite understand the Red-Black tree and I believe STL map is a very very well tested container, so the problem must lie in my code so that I can look out for it.
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Feb 7, 2012
Code:
it = m_CoopTable->m_SparseMap.find(s);
if (it != NULL) //Error
{
return false;
}
This gives me compile-time error. it is an iterator to a hash_map
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May 22, 2014
In order to parse mathematical expressions I am trying regular expressions and a recursive algorithm, but I have a problem with the four basic operations: +, -, *, /.
Trying to analyze a string like "a+(b+c)", if I use the pattern for a sum "(.+)+(.+)" the program matches it recognizing as subpatterns: "a+(b" and "c". How could I achieve the program to try also the other possibility?
I think that it would be great something like an regex_iterator which worked with regex_match instead of regex_search. I mean, an iterator that iterates over all the possible ways to match a given regular expression and a given string. This way I could loop through all these possibilities until the two subpatterns produced were correct mathematical expressions.
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