class, struct, vector, map

Question

class, struct, vector, map

Markus Freitag 3,791

Hello,

class PanelData  
{  
	 // **   
	 CString Price;  
	 CString IdentNo;  
  
	 bool    PanelReported;  
};  
vector<PanelData>      m_dequePanelData;  
  
// ******** OR *******  
  
struct PanelData  
{  
	 // **   
	 CString Price;  
	 CString IdentNo;  
  
	 bool    PanelReported;  
  
};  
vector<PanelData>      m_dequePanelData;

If I have a vector a map, and create the elements with class, is that by reference or by value?
Which is generally preferable?

David Lowndes 4,726 Reputation points

2022-10-14T08:43:16.01+00:00

Your question isn't clear to me.
If you're wondering whether there's some similar reference/value difference between class/struct as in C#, then no, the only difference between class/struct that you have above is that for class, the members aren't public as they are for a struct.

Accepted answer

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Your answer

David Lowndes 4,726 Reputation points

2022-10-14T08:43:16.01+00:00

Your question isn't clear to me.
If you're wondering whether there's some similar reference/value difference between class/struct as in C#, then no, the only difference between class/struct that you have above is that for class, the members aren't public as they are for a struct.

Answer 1

YujianYao-MSFT 4,296 Microsoft External Staff

Hi @Markus Freitag ,

There are several ways to initialize the map:

map<string, int> m1;  
m1[string("abc")] = 1;  
m1[string("defg")] = 2;  
   
  
map<string, int> m2;  
m2.insert({ string("abc"), 1 });  
m2.insert(make_pair(string("defg"), 2));  
m2.insert(pair<string, int>(string("hijk"), 3));

References are often used for function parameter lists and function return values.

Best regards,

Elya

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Markus Freitag 3,791 Reputation points

2022-10-19T05:24:01.743+00:00
Thanks for your answer!

map<string, PanelData> m1; PanelData pD; pD.IdentNo = "20221019-0127"; m1[string("abc")] = pD; // I think it is by value. // How is looking for by reference?

Can you post an example with a struct as value?

By reference.
I change the pD.IdentNo = "20221019-0127--NEW";
In my map is at moment the old value. Sometimes good, sometimes bad.

YujianYao-MSFT 4,296 Microsoft External Staff

Hi @Markus Freitag ,

I wrote a sample to show you how to use reference, please feel free to contact me if you have any questions.

#include<iostream>  
#include<map>  
#include<string>  
using namespace std;  
class PanelData {  
  // **  
 public:  
  
  string Price;  
  string IdentNo;  
  
  bool PanelReported;  
};  
void ins(map<string, PanelData> &m1) {  
  PanelData pD;  
  pD.IdentNo = "20221019-0127";  
  m1[string("abc")] = pD;  
  
}  
  
int main() {  
  map<string, PanelData> m1;  
  ins(m1);  
  map<string, PanelData>::iterator iter;  
  for (iter = m1.begin(); iter != m1.end(); iter++)  
    cout << iter->first << endl << iter->second.IdentNo << endl;   
  return 0;  
}

Best regards,

Elya

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Markus Freitag 3,791 Reputation points

2022-10-19T18:52:18.46+00:00
please feel free to contact me if you have any questions.

First, thanks! I have a small question.

abc 20221019-0127 abd 20221019-0128 abe 20221019-0129 abf 20221019-0130 abg 20221019-0131

I fill that in the list, in the map.

After I put the last one in the map I change the id.
a) change should also be in the map too, by reference
b) not by value.
Can you show both variants? Would be great.

YujianYao-MSFT 4,296 Microsoft External Staff

Hi @Markus Freitag ,

I wrote the ModifyMap() function, which can change the value of the map according to the key value, hope it helps.

void ModifyMap(string key, string value, map<string, PanelData> &m1) {  
  map<string, PanelData>::iterator it;  
  std::map<int, string> m2;  
  it = m1.find(key);  
    it->second.IdentNo = value;  
    
}  
int main() {  
  map<string, PanelData> m1;  
  ins(m1);  
  map<string, PanelData>::iterator iter;  
  ModifyMap("abc", "20221019 - 0128", m1);  
  for (iter = m1.begin(); iter != m1.end(); iter++)  
    cout << iter->first << endl << iter->second.IdentNo << endl;   
  return 0;  
}

Markus Freitag 3,791 Reputation points

2022-10-20T05:26:48.497+00:00
Hi @YujianYao-MSFT ,

Thanks for the answer.
I want to see the difference between By Value and By Reference.
Hope you understand my goal. Maybe you can test with a MFC dialog application.

Please look here.

//Header file MFC dialog application class CMFCTestDlg : public CDialogEx { public: void InsertTest(map<string, PanelData> &m1); void ModifyMap(string key, string value, map<string, PanelData> &m1); CPP

Thanks for your help!

YujianYao-MSFT 4,296 Microsoft External Staff

Hi @Markus Freitag ,

I'm sorry I didn't understand your idea at first.

I wrote a function that doesn't use a reference, so ModifyMapNoReference() doesn't modify the value of pD.IdentNo. If you want to modify variables through formal parameters, you need to use reference parameters.

void ModifyMapNoReference(string key, string value, map<string, PanelData> m1) {  
  map<string, PanelData>::iterator it;  
  std::map<int, string> m2;  
  it = m1.find(key);  
  it->second.IdentNo = value;  
}  
int main() {  
  map<string, PanelData> m1;  
  ins(m1);  
  map<string, PanelData>::iterator iter;  
  ModifyMapNoReference("abc", "20221019 - 0128", m1);//20221019 - 0128 does not modify the value of pD.IdentNo  
  for (iter = m1.begin(); iter != m1.end(); iter++)  
    cout << iter->first << endl << iter->second.IdentNo << endl;   
  ModifyMap("abc", "20221019 - 0128", m1);//20221019 - 0128 modify the value of pD.IdentNo  
  for (iter = m1.begin(); iter != m1.end(); iter++)  
    cout << iter->first << endl << iter->second.IdentNo << endl;   
  return 0;  
}

We can see that after calling ModifyMapNoReference(), the value of map is not really changed.

Markus Freitag 3,791 Reputation points

2022-10-22T06:34:25.497+00:00
Hello Elya,

Thanks for the sample.

The goal is by reference.
Maybe we have a misunderstanding.
I explain as I understand by reference.

abc 20221019-0127 abd 20221019-0128 abe 20221019-0129 abf 20221019-0130 abg 20221019-0131

abf 20221019-0130 is inside the map --> IO

In my main function I now change the value to

Change to-20221019-0130-NEW

.
Now I automatically guess the changed value in the map.
I hope you understood my goal and can publish a small example.
I think it has to be solved with & or pointer *.
What is the exact syntax? Thanks in advance.

I hope you understood what I'm looking for. About a short feedback, thanks of course.

YujianYao-MSFT 4,296 Microsoft External Staff

Hi @Markus Freitag ，

Is this the sample you need? Hope it helps.

  //ONE  
  map<string, string> *m1;  
  map<string, string> m2;  
  m2.insert(std::make_pair("abc", "20221019-0127"));  
  m1 = &m2;  
  (*m1)["abc"] = "change to 20221019 - 0127 ";  
  
  //TWO  
  string *a=new string;  
  string b = "20221019-0127";  
  a = &b;  
  map<string, string*> m3;  
  m3.insert(std::make_pair("abc", a));  
  b = "change to 20221019 - 0127 ";  
  delete a;

Markus Freitag 3,791

That looks good now. Thanks for your help!

delete a;

Only when I call the delete, there is an exception. Do you have an explanation?

//ONE  
map<string, string> *m11;  
map<string, string> m22;  
m22.insert(std::make_pair("abc", "20221019-0127"));  
m22.insert(std::make_pair("abd", "20221019-0128"));  
m22.insert(std::make_pair("abe", "20221019-0129"));  
m11 = &m22;  
(*m11)["abc"] = "change to 20221019 - 0127 ";  
  
//TWO  
string *a = new string;  
string b = "20221019-0127";  
a = &b;  
map<string, string*> m33;  
m33.insert(std::make_pair("abc", a));  
b = "change to 20221019 - 0127 ";  
delete a;

David Lowndes 4,726 Reputation points

2022-10-24T19:05:55.34+00:00
Let's analyse what you've written:

string *a = new string;

a is a pointer to a string you've allocated on the heap.

string b = "20221019-0127"; a = &b;

b is a string, and a now points to b, so you've lost the string object you allocated on the heap.

b = "change to 20221019 - 0127 "; delete a;

You've tried to delete a which is pointing to b which wasn't allocated by your code, so things go bang.

Generally with modern C++, avoid writing new/delete yourself - there really is little need for them.

Markus Freitag 3,791

You've tried to delete a which is pointing to b which wasn't allocated by your code, so things go bang.

Generally with modern C++, avoid writing new/delete yourself - there really is little need for them.

Very good analysis, thank you.

Yes, but what is the solution?

How would you implement it?

Ups you mean only with new. Not use new for string, right?

What you think about solution number 4?
I think the best way, isn't?

My summary.

//THREE solution  
string *a2 = NULL; //new string;  
string b2 = "20221019-0127";  
a2 = &b2;  
map<string, string*> m333;  
m333.insert(std::make_pair("abc", a2));  
b2 = "change to 20221019 - 0127 ";  

// _T for UniCode  C2665    "AfxMessageBox": TCHAR Typ ist Unicode (wchar_t*) oder MBCS (char*)   
AfxMessageBox(_T("Ready"));  

// The fourth solution.  
string b3 = "20221019-0127";  
map<string, string*> m3333;  
m3333.insert(std::make_pair("abc", &b3));  
b3 = "change to 20221019 - 0127 ";  

// _T for UniCode  C2665    "AfxMessageBox": TCHAR Typ ist Unicode (wchar_t*) oder MBCS (char*)   
AfxMessageBox(_T("Ready 2"));

David Lowndes 4,726 Reputation points

2022-10-25T21:44:41.1+00:00

I can't tell you what to do for the best because I don't know precisely what you intend in the wider sense. A few snippets are clearly not representative of a real application.

However, one thing that may be wise, would be to define your map as having a reference to a string rather than a pointer - the key difference is that with a reference (which is essentially a pointer) you can't (unless you try hard) have a potential null as you can with a pointer.

I'd also question why you're storing references/pointers too - you run the risk of the reference becoming invalid if the thing that's being referenced has a lesser lifetime scope than your map object. Do you perhaps need a reference to the string contained in the map rather than the other way round that you've currently got it? Ultimately, I don't know!
YujianYao-MSFT 4,296 Reputation points Microsoft External Staff

2022-10-26T05:47:07.43+00:00

Hi @Markus Freitag ，

Generally speaking, an issue only focuses on one problem. At present, the center of this issue has been changed from map to pointer. If the original question has already been resolved, I suggest you mark this reply as an answer and reopen an issue.
Markus Freitag 3,791 Reputation points

2022-10-26T18:48:42.52+00:00

Generally speaking, an issue only focuses on one problem. At present, the center of this issue has been changed from map to pointer.
If the original question has already been resolved, I suggest you mark this reply as an answer and reopen an issue.

you are right.

Answer 2

Guido Franzke 2,191

Hello,
in the class your members are private. In the struct, your members are public. That's all.
What do you mean with "by reference" or "by value" in a map or vector? The elements are totally stored in both containers.
Regards, Guido

Markus Freitag 3,791 Reputation points

2022-10-14T18:25:40.957+00:00
What do you mean with "by reference" or "by value" in a map or vector? The elements are totally stored in both containers.

Yes for map and vector.
In C# all is by reference, easy. In C++ is that difficult.

How would you fill a map?

by reference

by value std::vector<PanelData> result;
std::map<CString, PanelData> dictKeyValue;

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