"auto" declaration issue in C++

CFLam (45)
Hello all,
I have a problem in using "auto" declaration. I write a program in Visual Studio 2017 as follow:
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#include "stdafx.h"
#include <stdio.h>
#include <iostream>
#include <vector>

using namespace std;

class MyClass
{
public:
	struct mystruct {
		vector<int> vi;
	};
	vector<mystruct> TheStructV;
	void AddStructV() {
		TheStructV.push_back(mystruct());
	};
};

int main()
{

	MyClass MyObj[3];
	for (int a = 0; a < 3; a++) {
		MyObj[a].AddStructV();
		for (int i = 1; i <= 5; i++) {
			MyObj[a].TheStructV[MyObj[a].TheStructV.size() - 1].vi.push_back(i * 10 + idx);
		}
		idx++;
	}
	for (int b = 0; b<3; b++) {
		cout << "MyObj[" << b  << "] struct vector size:" << MyObj[b].TheStructV.size() << endl;
		cout << "MyObj[" << b  << "] struct vi size:" << MyObj[b].TheStructV[0].vi.size() << endl;
	}
	for (int i = 0; i < 3; i++) {
		cout << "MyObj[" << i  << "].vi:";
		for (int j = 0; j < 5; j++) {
			cout << MyObj[i].TheStructV[0].vi[j] << "-";
		}
		cout << endl;
	}
	return 0;
}

It works as expected and the output is:

MyObj[0] struct vector size:1
MyObj[0] struct vi size:5
MyObj[1] struct vector size:1
MyObj[1] struct vi size:5
MyObj[2] struct vector size:1
MyObj[2] struct vi size:5
MyObj[0].vi:11-21-31-41-51-
MyObj[1].vi:12-22-32-42-52-
MyObj[2].vi:13-23-33-43-53-

However, if I change to:
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#include "stdafx.h"
#include <stdio.h>
#include <iostream>
#include <vector>

using namespace std;

class MyClass
{
public:
	struct mystruct {
		vector<int> vi;
	};
	vector<mystruct> TheStructV;
	void AddStructV() {
		TheStructV.push_back(mystruct());
	};
};
int main()
{

	MyClass MyObj[3];

	int idx = 1;
	for (auto M : MyObj) {
		M.AddStructV();
		for (int i = 1; i <= 5; i++) {
			M.TheStructV[M.TheStructV.size() - 1].vi.push_back(i * 10 + idx);
		}
		idx++;
	}
	for (int b = 0; b<3; b++) {
		cout << "MyObj[" << b << "] struct vector size:" << MyObj[b].TheStructV.size() << endl;
		cout << "MyObj[" << b << "] struct vi size:" << MyObj[b].TheStructV[0].vi.size() << endl;
	}
	idx = 1;
	for (auto MC : MyObj) {
		cout << "MyObj[" << idx - 1 << "].vi:";
		for (auto thisStruct : MC.TheStructV) {
			cout << thisStruct.vi[0] << "-";
			cout << thisStruct.vi[1] << "-";
			cout << thisStruct.vi[2] << "-";
			cout << thisStruct.vi[3] << "-";
			cout << thisStruct.vi[4] << "-";
		}
		cout << endl;
		idx++;
	}

	return 0;
}


It compile without problem but got error when run and the output is:

MyObj[0] struct vector size:0

Program stuck here.
Seems that I am missing something, anyone help? I tried replace auto M with MyClass M and still the same. Thanks a lot.

Regds
LAM Chi-fung
Last edited on
poteto (525)
Read the sticky on how to code tag.
Make your autos a reference like: auto& this will prevent unnecessary copying.
When you call M.AddStructV(); you are actually adding it to a copy.
CFLam (45)
I can't get the "format" work, anyone point me the right way ? thx
poteto (525)
Oh, sorry I had to read what I said before understanding what you said!
I didn't know the sticky doesn't tell how to format.
You need to press the <> button in the edit page.
and re-copy and paste your code with spaces.
Last edited on
CFLam (45)
@poteto
thanks for your help, eventually I got it works (both the program and format).
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