'auto' declaration issue in C++ - c++

Question

I have a problem in using auto declarations. I write a program in Visual Studio 2017 as follow:
#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 the code to this:
#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 compiles without problem, but I get an error when I run it, and the output is:
MyObj[0] struct vector size:0
The program is stuck here.
It seems that I am missing something. I even tried replacing auto M with MyClass M, but still the same problem.

Answer 1

This line will make a copy of your object
for (auto M : MyObj)
change it to a reference so you can modify it
for (auto& M : MyObj)