User input in default constructors (C++). Can this be done? - c++

Question

This was the question: Write a program in C++ to enter two sequence separately and find total length of both sequences using constructor.
This is the program that I wrote. It runs but did I use constructors properly? Can we do this? Can we take user input in default constructors?
#include <iostream>
#include <string.h>
using namespace std;
class merge {
private:
int i;
char seq1[5];
int len, len1, len2;
char seq2[5];
public:
merge();
};
merge::merge() {
cout << "Enter first sequence: ";
cin >> seq1;
cout << "Enter second sequence: ";
cin >> seq2;
len1 = strlen(seq1);
cout << "Length of first sequence: " << len1 << "\n";
len2 = strlen(seq2);
cout << "Length of first sequence: " << len2 << "\n";
strcat (seq1, seq2);
cout << "Merged sequence: " << seq1 << "\n";
len = strlen(seq1);
cout << "Total length: " << len;
}
int main() {
merge o;
}

Answer 1

User input in default constructors (C++). Can this be done?
Technically yes. Constructor body is mostly the same as any other function, and I/O is possible.
Practically, I/O in a constructor would be a bad design.
So we can, but we do not do this.

Answer 2

Can we take user input in default constructors?
Yes you can do it, but you shouldn't.
There are many ways to explain why. For example, consider what it means to be default-constructible. From cppreference about the named requirement DefaultConstructible:
Specifies that an instance of the type can be default constructed.
Requirements
The type T satisfies DefaultConstructible if
Given
u, an arbitrary identifier
The following expressions must be valid and have their specified
effects
T u The object u is default-initialized
T u{} The object u is value-initialized or aggregate-initialized.
T() / T{} A temporary object of type T is value-initialized or aggregate-initialized.
Your merge is DefaultConstructible. But is it really?
Consider what happens when you add a second constructor:
merge::merge(std::string seq1, std::string seq2);
Now I can choose whether I want to use the default constructor to take input from user, or get the sequences from elsewhere and call the other constructor. Sounds good, no?
(It isn't. It just helps me to emphasize the problem with your default constructor in the following examples. Bear with me.)
Code that is fine with a default constructible type T, but not quite with your merge is...
example std::vector
using T = merge;
std::vector<T> v(10);
v[0] = T("1,2,3,4","4,5,6,7,8");
// ...
Because I want to avoid reallocations of the vector and because I know that T can be default constructed, I created a vector with 10 elements even though I want to create the actual elements only later. I didn't want to ask for user input 10 times when I wrote std::vector<T> v(10); but thats the only way to resize a vector that contains elements of your class.
example: std::map
std::map<int,T> m;
m[1] = T("1,2,3,4","4,5,6,7,8");
I want to add an element to the map (if it didnt exist before). std::map::operator[] tries to find an element with the given key (1). It does not find one, hence it inserts a new element with that key and a default constructed T. Then it returns a reference to that T.
I could have used insert, but I was lazy and used []. As a consequence, this code asks the user for input, but thats not what I wanted.
Here is a demo of the two issues I tried to explain above: https://godbolt.org/z/fj5qoa
Formally, your merge is default-constructible, but for most practical purposes it isn't. A default constructor is expected to construct an object in a default state. Reading user input is surprising and many types and algorithms that require that your type is default constructible will not work as expected, even though you provided a default constructor.