What could be a better solution to calling a function recursively? - c++

Question

This is a simple function which will take value from user and if value is invalid then the function will call itself recursively until a valid input is provided.
#include<iostream>
using namespace std;
void getnum(){
int num;
string strnum;
getline(cin, strnum);
try{
num = stoi(strnum);
}
catch(invalid_argument &ia){
cout<<"Invalid argument\n";
getnum();
}
cout<<"\n"<<num;
}
int main(){
getnum();
return 0;
}
/*output(input: abc,abc,abc,4):
4
2494464
2494464
4201200
*/
Using the recursive approach the program is creating a new instance of the function every time an invalid argument is passed. After receiving a valid argument, function is printing multiple values(garbage values) of num due to multiple instances created.
The problem is that I want only the last value(correct one) to be printed. So I tried setting a 'flag' to control the execution of cout<<"\n"<<num.
#include<iostream>
using namespace std;
void getnum(){
int flag = 0;
int num;
string strnum;
getline(cin, strnum);
try{
flag = 1;
num = stoi(strnum);
}
catch(invalid_argument &ia){
flag = 0;
cout<<"Invalid argument\n";
getnum();
}
if(flag)
cout<<"\n"<<num;
}
int main(){
getnum();
return 0;
}
/*output(input:abc,abc,abc,4)
4 */
It solves my problem but still multiple instances are being created which I think is wastage of memory.
Is there any better way to do this without using a lot of memory(recursion)?

Answer 1

You get multiple outputs because you print outside "the happy path" - move printing inside the try block.
It's even clearer to put the entire "happy path" inside the try:
void getnum(){
try {
string strnum;
getline(cin, strnum);
int num = stoi(strnum);
cout<<"\n"<<num;
}
catch(invalid_argument &ia){
cout<<"Invalid argument\n";
getnum();
}
}
The idiomatic solution is to loop rather than recurse:
void getnum(){
while (true)
{
try {
string strnum;
getline(cin, strnum);
int num = stoi(strnum);
cout << "\n" << num;
return;
}
catch (invalid_argument &){
cout<<"Invalid argument\n";
}
}
}