Defaulting to user input using a lambda when parameter is not explicitly passed

Question

I wrote the following code (Still wondering about its uses...) to default to user input if the parameter is not passed.

#define _CRT_SECURE_NO_WARNINGS
#include <iostream>

unsigned getInput() {
    unsigned input;
    std::cin >> input;
    return input;
}

void foo(unsigned number = getInput()) {
    std::cout << number << "\n";
}

int main() {
    foo(1); //prints 1
    foo();  //defaults to user input    
    return 0;
}

What I wanted to ask was, is there any way we can convert the getInput() function to a lambda? Something on the lines of

void foo(unsigned number = { []() {unsigned num = 0; std::cin >> num; return num; } }) {
    std::cout << number << "\n";
}

Also, how does one achieve similar functionality in python?

score 4 · Accepted Answer · answered May 21 '20 at 16:16

Yes, you can use a lambda. What you need to do is call the lambda after you define it like

void foo(unsigned number = []() {unsigned num = 0; std::cin >> num; return num; }()) 
//                         ^                   define lambda                    ^^^
//                                                               call lambda_____|

score 2 · Answer 2 · answered May 21 '20 at 16:15

2

Sure, but you are missing the call to the lambda:

void foo(unsigned number = [] {
               unsigned num = 0; 
               std::cin >> num; 
               return num; 
              }()   // <- call here
        ) 
{
    std::cout << number << "\n";
}

answered May 21 '20 at 16:15

cigien

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Thanks a lot. Also, is `[]()` not always necessary when defining a lambda? I see you haven't used the parenthesis after `[]` – kesarling He-Him May 21 '20 at 16:20
That's correct. The `()` after `[]` is optional in a lambda that takes no arguments. – cigien May 21 '20 at 16:21

Defaulting to user input using a lambda when parameter is not explicitly passed

2 Answers2

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