Is there a way to set the value of a variable to empty in C++?

Question

I have a program which prints a statement based on inputs. The print statement prints using all inputs, but I only want it to print inputs which are actually given. For instance:

std::cout << "-Number: " << number < "-Letter: " << letter << "-String: " << str << "-Sum: " << sum << std::endl;

(for the purpose of demonstration, the variables above are arbitrary, this is just to show a point)

So this print statement is called after every iteration of a loop. The values can be anything, but I do not want it to print if a value was not received. Ideally, this would be done like so:

// Get input... Then print statement... 
// If input was not received for a value (i.e. it equals none) then skip that value
std::cout << "-Number: " << number < "-Letter: " << letter << "-String: " << str << "-Sum: " << sum << std::endl; 
number = none, letter = none, str = none, sum = none // reset inputs and repeat loop

(Once again, the above is pseudo-code for the purpose of demonstration)

Is this possible in C++?

You can use default initialization: `number = {}, str = {}`... — Daniel, Aug 03 '20 at 18:02
@Dani Is that any different than initializing to 0 or containing an uninitialized value? — EthanR, Aug 03 '20 at 18:04
@EthanR No, as already stated the way to get what you want is `std::optional`. — john, Aug 03 '20 at 18:05
"default initialization" is something different, you should find the definition in the link I gave you. However, that is initialization, not assignment. Also, it doesn't magically extend the possible values of a variable by a value that means "none". — Ulrich Eckhardt, Aug 03 '20 at 18:06
The difference is that 0 for number and “” for string are not the same, while {} works for all defaultly constructible types. — Daniel, Aug 03 '20 at 18:07

Thomas Caissard · Answer 1 · 2020-08-03T18:32:50.187

1

If you have C++17 or a later version see optional. Prior to that, you would need to wrap your type into a proper struct that would mimic what optional does (of which you can find various implementation on the net, e.g. https://github.com/TartanLlama/optional).

In your example, you could do:

std::optional<int> number;

// Puts a value inside number
if (some condition)
    number.value() = 44;
// Set number to invalid
else number.reset();

std::cout << "Number: " << (number ? number.value() : "Some Default Value You want to Show" << std::endl;

std::optional has an operator bool() which tells you if you correctly have a value in your std::optional. Also note that your std::optional is invalid by default if you call the default constructor (see https://en.cppreference.com/w/cpp/utility/optional/optional).

edited Aug 03 '20 at 18:32

answered Aug 03 '20 at 18:13

Thomas Caissard

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Can you show a minimal example including a cout? You don't have to use all of the variables of the question. – drescherjm Aug 03 '20 at 18:15
There are already plenty of examples on the net: https://stackoverflow.com/questions/16860960/how-should-one-use-stdoptional :) – Thomas Caissard Aug 03 '20 at 18:18
1

I know there are examples for how to use optional but that example did not show what the OP wants to do. – drescherjm Aug 03 '20 at 18:22

Is there a way to set the value of a variable to empty in C++?

1 Answers1