Setting default value for Bool by reference

Question

How do i assign a default value to a bool that is a reference?

I have a function like this :

void someFunction(bool a = true, bool& b)
{
 if(a)
   b = false;
 else
   std::cout << "nothings changed" << std::endl;
}

How should i assign the default value to b in this context?

void someFunction(bool a = true, bool& b = false)

will not work. So how should it be done?

http://stackoverflow.com/questions/1059630/default-value-to-a-parameter-while-passing-by-reference-in-c could be of some use. — another.anon.coward, Feb 15 '12 at 12:01
if `b` is supposed to be an *out* variable (i.e. you are returning some data with it) then you can not do it using references. — Naveen, Feb 15 '12 at 12:03
i dont understand. it works fine if i dont assign the default value. but life would be so much easier if i could. — Prasanth Madhavan, Feb 15 '12 at 12:04
You don't _need_ to give a default value to a reference-type argument, because the variable you call the function with already has a value when you make the call! — Mr Lister, Feb 15 '12 at 12:04
i dont want to use the variable all the time. i need to ignore it in some situations. that is why i need a default value for it, so that i can call the fun with one or no value at all.. — Prasanth Madhavan, Feb 15 '12 at 12:08
Then use a pointer in the function (with default value NULL) and pass the address of b — Mr Lister, Feb 15 '12 at 12:09
What's the point of calling a function that changes a variable if you don't actually want to change a variable? Sounds like a strange design to me. — fredoverflow, Feb 15 '12 at 12:10

score 5 · Accepted Answer · answered Feb 15 '12 at 12:03

You cannot initialize non-const references with values. You need a variable:

bool dummy_variable;

void someFunction(bool a = true, bool& b = dummy_variable)
{
    // ...
}

But it would probably make a lot more sense to simply return the bool:

bool someFunction(bool a = true)
{
    return !a;
}

Then the client can still decide for himself if he wants to ignore the result or not.

score 0 · Answer 2 · answered Mar 23 '22 at 12:51

Why not use a pointer?

#include <iostream>

using namespace std;

void someFunction(bool a = true, bool* b = nullptr)
{
    if (b != nullptr) {
        *b = a; 
    }
}

int main()
{
   bool res = true;
   someFunction();
   cout << res << endl; // true
   someFunction(false);
   cout << res << endl; // true
   someFunction(false, &res);
   cout << res << endl; // false

   return 0;
}

score 0 · Answer 3 · answered Feb 15 '12 at 12:04

0

You cannot bound temporary object to a non-const reference. You need to use non-temp object:

bool bDefaultVal = false;

void someFunction(bool a = true, bool& b = bDefaultVal)
{
    ...
}

answered Feb 15 '12 at 12:04

Bojan Komazec

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Setting default value for Bool by reference

3 Answers3