C++ How to make std::function to return a boolean value

Question

I have the following small code example:

std::function<bool()> isSet;

bool check() {
   bool tmp;
   return tmp;
}

std::function<bool()> isSet {
    return check();
}

This gives me the following error message

could not convert 'isSet()' from 'bool' to 'std::function<bool()>'

Can anyone explain why and how I to fix it?

Your `check` function returns an uninitialized value. It isn't clear what you're actually trying to do. — interjay, Nov 27 '18 at 13:35

score 5 · Accepted Answer · answered Nov 27 '18 at 13:36

5

When you initialize a std::function you just give it the function you want it to use. That would look like

bool check() {
   bool tmp;
   return tmp;
}

std::function<bool()> isSet { check };

What you almost did was make a lambda, which you could do, but the syntax would be

std::function<bool()> isSet { [](){ return check(); } };

Do note that check returns an uninitialized variable which is undefined behavior: Why is returning a non-initialized value considered undefined behavior?

answered Nov 27 '18 at 13:36

NathanOliver

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Thanks,, yes,, it was a 'typo' in check to return a uninitialized. I just so sorry that there are people hanging around on stackoverflow more interested in finding errors than helping..... YOU helped alot..... Thanks. – lg.lindstrom Nov 27 '18 at 14:32

score 4 · Answer 2 · answered Nov 27 '18 at 13:35

std::function<bool()> isSet is a regular old object, not a function. You are supposed to initialize it with a functor it can hold, not provide function block to execute.

Simply fix the initialization

std::function<bool()> isSet { check }; // Hold a pointer to check

C++ How to make std::function to return a boolean value

2 Answers2