Warning C4800: 'int' : forcing value to bool 'true' or 'false' (performance warning)

Question

I have this problem in my code:

bool CBase::isNumber()
{
return (id & MID_NUMBER);
}

bool CBase::isVar()
{
return (id & MID_VARIABLE);
}

bool CBase::isSymbol()
{
return (id & MID_SYMBOL);
}

id or MID_SYMBOL must be of type BOOL. which is just #define to int type in Windows. Your return value is BOOL and it is converted into bool(true or false ). — CreativeMind, Jan 04 '14 at 10:23
`(id & MID_NUMBER) != 0;` is the general way to avoid the warning. — Retired Ninja, Jan 04 '14 at 10:24
One of the warnings I usually turn off: #pragma warning(disable: 4800) — , Jan 04 '14 at 10:24
I took your title and put it into Google, several of the first results explain the warning. — GManNickG, Jan 04 '14 at 10:24
possible duplicate of [What is the performance implication of converting to bool in C++?](http://stackoverflow.com/questions/206564/what-is-the-performance-implication-of-converting-to-bool-in-c) — Retired Ninja, Jan 04 '14 at 10:27

score 56 · Answer 1 · answered Jan 04 '14 at 10:28

56

FYI: Casts won't hide the warning by design.

Something like

return (id & MID_NUMBER) != 0;

should clearly state "I want to check whether this value is zero or not" and let the compiler be happy

answered Jan 04 '14 at 10:28

Marco A.

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score 2 · Answer 2 · answered Jan 04 '14 at 10:24

2

Use the !! idiom eg

bool CBase::isNumber()
{
    return !!(id & MID_NUMBER);
}

answered Jan 04 '14 at 10:24

cup

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Just to please the compiler !?? – Jan 04 '14 at 10:25
And why 2 ops instead of 1 (even if it is optimized)? – deviantfan Jan 04 '14 at 10:25
@Dieter Lücking: Non-standard workarounds are probably worse. – deviantfan Jan 04 '14 at 10:26
9

@deviantfan The warning itself is useless – Jan 04 '14 at 10:26
@DieterLucking: It can not be useless as it is warning. – CreativeMind Jan 04 '14 at 10:28
1

@Lücking: Why? Of course the compiler can generate the correct stuff without changed code, but maybe the coder has not intended a implicit cast and finds a semantic error because of the warning? – deviantfan Jan 04 '14 at 10:28
1

First inverts it so you get the inverse of what you actually wanted, second inverts that so you get what you actually wanted. Useful when you have a mix of BOOL (windows), Bool (xwindows) and bool (C++) – cup Jan 04 '14 at 10:30
4

@deviantfan : The warning is not about semantics, it is about performance, which is specious. – ildjarn Jan 04 '14 at 10:30
I give you a +1. This was useful for me, since I have this warning in templated code where I cannot compare against 0 but !! works. – André Bergner Apr 10 '17 at 11:32

score 2 · Answer 3 · answered Sep 22 '14 at 09:17

Where's the declaration of id and MID_NUMBER? Are you sure they are not windef-style BOOLs rather than (lowercase) bool's? BOOL's have been in windef for decades typedef'd as an int; they pre-date proper C++ bool's and a lot of developers still use them.

Warning C4800: 'int' : forcing value to bool 'true' or 'false' (performance warning)

3 Answers3

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