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

问题

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);
}

回答1:

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

回答2:

Use the !! idiom eg

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

回答3:

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.

来源：https://stackoverflow.com/questions/20919650/warning-c4800-int-forcing-value-to-bool-true-or-false-performance-warn