我可以在C ++中将int与布尔值相乘吗？

Question

I have a widget in my GUI that displays chart(s). If I have more than one chart there will be a legend shown in a rectangle on the GUI.

I have a QStringlist (legendText) which holds the text of the legend. If there is no legend required, legendText would be empty. If there will be a legend, legendText would hold the text.

For finding the height of the rectangle around the legend I would like to do the following:

 int height = 10;
 QStringList legendText;
 ...
 height = height * (legendText->size() > 0);
 ...

Is this a good idea/ good style to multiply an int with a boolean ? Will I run into problems with that?

Answer 1

This is technically fine, if a bit unclear.

The bool will be promoted to an int , so the result is well-defined. However, looking at that code I don't instantly get the semantics you are trying to achieve.

I would simply write something like:

height = legendText->isEmpty() ? 0 : height;

This makes your intent far clearer.

Answer 2

It's perfectly fine according to the standard (§4.5/6):

A prvalue of type bool can be converted to a prvalue of type int , with false becoming zero and true becoming one.

However, I suggest using isEmpty instead of comparing size to zero height = height * (!legendText->isEmpty());

Or use the conditional operator as the other answers suggest (but still with isEmpty instead of .size() > 0 )

Answer 3

您可以使用条件（三元）运算符：

height = ( legendText->size() >0 ) ? height : 0 ;

Answer 4

Maybe this?

if(legendText->isEmpty())
{
   height = 0;
}

or

int height = legendText->isEmpty() ? 0 : 10;

Answer 5

Some people may find following information useful (following code should be considered in high performance programs where every clock cycle matters and it's purpose here is to show alternative techniques, I wouldn't use it in this particular situation).

If you need fast code without branches you can implement int multiplication with boolean using bitwise operators.

bool b = true;
int  number = 10;
number = b*number;

can be optimized to:

number = (-b & number);

If b is true then -b is -1 and all bits are set to 1 . Otherwise all bits are 0 .
Boolean NOT ( !b ) can be implemented by XOR'ing b with 1 ( b^1 ).
So in your case we get following expression:

height = (-(legendText->isEmpty()^1) & height);