Double comparison

问题

Can I do this in C++?

if (4<5<6)
 cout<<"valid"<<endl;

i.e a double comparison? Since I know that I can

bool a;
a = 1+2<3+4<5>6;//etc

回答1:

Yes, you can do it, but it won't be what you expect. It's parsed as

if ( (4<5) < 6 )

which yields

if ( 1 < 6 ) 

because 4<5 evaluates to true which is promoted to 1, which yields, obviously, true.

You'll need

if ( (4<5) && (5<6) )

Also, yes, you can do

a = 1+2<3+4<5>6;

but that as well is parsed as

a = ((1+2)<((3+4)<5))>6;

which will evaluate to false since (1+2)<((3+4)<5) yields a boolean, which is always smaller than 6.

回答2:

It compiles but won't do what you expect -

if( 4 < 5 < 2) 

same as

if( (4 < 5) < 2)

same as

if( (1 < 2) )  //1 obtained from cast to boolean

which is of course true, even though I imagine you were expecting something quite different.

回答3:

It may be clumsy but this will work:

int i, j, k;
i = 4; j = 5; k = 6;
if ( (i < j) && (j < k) )
{
    cout << "Valid!" << endl;
}

来源：https://stackoverflow.com/questions/12961717/double-comparison