No Compile Error for if(x=1)

**motocross1** · 01-13-2012

I discovered that VC++ Express 2008 complies:
if(x=1)doTask();

*Note: (x=1) not (x==1)

Is there a way to get VC++ to give me a warning for this?
And, why is this is allowed in the first place?
Thanks

**manasij7479** · 01-13-2012

Originally Posted by motocross1

And, why is this is allowed in the first place?

Because it is valid code...and can be useful if you want to put the result of a condition into a variable and test it for true/false at the same time.

**motocross1** · 01-13-2012

I'm not sure I understand, but do you mean...
I can do math inside an IF and the IF is true when the math result is not 0 and false when the result is 0?

And if this is what you mean, then...
Shouldn't I test: if((x=y+z)!==FALSE) or if((x=y+z)===FALSE)
Also, is this okay too: if(x++) and if(x+=3)

I'm just curious to know.
Not as if I will start using such code.
I think it is ugly.

**iMalc** · 01-13-2012

Make sure that your project is set to warning level 4. I believe that should pick it up.

**Elysia** · 01-13-2012

Originally Posted by motocross1

And, why is this is allowed in the first place?
Thanks

A typical use is:
if ( (result = SomeFunction()) == SOME_VALUE )

**manasij7479** · 01-13-2012

Originally Posted by motocross1

I'm not sure I understand, but do you mean...
I can do math inside an IF and the IF is true when the math result is not 0 and false when the result is 0?

And if this is what you mean, then...
Shouldn't I test: if((x=y+z)!==FALSE) or if((x=y+z)===FALSE)
Also, is this okay too: if(x++) and if(x+=3)

I'm just curious to know.
Not as if I will start using such code.
I think it is ugly.

I meant... code like

Code:

if(someflag = test_condition() != some_value)
...

is often used.
It doesn't look too ugly to me .

Edit: Just noticed Post #5 ...

**oogabooga** · 01-13-2012

@manasij, you've forgotten a set of parens:

Code:

if((someflag = test_condition()) != some_value)

But this isn't really the same as what the OP posted.
The case of this:

Code:

if (x = a)
  ...

is really just a case of this

Code:

if (b)
  ...

In such a case, where no logical test is explicitly performed, it is interpreted as if it were written:

Code:

if (b != 0)
  ...

if ((x = a) != 0)
  ...

**manasij7479** · 01-13-2012

Originally Posted by oogabooga

@manasij, you've forgotten a set of parens:

I thought they weren't necessary... as assignment has a lower precedence than the comparisons.

**grumpy** · 01-13-2012

Originally Posted by motocross1

And, why is this is allowed in the first place?

Because assignment operations yield a value.

Apart from particular options with your compiler (iMalc mentioned warning level 4) you can also trigger warnings from such things by swapping left and right operands.

This will not compile with any compiler

Code:

if (1 = x)

but this will

Code:

if (1 == x)

The weakness of this technique is that it only works if one of the operands is an rvalue (i.e. it cannot be on the left hand side of an assignment). So it won't catch

Code:

if (some_variable = another_variable) ....

because the assignment is still valid.

**oogabooga** · 01-13-2012

Originally Posted by manasij7479

I thought they weren't necessary... as assignment has a lower precedence than the comparisons.

Sorry manasij, I thought you meant it the other way around (like Elysia's post).

**motocross1** · 01-13-2012

iMalc,
Thank you.

Level 4 did it.
And I actually found a semi-copycat post when Googling "VC++ warning level":
VC++ General: How to avoid making an assignment when the intention is a comparison? - CodeGuru Forums

Of course, WOW, the list of warnings is HUGE.
I guess I'm about to really learn C++ now.

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