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up vote 32 down vote favorite
6

What's going on here?

if(int a = Func1())
{
    // Works.
}

if((int a = Func1()))
{
    // Fails to compile.
}

if((int a = Func1())
    && (int b = Func2()))
)
{
    // Do stuff with a and b.
    // This is what I'd really like to be able to do.
}

Section 6.4.3 in the 2003 standard expains how variables declared in a selection statement condition have scope that extends to the end of the substatements controlled by the condition. But I don't see where it says anything about not being able to put parenthesis around the declaration, nor does it say anything about only one declaration per condition.

This limitation is annoying even in cases where only one declaration in the condition is required. Consider this.

bool a = false, b = true;

if(bool x = a || b)
{

}

If I want to enter the 'if"-body scope with x set to false then the declaration needs parenthesis (since the assignment operator has lower precedence than the logical OR), but since parenthesis can't be used it requires declaration of x outside the body, leaking that declaration to a greater scope than is desired. Obviously this example is trivial but a more realistic case would be one where a and b are functions returning values that need to be tested

So is what I want to do non-conformant to the standard, or is my compiler just busting my balls (VS2008)?

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5  
"If I want to enter the loop with" <-- your examples have if. if is not a loop, it is a conditional. – crashmstr Oct 20 '11 at 13:48
2  
@crashmstr: true, but the conditions for while are the same as for if. – Mike Seymour Oct 20 '11 at 14:08
1  
Can't this be done with the comma operator? I mean: if (int a = foo(), int b = bar(), a && b)? If comma operator is not overloaded, the standard says that the expressions are evaluated from left to right, and the result value is the last expression. It works with for loops initialization, why not here? – Archie Oct 20 '11 at 14:46
@Archie: I just tried this, I couldn't get it to work. Maybe you can provide a working example? – James Johnston Oct 20 '11 at 15:10
@JamesJohnston: I just also have tried, and it doesn't seem to work. That idea just came from the top of my head, I was suggested by how if works, and it seems to be wrong assumption. – Archie Oct 20 '11 at 15:52

5 Answers

up vote 25 down vote accepted

The condition in an if or while statement can be either an expression, or a single variable declaration (with initialisation).

Your second and third examples are neither valid expressions, nor valid declarations, since a declaration can't form part of an expression. While it would be useful to be able to write code like your third example, it would require a significant change to the language syntax.

I don't see where it says anything about not being able to put parenthesis around the declaration, nor does it say anything about only one declaration per condition.

The syntax specification in 6.4/1 gives the following for the condition:

condition:
    expression
    type-specifier-seq declarator = assignment-expression

specifying a single declaration, with no parentheses or other adornments.

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up vote 37 down vote

I think you already hinted at the issue. What should the compiler do with this code?

if (!((1 == 0) && (bool a = false))) {
    // what is "a" initialized to?

The "&&" operator is a short-circuit logical AND. That means that if the first part (1==0) turns out to be false, then the second part (bool a = false) should be not be evaluated because it is already known that the final answer will be false. If (bool a = false) isn't evaluated, then what to do with code later on that uses a? Would we just not initialize the variable and leave it undefined? Would we initialize it to the default? What if the data type was a class and doing this had undesirable side effects? What if instead of bool you used a class and it had no default constructor such that the user must provide parameters - what do we do then?

Here's another example:

class Test {
public:
    // note that no default constructor is provided and user MUST
    // provide some value for parameter "p"
    Test(int p);
}

if (!((1 == 0) && (Test a = Test(5)))) {
    // now what do we do?!  what is "a" set to?

Seems like the limitation you have found seems perfectly reasonable - it prevents these kinds of ambiguities from happening.

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Good point. You might want to mention short-circuiting explicitly, in case the OP or others are not familiar with it. – Chris Cooper Oct 20 '11 at 13:52
3  
I hadn't thought of that. Although in the example you provided the short circuiting prevents the conditional statement scope from being entered, in which case the part of the expression that declares the varible not being processed is not an issue, since its scope is limited to that of the conditional statement. In which case wouldn't it be better if the compiler raised an error only in those cases where there is a possibilty of the conditional statement scope being entered when a part of the expression that declared a variable was not processed? Which was not the case in the examples I gave. – Neutrino Oct 20 '11 at 14:35
@Neutrino At first sight you idea sounds a bit like the SAT-problem, which is not that easy to solve, at least in the general case. – Christian Rau Oct 20 '11 at 14:37
@Neutrino: Edited my examples so that the expression evaluates to true instead of false. I think my points still stand. While you are right and the compiler could theoretically do a static analysis to allow variable declarations in these specific cases, that adds a lot of complexity to the language with little benefit (how should the static analysis be done? this has to be defined for portability). The proposed feature starts at -100 points and probably doesn't have enough benefits to implement it: blogs.msdn.com/b/ericgu/archive/2004/01/12/57985.aspx – James Johnston Oct 20 '11 at 14:57
4  
What you explain about all the problems with having multiple variable declarations in the if condition and the fact that you can only use them in a restricted way makes me wonder why on earth this kind of declaration was introduced in the first place. I had never ever felt the need for such a syntax before I saw it in some code example. I find this syntax rather clumsy and I think that declaring the variable before the if block is much more readable. If you really need to restrict the scope of that variable you can put an extra block around the if block. I have never used this syntax. – Giorgio Oct 20 '11 at 15:53
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up vote 12 down vote

If you want to enclose variables in a narrower scope, you can always use additional { }

//just use { and }
{
    bool a = false, b = true;

    if(bool x = a || b)
    {
        //...
    }
}//a and b are out of scope
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3  
+1. Additionally, I would move the declaration of x to the surrounding block: why should it have a special status wrt a and b? – Giorgio Oct 20 '11 at 16:05
up vote 8 down vote

The last section already works, you just have to write it slightly different:

if (int a = Func1())
{
   if (int b = Func2())
   {
        // do stuff with a and b
   }
}
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up vote 0 down vote

One thing to note, also is that the expressions inside the larger if-block

if (!((1 == 0) && (bool a = false)))

are not necessarily guaranteed to be evaluated in a left-to-right fashion. One rather subtle bug that I had back in the day had to do with the fact that the compiler was actually testing right-to-left instead of left-to-right.

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