Pro/con: Initializing a variable in a conditional statement

Question

In C++ you can initialize a variable in an if statement, like so:

if (CThing* pThing = GetThing())
{
}

Why would one consider this bad or good style? What are the benefits and disadvantages?

Personally i like this style because it limits the scope of the pThing variable, so it can never be used accidentally when it is NULL. However, i don't like that you can't do this:

if (CThing* pThing = GetThing() && pThing->IsReallySomeThing())
{
}

If there's a way to make the above work, please post. But if that's just not possible, i'd still like to know why.

Question borrowed from here, similar topic but PHP.

Answer 1

The important thing is that a declaration in C++ is not an expression.

bool a = (CThing* pThing = GetThing()); // not legit!!

You can't do both a declaration and boolean logic in an if statement, C++ language spec specifically allows either an expression or a declaration.

if(A *a = new A)
{
    // this is legit and a is scoped here
}

How can we know whether a is defined between one term and another in an expression?

if((A *a = new A) && a->test())
{
    // was a really declared before a->test?
}

Bite the bullet and use an internal if. The scope rules are useful and your logic is explicit:

if (CThing* pThing = GetThing())
{
    if(pThing->IsReallySomeThing())
    {
    }
}