如何在C ++中使用返回值创建内联作用域？

Question

I have a larger if expression let's say of the kind

if (a && (c(b.getObject()) || d(b.getObject()))
{
    ...
}

is there a way to declare b.getObject() as local reference only if a is true ? I'm looking for a kind of lambda function perhaps like this:

if (a && { Object & o(b.getObject()); return (c(o) || d(o))} )
{
    ...
}

but this obviously doesn't work. Of course I could nest it in another if block, but is there a more "local" way to do it?

edit:
One argument against nested if blocks is that they do not allow one combined else block.

Answer 1

Define a function that does the two checks, and call it so you only evaluate b.getObject() once.

auto e = [](const Object& o) { return c(o) || d(o); };
if (a && e(b.getObject()))
{
    ...
}

Prior to C++11 you would have to write e as a function at namespace scope, or as a variable of a local class type with an operator() .

Also, G++ has a non-standard "statement expression" extension that allows what you want, with slightly different syntax:

if (a && ({ const Object& o = b.getObject(); c(o) || d(o); }))
{
  ...
}

However, I think generally a nested if is much cleaner and easier to read than either of these alternatives.

Answer 2

Please don't do this, but it works...

Object x;

if (a && (x = b.getObject(), c(x) || d(x)))
{
}

Answer 3

You may use a lambda for that:

if (a && [&b](){ const Object& o = b.getObject(); return c(o) || d(o); }() ) {
    ...
}

but declaring the function outside the condition seems cleaner.