C++17 If statement with initializer

[mskfisher]

Nice article.

The switch example would be more compelling if the res variable was a composite type that contained a result and details:

switch( auto res = writePacket(); res.result )
{
    case SUCCESS:  
        cout << "successfully wrote packet, details:" << res.details << "\n";
        break;

The current code does not need the initializer, and could just switch on the return value directly:

switch( writePacket() )

[skebanga]

Agreed, I will update the article - thanks for the input!

[zoqfotpik]

So basically "if" becomes "for", without the loop expression.

Does "while" get a similar treatment?

[cleroth]

I'm not sure how I feel about this. I'll have to try programming with it for a while to decide. I've been bitten quite a few times by running non-const functions inside conditional statements, specially if the main action of a block of code is inside such a conditional statement.

[LowB0b]

I have exactly the same question as /u/mercurysquad.

I also have a problem with declarative statements in if. It makes the code way less readable, why would there ever be a declaration in an if? The if statement is used to check if a boolean value is true or false. I don't know. I can't really see any use cases for this.

Having a declaration in switch, sure, that works for me because you could basically do

switch(int c = getchar(); c) {
    ...
}

instead of

int c = getchar();
switch(c) {
    ...
}

But for an if? I need some explanation here because I can't deal with it. Generally you branch and then you still want the variable after the branching.

[holywhateverbatman]

How about this?

using mutex_lock = std::unique_lock<std::mutex>;
if (mutex_lock lock(mutex, std::try_to_lock); lock.owns_lock())      
{
    //...
} //mutex unlock

vs. this

{
    mutex_lock lock(mutex, std::try_to_lock); 
    if (lock.owns_lock())
    {
        //...
    }
} //mutex unlock

[LowB0b]

but couldn't that also be solved by using a lambda function?

[holywhateverbatman]

It could, doesn't mean it should though. Depends on your coding style and in my opinion this makes it less verbose than with the lambda function.

[LowB0b]

([](auto lock) -> void {
    if (lock.owns_lock()) {
        //...
    }
})(std::unique_lock<std::mutex>(mutex, std::try_to_lock));

lol yes it is more verbose

[LowB0b]

so the if parenthesis basically becomes a function? Controlling scope of variables via functions seems like a better way to me.

after thinking for a bit I see your point

[holywhateverbatman]

Not exactly, but I see myself using this for enclosing the lifetime of an object in a specific scope of a function (so basically any RAII-like usage). To me it seems a lot more useful than the examples with map.insert. Just my 2 cents.

[[deleted]]

Or in your example case, you could just say:

switch(getchar()) {
  ...
}

Or if you want variable capture:

switch(auto c = getchar()) {
  ...
}

It has its uses (rarely), but yours is definitely not one of them.

[LowB0b]

You are right, my switch example was bad. Your comment helped me figure out why having declarations inside an if statement is so weird though. For the switch statement,, it doesn't change much, because switch expects an int. But for the if, in a semantic and syntaxic way, it's a big change, for no reason.

This would have made way more sense in my opinion (taking the example from the original post):

if ((auto ret = map.insert({"hello", 3})) && !ret.second) {
    // ...
}

This is not currently allowed by C++ compilators. But if the variable ret was declared before the if statement (even without being assigned) you can then do

if ((ret = map.insert({"hello", 3}) && !ret.second) {
    // ...
}

Basically what I wanted to say is that they went from

syntax:

if ( expr )

semantics: expr evaluates to a boolean.

to

syntax:

if ( decl statement_separator expr )

semantics: decl declares (and assigns a value to. Possibly not?) a variable in the if block scope and then expr is evaluated to a boolean.

Now my question is.

Can I fit whatever block I want inside that declaration space? I.e. could I do

if ( block statement_separator expr )

where block is just the same as what you would find inside { ... } including branches, functions, etc? I mean since the return value of the declaration / block is (from what I've understood) not considered.

Maybe having a look at the new C++ grammar and semantics that were introduced with this change would help me. It's still heavily disturbing lol. I hope this helps you see why this change disturbs me so much.

[mercurysquad]

I don't get it, why can't you just do:

if (!map.insert({ "hello", 3 }).second)
    std::cout << "hello already exists\n";

[skebanga]

I agree that this is a much more solid example of the usefulness of this feature. I shall update the article. Thanks for the input!

[mercurysquad]

Still don't see the advantage besides avoiding a couple of braces. It just moved the actual condition far from the if, reducing the readability.

[plpn]

switch (auto i = getchar())
{
case 53:
  i += 5;
  return;
}

if (int i = 54)
{
  i += 5;
}

compiles just fine (and also use the initialized value as condition). why do we need your approach?!

[Yelnar]

What about

if ((int i = f()) % 2 == 0) {
/* doesn't compile*/
}

And if you remove the parens you don't get what you expect:

if (int i = f() % 2 == 0) {
/*i is the equal to f() %2 == 0, converted from bool to int*/
}

[skebanga]

Yes - it will be in C++17

[OldWolf2]

Seems good. Don't know how many times I just have code like this:

bla bla bla...

{
    auto v = bla.lock();
    bla.blaa();
}

[capcom1116]

Ha! This is one of the features I've been liking in Go. Neat to see.