Is a declaration valid inside an if block with no actual block?

Question

Is the following code valid? If so, what is the scope of x?

int main()
{
   if (true) int x = 42;
}

My intuition says that there is no scope created by the if because no actual block ({}) follows it.

Hm, my intuition is that the scope doesn't depend on the {} being present, or in other words that for a single statement the presence of { } is optional. — PlasmaHH, Mar 22 '13 at 11:17
It should only exist for the scope of conditional, single statements don't require the `{}`. — Nicholas Smith, Mar 22 '13 at 11:19
http://stackoverflow.com/questions/14797810/why-are-my-structs-members-not-properly-initialised-using#comment20723793_14797810 — Lightness Races in Orbit, Mar 22 '13 at 11:31
@LightnessRacesinOrbit: sure, I could link to your frankly excellent answer, or dig through the C++ language specification to find where it talks about scope definition and block definition. — Nicholas Smith, Mar 22 '13 at 12:00
I actually do this quite often, to create a line I can stick a breakpoint on. Of course you need to put the statement on a different line to the `if`. — Jack Aidley, Mar 22 '13 at 16:06
@JackAidley Why not put a breakpoint on an actual line of real code? — Lightness Races in Orbit, Mar 22 '13 at 16:24
@LightnessRacesinOrbit: Because I don't necessarily want to break on a real line of code. For example, I might want to break into a loop when `foo[i]==356`; adding two lines of code going `if (foo[i]==256) int spug=7;` (it's always `spug=7`, I don't know why) let's me break when into a complex loop at the point I want. — Jack Aidley, Mar 22 '13 at 21:29

Lightness Races in Orbit · Accepted Answer · 2013-03-22T11:21:18.917

GCC 4.7.2 shows us that, while the code is valid, the scope of x is still simply the conditional.

Scope

This is due to:

[C++11: 6.4/1]: [..] The substatement in a selection-statement (each substatement, in the else form of the if statement) implicitly defines a block scope. [..]

Consequently, your code is equivalent to the following:

int main()
{
   if (true) {
      int x = 42;
   }
}

Validity

It's valid in terms of the grammar because the production for selection statements is thus (by [C++11: 6.4/1]):

selection-statement:
  if ( condition ) statement
  if ( condition ) statement else statement
  switch ( condition ) statement

and int x = 42; is a statement (by [C++11: 6/1]):

statement:
  labeled-statement
  attribute-specifier-seq_opt expression-statement
  attribute-specifier-seq_opt compound-statement
  attribute-specifier-seq_opt selection-statement
  attribute-specifier-seq_opt iteration-statement
  attribute-specifier-seq_opt jump-statement
  declaration-statement
  attribute-specifier-seq_opt try-block

@Downvoter: Please do explain why you think that this answer is incorrect. I look forward to hearing your views. Thank you. — Lightness Races in Orbit, Mar 22 '13 at 11:25

score 2 · Answer 2 · edited Mar 22 '13 at 11:20

2

My Visual studio says that time of life of your variable x is pretty small - just while we are inside operator if, so x vill be destroyed when we are out of if condition, and there is absolutely no meaning to declare variables like this.

edited Mar 22 '13 at 11:20

R. Martinho Fernandes

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answered Mar 22 '13 at 11:20

Anton Kizema

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2

Your English is fine, btw. – R. Martinho Fernandes Mar 22 '13 at 11:20
4

What if the variable constructor has side effects? (I know, I am being picky, but *there is absolutely no meaning* is not 100% correct) – Bartek Banachewicz Mar 22 '13 at 11:20
no "side effects" are being shoun by VS, just that the variable x is being destroyed after we exit "if" – Anton Kizema Mar 22 '13 at 11:22
4

No, you didn't understand @Anton. Here we are constructing `int`, but if I, did, for example `if(cond) ObjWithSideEffectsCtor o;` it would matter. – Bartek Banachewicz Mar 22 '13 at 11:24
Or, for example, if you did chaining, like `new internetAPIhandlerobject().sendAPImessage("some message")`; – Yamikuronue Mar 22 '13 at 16:29

Is a declaration valid inside an if block with no actual block?

2 Answers2

Scope

Validity

Linked