structured binding initializer inside if statement does not compile

Question

Reading up on C++17 and now multiple initializations inside if statement is possible:

if (int x = func(), y = func2(); x > 0 && y > 0)
{
}

Nice one, also in combination with another feature in C++17, structured bindings:

if (auto[iter, success] = set.insert("Hello"); success)
{   }
else    
{   }

But combining both features does not compile in VisualStudio 2017.

if (auto[iter, success] = set.insert("Hello"), [iter2, success2] = set.insert("Foo"); success && success2)
{}
else
{}

missing ';' before ','

Is this is a bug in VS2017 or is this not possible?

`auto [iter, success, iter2, success2] = std::tuple_cat(set.insert("Hello"), set.insert("Foo"))` — Jarod42, Sep 11 '17 at 13:09
Strictly speaking this has nothing to do with the new `if` initializer declaration. This won't work outside an `if` too.. — StoryTeller - Unslander Monica, Sep 11 '17 at 13:10

score 18 · Accepted Answer · edited Sep 11 '17 at 13:36

MSVC is right on the money in this one. This stems for the grammar alone:

selection-statement:  
    if ( init-statement condition )

init-statement:
    simple-declaration

simple-declaration:
    decl-specifier-seq init-declarator-list;
    decl-specifier-seq ref-qualifier [ identifier-list ] initializer ;

It's summarized above from all over the standard (with some optional things removed for brevity), but the start point is at [stmt.select]/1.

The crux of the matter is that a simple-declaration is either a comma separated list of declarators that introduce objects of the same type¹ (decl-specifier-seq init-declarator-list;) or a single structured binding (the second, rather verbose line under simple-declaration).

_{¹ Not strictly the same type (int x, *y;), but the the point should be clear.}

structured binding initializer inside if statement does not compile

1 Answers1