One liner tuple/pair unpack in c++ with reusing same variable multiple times

Question

I have already seen Is there a one-liner to unpack tuple/pair into references? and know how the unpack values from tuple/pairs in a single line like following

auto [validity, table] = isFieldPresentAndSet(r, "is_federated");

here isFieldPresentAndSet returns a tuple.

Now I want to reuse these two variable in multiple successive calls of isFieldPresentAndSet like following

auto [validity, table] = isFieldPresentAndSet(r, "is_federated");
auto [validity, table] = isFieldPresentAndSet(r, "gslb_sp_enabled");

and then check the value for the validity and table. But this gives me compile error because I am redefining the validity and table variable second time. If the change the second line to

[validity, table] = isFieldPresentAndSet(r, "gslb_sp_enabled"); 

or

validity, table = isFieldPresentAndSet(r, "gslb_sp_enabled"); 

It still gives me compile error.

Is there any way to do this??

Kentiga

add {} around the block to make the vars local to that scope? – Thirteenth

Nope. I have to put both tuple unpacking into separate scope. But the checking after calling the function would happen outside. – Kentiga

std::tie(validity, table) = isFieldPresentAndSet(r, "gslb_sp_enabled"); ? – Sinatra

Unclear to me because that means you unpack multiple times and check once??? – Thirteenth

@Thirteenth yes that's what I am doing – Kentiga

structured_binding is C++17 though, tags fixed. – Sinatra

@Sinatra yes I could use tie but I was looking for more cleaner solution. Thanks though! – Kentiga

If you check once, you get only the result of last unpacking... really? :-) – Thirteenth

Answer 1

You can use std::tie. It returns a tuple of references, which makes the assignment possible:

std::tie(validity, table) = isFieldPresentAndSet(r, "gslb_sp_enabled");

Amaro