Using std::tie as a range for loop target

Question

I want to do something like the following:

//std::vector<std::pair<TypeA, TypeB>> someInitializingFunction(); {   TypeA a;   TypeB b;    for (std::tie(a, b) : someInitializingFunction()) {     // do stuff;   } } 

However, this is not valid code because, as the standard says, a range based for loop is defined as equivalent to:

{   auto && __range = range-init;   for ( auto __begin = begin-expr,       __end = end-expr;       __begin != __end;       ++__begin ) {     for-range-declaration = *__begin;     statement   } } 

Where a for-range-declaration is defined as:

for-range-declaration: attribute-specifier-seq_{opt} decl-specifier-seq declarator

So what's holding me back is that decl-specifier-seq is not marked as optional?

Therefore, it seems that I must fall back on old style for loops for this one a la:

std::vector<std::pair<TypeA, TypeB>> myList = someInitializingFunction();  {   TypeA a;   TypeB b;    for (auto it = myList.begin(); it != myList.end(); ++it) {     std::tie(a, b) = *it;     // do stuff;   } } 

But it seems a bit messy syntactically for what seems to intuitively be a rather common task, unpacking the result of a function call, which is valid in many other contexts.

Is there a proposal to add something this to the language? Is this even reasonable idea? Is there a better way to do this that I'm overlooking? Am I misreading the standard?

Obviously, I could put together my own function to do this, but that's also a bit messy to use as well.

Answer 1

As of 03-21-2017, Structured Bindings are part of C++.

This allows directly doing the following:

//std::vector<std::pair<TypeA, TypeB>> someInitializingFunction(); for (auto [a, b] : someInitializingFunction()) {   // do stuff; }