What's the best way to return a tuple from function in C++11?

Question

I want to return some values from a function and I want to pack it in a tuple. So I have two possibilities for function declaration:

std::tuple<bool, string, int> f() {   ...   return std::make_tuple(false, "home", 0); } 

and

std::tuple<bool, string, int> f() {   ...   return std::forward_as_tuple(false, "home", 0); } 

These functions are equivalents? Between these functions which do you prefer?

Answer 1

std::forward_as_tuple() creates a tuple of references. Since you are returning a tuple<bool, string, int> anyway, the two end up being equivalent in this case, but I think the first approach is clearer - using forward_as_tuple() when you are not forwarding anything is confusing.

Also, as mentioned by Sebastian Redl in the comments, make_tuple() would allow the compiler to perform copy elision - per paragraph 12.8/31 of the C++11 Standard, while forward_tuple() would not (since what it returns does not have the same type as the function's return type).

Answer 2

I prefer,

std::tuple<bool, std::string, int> f() {   ...   return { false, "home", 0 }; } 

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EDIT 1

The above code is actually compiling for me under clang/libc++ trunk. As @AndyProwl commented in comments section, this shouldn't since std::tuple constructor is explicit and returning through initialization-list syntax is in copy-initialization context, hence copy-list-initialization, which fails when an explicit constructor is matched.

I don't know the reason why clang/libc++ is passing, I suppose it to be a bug in libc++. Anyway, it's sad one can't do that for tuples...

I think I realized how sad (for me, at last) it's, generally. I was becoming used to that syntax, but one is forced to know beforehand whether or not the returning type contains an explicit constructor anytime for it to work.

EDIT 2

This is indeed a libc++ extension, for more information, checkout Howard Hinnant answer here: https://stackoverflow.com/a/14963014.

It's also currently open in the libc++ bug list: http://llvm.org/bugs/show_bug.cgi?id=15299.

This is the relevant proposal: Daniel Krügler, Improving pair and tuple.

In short this is what happens with libc++:

#include <tuple> #include <string>  struct S {     explicit S(int) {} };  int main() {     std::tuple<int, std::string> t1 = { 1, "hello" }; // ok     std::tuple<std::string> t2      = "hello";        // ok     std::tuple<int, S> t3           = { 1, 1 };       // fail: an *element* is to be constructed explicitly }