Why does auto x{3} deduce an initializer_list?

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To make long story short:

  • a braced initializer expression {} has no type by itself
  • auto has to infer type information
  • int{3} obviously means “create an int var with value taken from initializer list”, thus its type is just int and can be used in any wider context (int i = int{3} will work and auto i = int{3} can deduce type, because right side is obviously of type int)
  • {3} by itself has no type (it can’t be int, because it’s not a value but an initializer list), so auto wouldn’t work — but, because committee considered that auto should still work in this case, they decided that the “best” type for (yeah, typeless by definition) initializer list would be… std::initializer_list, as you already probably guessed.

But, as you pointed out, this made the whole behaviour of auto quite semantically inconsistent. That’s why there were proposals to change it — namely N3681, N3912 and N3922 — submitted to the committee. Former proposal was REJECTED as FI3 due to no committee consensus on this matter, http://isocpp.org/files/papers/n3852.html#FI3 , current (N3922) got adopted ca. Q1 of 2015;

tl;dr you may assume that standards-compliant compilers1 with bleeding-edge C++ support2 either have the new, more sane-ish semantics already in place, or will have it shortly.

The Standardization Committee acknowledged the problem by adopting N3922 into draft C++17.

— so it’s 

auto x1 = { 1, 2 }; // decltype(x1) is std::initializer_list<int>
auto x2 = { 1, 2.0 }; // error: cannot deduce element type
auto x3{ 1, 2 }; // error: not a single element
auto x4 = { 3 }; // decltype(x4) is std::initializer_list<int>
auto x5{ 3 }; // decltype(x5) is int

now, for better or worse.

further reading:

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3681.html

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3912.html

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3922.html

http://scottmeyers.blogspot.com/2014/03/if-braced-initializers-have-no-type-why.html

http://herbsutter.com/2014/11/24/updates-to-my-trip-report/

1GCC 5.1 (& up) apparently uses N3922 even in C++11/C++14 mode

2Clang 3.8, with the caveat

This is a backwards-incompatible change that is applied to all language versions that allow type deduction from auto (per the request of the C++ committee).

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