Why can't MSVC and GCC initialize a struct with a field with a default value

Question

int main() {
  struct WorkItem {
    int node;
    unsigned predecessorIndex = 0;
  };  

  auto x = WorkItem { 0 };

  return 0;
}

This code compiles fine with Clang, but not with GCC:

source_file.cpp: In function ‘int main()’:

source_file.cpp:9:25: error: no matching function for call to ‘main()::WorkItem::WorkItem()’ auto x = WorkItem { 0 }; ^

source_file.cpp:9:25: note: candidates are:

source_file.cpp:4:10: note: main()::WorkItem::WorkItem() struct WorkItem { ^

source_file.cpp:4:10: note: candidate expects 0 arguments, 1 provided

source_file.cpp:4:10: note: constexpr main()::WorkItem::WorkItem(const main()::WorkItem&)

source_file.cpp:4:10: note: no known conversion for argument 1 from ‘int’ to ‘const main()::WorkItem&’

source_file.cpp:4:10: note: constexpr main()::WorkItem::WorkItem(main()::WorkItem&&)

source_file.cpp:4:10: note: no known conversion for argument 1 from ‘int’ to ‘main()::WorkItem&&’

Or MSVC:

source_file.cpp(9): error C2440: 'initializing': cannot convert from 'initializer list' to 'main::WorkItem'

source_file.cpp(9): note: No constructor could take the source type, or constructor overload resolution was ambiguous

Is Clang incorrectly compiling this code, or are MSVC and GCC wrong, from a standards point of view? Also, why does removing the = 0 allow GCC and MSVC to compile? E.g.

int main() {
  struct WorkItem {
    int node;
    unsigned predecessorIndex = 0;
  };  

  auto x = WorkItem { 0 };

  return 0;
}

GCC version: 4.9.3 Clang version: 3.7.0 MSVC version: 19.00.24215.1

Show the entire compile command, including all options. The -std= option is especially going to affect the result. Also, that's a very old version of g++. — Ben Voigt
– Ben Voigt, Commented Dec 27, 2016 at 15:49

Nicol Bolas · Accepted Answer · 2016-12-27 15:49:40Z

6

GCC version: 4.9.3

Aggregate initialization with default member initializers is a C++14 feature which GCC does not support until GCC 5.x.

MSVC version: 19.00.23506

I believe this is Update 1 of VC 2015. Aggregate initialization with default member initializers is a C++14 feature that VC doesn't support until 2017.

answered Dec 27, 2016 at 15:49

Nicol Bolas

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4 Comments

H Bellamy Over a year ago

I got the MSVC version wrong: 19.00.24215.1 is the correct one

Kerrek SB Over a year ago

In other words, in C++11 WorkItem is not an aggregate, but in C++14 it is.

Nicol Bolas Over a year ago

@HBellamy: That's 2015 Update 3 (it'd be much easier if you just listed the year and update number. Using versions makes me have to go look up what they mean). It's still not VC 2017, which is when VC implemented this feature.

H Bellamy Over a year ago

Thanks! I can confirm this now here: learn.microsoft.com/en-gb/cpp/cpp-conformance-improvements-2017

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