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c++c++11initializationinitializer-list
VP.
VP.

Reputation: 16735

C++11 POD structure initializing error

I have confusing situation with simple code:

struct Item {
    size_t span{};
};

int main() {
    Item item{1}; // error is here
    return 0;
}

While compiling this I have following error:

test.cpp: In function ‘int main()’:
test.cpp:8:13: error: no matching function for call to ‘Item::Item(<brace-enclosed initializer list>)’
     Item i{1};
             ^
test.cpp:8:13: note: candidates are:
test.cpp:3:8: note: constexpr Item::Item()
 struct Item {
        ^
test.cpp:3:8: note:   candidate expects 0 arguments, 1 provided
test.cpp:3:8: note: constexpr Item::Item(const Item&)
test.cpp:3:8: note:   no known conversion for argument 1 from ‘int’ to ‘const Item&’
test.cpp:3:8: note: constexpr Item::Item(Item&&)
test.cpp:3:8: note:   no known conversion for argument 1 from ‘int’ to ‘Item&&’

Why g++ tries to find a ctor for initializer list in this case instead of simple C-style structure object creating?

If I remove {} from size_t span{} it compiles successfully.

It also happens if I change the line to size_t span = 0 so it seems to be some initialization in declaration issue which exists since c++11.

Upvotes: 2

Views: 205

Answers (1)

Angew is no longer proud of SO
Angew is no longer proud of SO

Reputation: 171127

Usign Item item{1}; means you're doing list-initialisation (of item). List initialisation is defined as follows:

  • if the type is an aggregate, aggregate initialisation (what you refer to as "C-style struct object creating") happens
  • ...
  • if the type is a class, constructors are considered

Your class has no constructors. It is also not a (C++11) aggregate, because it contains an initialiser for a non-static data member.

Note that this restriction (member initialisers) was lifted in C++14, so Item is a C++14 aggregate and your code, while not valid C++11, is valid C++14.

Upvotes: 5

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