Clang vs GCC vs MSVC - 使用大括号括起初始化列表创建用户定义类型？

Question

Was experimenting with brace-enclosed initializer list and compilers seems to behave differently (well, it's a fairly new feature), but was wondering what should be the correct behavior and especially if 'c3' mentioned below should actually compiles with private, default initialized variable?

int main() {
    struct c1 {
        bool b;
        char c;
        int i;
    };

    auto c1_ = c1{true, '0', 0};
    //clang-3.6 - okay
    //gcc-4.9   - okay
    //vs-2013   - okay

    // ------------------------------------------

    struct c2 {
        bool b;
        char c;
        int i = 0; // with default value
    };

    auto c2_ = c2{true, '0', 0};
    //clang-3.6 - okay
    //gcc-4.9   - error: no matching function for call to ‘c2::c2(<brace-enclosed initializer list>)’
    //vs-2013   - error C2440: '<function-style-cast>' : cannot convert from 'initializer-list' to 'c2'

    auto c2__ = c2{true, '0'};
    //clang-3.6 - okay
    //gcc-4.9   - error: no matching function for call to ‘c2::c2(<brace-enclosed initializer list>)’
    //vs-2013   - error C2440: '<function-style-cast>' : cannot convert from 'initializer-list' to 'c2'

    // ------------------------------------------

    struct c3 {
        bool b;
        char c;
    private:
        int i = 0; // with private default value
    };

    auto c3_ = c3{true, '0'};
    //clang-3.6 - error: no matching constructor for initialization of 'c3'
    //gcc-4.9   - error: no matching function for call to ‘c3::c3(<brace-enclosed initializer list>)’
    //vs-2013   - error C2440: '<function-style-cast>' : cannot convert from 'initializer-list' to 'c3'
}

Answer 1

This is aggregate initialization.

C++11 paragraph that defines an aggregate says (n3337, §8.5.1/1):

An aggregate is an array or a class (Clause 9) with no user-provided constructors (12.1), no brace-or-equal initializers for non-static data members (9.2), no private or protected non-static data members (Clause 11), no base classes (Clause 10), and no virtual functions (10.3).

This means that c2 is not an aggregate and auto c2_ = c2{true, '0', 0}; should trigger a diagnostic in C++11 (and gcc is right).

However, with clang, you seem to compile in C++14 mode, which removed the "no brace-or-equal initializers" limitation (n3690, §8.5.1/1):

An aggregate is an array or a class (Clause 9) with no user-provided constructors (12.1), no private or protected non-static data members (Clause 11), no base classes (Clause 10), and no virtual functions (10.3).

So, auto c2_ = c2{true, '0', 0}; is okay in C++14.

About auto c2__ = c2{true, '0'}; , §8.5.1/7 (n3690) says:

If there are fewer initializer-clauses in the list than there are members in the aggregate, then each member not explicitly initialized shall be initialized from its brace-or-equal-initializer or, if there is no brace-or-equal-initializer , from an empty initializer list (8.5.4).

Which means it's ok and clang is right to compile it in C++14 mode.

c3 is not an aggregate (because of private member) neither in C++11 nor in C++14 so all compilers are correct to reject it.

Answer 2

 auto c2_ = c2{true, '0', 0};
 auto c2__ = c2{true, '0'};

These are fine in C++14. The in-class initialization does not stop the type from being an aggregate in C++14, and the in-class value will be used when a value is not provided by the brace-initializer.

auto c3_ = c3{true, '0'};

This is an error, because aggregates cannot have private: or protected: members. Since it's not an aggregate you only have the implicitly declared default constructor. You can do the following:

auto c3_ = c3{};

Answer 3

c2 is not an aggregate in C++11, but is in C++14. So the code should not compile in the former, and should in the latter. It looks like clang is compiling in C++14 mode.

c3 is not an aggregate due to the presence of a private data member, so it should not compile, regardless of the standard..