maxbc maxbc - 4 months ago 14
C++ Question

Inheriting constructors and brace-or-equal initializers

I don't understand why you can't compile a class which has both a member (not default constructible) with a brace-or-equal initializer and an inherited constructor. g++ says :


test.cpp:22:15: error: use of deleted function ‘Derived::Derived(float)’

Derived d(1.2f);

test.cpp:16:13: note: ‘Derived::Derived(float)’ is implicitly deleted

because the default definition would be ill-formed:

using Base::Base;

test.cpp:16:13: error: no matching function for call to ‘NoDefCTor::NoDefCTor()’

test.cpp:5:1: note: candidate:

NoDefCTor::NoDefCTor(int) NoDefCTor(int) {}


Code that fails to compile (under g++ 5.1):

struct NoDefCTor
{
NoDefCTor(int) {}
};

struct Base
{
Base(float) {}
};

struct Derived : Base
{
using Base::Base;
NoDefCTor n2{ 4 };
};

int main()
{
Derived d(1.2f);
}


Code that compiles, but never uses
NoDefCTor
's default constructor (despite apparently needing it!):

struct NoDefCTor
{
NoDefCTor(int) {}
NoDefCTor() = default;
};

struct Base
{
Base(float) {}
};

struct Derived : Base
{
using Base::Base;
NoDefCTor n2{ 4 };
};

int main()
{
Derived d(1.2f);
}


I don't really like the idea of having a default constructor when I don't need one. On a side note both versions compile (and behave) just fine on MSVC14.

dyp dyp
Answer

This is a gcc bug, #67054. Comparing the bug report by alltaken380 to the OP's case:

// gcc bug report                        // OP
struct NonDefault                        struct NoDefCTor
{                                        {
    NonDefault(int) {}                       NoDefCTor(int) {}
};                                       };

struct Base                              struct Base
{                                        {
    Base(int) {}                             Base(float) {}
};                                       };

struct Derived : public Base             struct Derived : Base
{                                        {
    NonDefault foo = 4;                      NoDefCTor n2{ 4 };

    using Base::Base;                        using Base::Base;
};                                       };

auto test()                              int main()
{                                        {
    auto d = Derived{ 5 };                   Derived d(1.2f);
}                                        }

We can even try this on recent gcc 6.0 versions, and it still fails to compile. clang++3.6 and, according to the OP, MSVC14 accept this program.

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