Thread: Class member initialization

Hi,

I've just figured out something I find quite astonishing:

Code:

struct base {
    base(std::string label);
};


struct type {
    type()=default; // makes errors below vanish
    type(std::size_t value);
};


int main()
{
    struct first: base {
        first(std::string label);
        
        type member{0};
    } first{"first"}; // successful compilation
    
    struct second: base {
        using base::base;
    } second{"second"}; // successful compilation
    
    struct third: base {
        using base::base;
        
        type member{0};
    } third{"third"}; // compilation failure if `type::type()' isn't defined
}

As you can see, inheriting the base constructor doesn't work when having defined a member variable of custom type. The interesting part is that the reason for this seems to be the initialization process of this variable.

Simplified compiler output:
- error: use of deleted function `third::third(std::string)'
- note: `third::third(std::string)' is implicitly deleted because the default definition would be ill-formed
- error: no matching function for call to `type::type()'

How can this happen? Why does the third class try to default-initialize the member variable? I'd be grateful for answers!

Sorry, I just wanted to demonstrate what I want to do. Hope this helps:

Code:

#include <cstddef>
#include <string>


struct base {
    base(std::string label) {}
};
 
 
struct type {
    type(std::size_t value) {}
};
 
 
int main()
{
    struct object: base {
        using base::base;
        
        type member{0};
    } object{"object"};
}

Code:

test.cc: In function ‘int main()’:
test.cc:20:20: error: use of deleted function ‘main()::third::third(std::string)’
     } third{"third"};
                    ^
test.cc:17:21: note: ‘main()::third::third(std::string)’ is implicitly deleted because the default definition would be ill-formed:
         using base::base;
                     ^
test.cc:17:21: error: no matching function for call to ‘type::type()’
test.cc:10:5: note: candidate: type::type(std::size_t)
     type(std::size_t value);
     ^
test.cc:10:5: note:   candidate expects 1 argument, 0 provided
test.cc:9:8: note: candidate: constexpr type::type(const type&)
 struct type {
        ^
test.cc:9:8: note:   candidate expects 1 argument, 0 provided
test.cc:9:8: note: candidate: constexpr type::type(type&&)
test.cc:9:8: note:   candidate expects 1 argument, 0 provided

What I don't understand is how the switch from a custom constructor to a inherited one can change member initialization when both do the same thing?

Could that mean it is a compiler-specific issue? I've just tried out clang - the same code compiled and linked without complaints!

Code:

$ g++ --version

g++ (GCC) 5.2.0
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Code:

$ clang++ --version

clang version 3.7.0 (tags/RELEASE_370/final)
Target: x86_64-unknown-linux-gnu
Thread model: posix

Okay, I'll try to build the current development version of GCC the next few days and see if the problem is fixed, otherwise I'll have a look at bugzilla. Thanks for your efforts, they're very much appreciated!