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c++incomplete-type

What is the difference between default constructed object return and empty braces return in C++?

In the following code:

#include <memory>

struct A;

std::unique_ptr<A> ok() { return {}; }

std::unique_ptr<A> error() { return std::unique_ptr<A>{}; }

Clang++ compiles fine ok() function and rejects error() function with the message:

In file included from /opt/compiler-explorer/gcc-11.1.0/lib/gcc/x86_64-linux-gnu/11.1.0/../../../../include/c++/11.1.0/memory:76:
/opt/compiler-explorer/gcc-11.1.0/lib/gcc/x86_64-linux-gnu/11.1.0/../../../../include/c++/11.1.0/bits/unique_ptr.h:83:16: error: invalid application of 'sizeof' to an incomplete type 'A'
        static_assert(sizeof(_Tp)>0,

Demo: https://gcc.godbolt.org/z/M8qofYbzn

If there any real difference between default constructed object return and empty braces return in C++ (both in general, and in this particular case)?

Solution

  • Neither should compile.

    The destructor for the result object is always potentially invoked. In the case of unique_ptr that requires A to be complete.