Hello,
what I would like to implement, is that a template that wraps another
type T would delegate a cast operator to another type Target to T if
it exists in T.
Example attached, showing something moderately simple as an example,
and most of the body is to try assertions which confirm that SFINAE
for these types work in general.
The failing line works if the cast is of the form
template <class Target> operator Target () const { return *get(); };
// or not const
... but of course fails for other types if T is not castable to
Target, failing in a bad way, usually when compiler finds the cast
ambiguous before knowing it actually wouldn't work because T is not
castable to Target. This way the cast operator actually matches any
type, so it is not very useful.
Is there any other way to use SFINAE for the cast operator? I tried
putting the invalid type in various places, but partial member
template specialization is not allowed, cast operator must take void
(not even a default) and so on. (tried these with 4.4.1 version I
think, but I don't expect such a change between 4.4.1 and 4.4.3).
$ g++ -v
Using built-in specs.
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info
--with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-bootstrap
--enable-shared --enable-threads=posix --enable-checking=release
--with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-languages=c,c++,objc,
obj-c++,java,fortran,ada --enable-java-awt=gtk --disable-dssi
--enable-plugin
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-1.5.0.0/jre
--enable-libgcj-multifile --enable-java-maintainer-mode
--with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--disable-libjava-multilib --with-ppl --with-cloog --with-tune=generic
--with-arch_32=i586 --build=x86_64-redhat-linux
Thread model: posix
gcc version 4.4.1 20090725 (Red Hat 4.4.1-2) (GCC)
$ g++ -Wall -c cast_wrapped.cpp
cast_wrapped.cpp: In function ‘int main(int, char**)’:
cast_wrapped.cpp:141: error: conversion from
‘WrappedPtr<CastableToString>’ to non-scalar type ‘std::string’
requested
Or another version:
$ ~/soft/gcc/exec/bin/g++ -v
Using built-in specs.
Target: x86_64-unknown-linux-gnu
Configured with: ../../src/gcc-4.4.3/configure
--prefix=/home/u2/soft/gcc/exec --enable-libssp
--with-mpfr=/home/u2/soft/gcc/exec --with-gmp=/home/u2/soft/gcc/exec
--with-cloog=/home/u2/soft/gcc/exec --with-ppl=/home/u2/soft/gcc/exec
Thread model: posix
gcc version 4.4.3 (GCC)
$ ~/soft/gcc/exec/bin/g++ -Wall -c cast_wrapped.cpp
cast_wrapped.cpp: In function ‘int main(int, char**)’:
cast_wrapped.cpp:141: error: conversion from
‘WrappedPtr<CastableToString>’ to non-scalar type ‘std::string’
requested
Edek
Attachment:
cast_wrapped.cpp
Description: Binary data
