Poco::Dynamic::Var doesn't cast to Poco::Data::BLOB

[TodorHryn]

Describe the bug
Compilation fails when trying to cast Poco::Dynamic::Var to Poco::Data::BLOB

To Reproduce

#include <Poco/Data/LOB.h>
#include <Poco/Dynamic/Var.h>

int main()
{
	std::vector<uint8_t> data = { 1, 2, 3 };
	Poco::Data::BLOB blob1(data);
	Poco::Dynamic::Var dvar(blob1);
	Poco::Data::BLOB blob2 = dvar;  //<- fails here
	return 0;
}

Compiler output:

1>  main.cpp
1>  In file included from /mnt/c/Users/User/source/repos/Test2/Test2/main.cpp:2:
1>  /mnt/d/Libraries/poco/Foundation/include/Poco/Dynamic/Var.h: In instantiation of ‘Poco::Dynamic::Var::operator T() const [with T = Poco::Data::LOB<unsigned char>]’:
1>  /mnt/c/Users/User/source/repos/Test2/Test2/main.cpp:9:27:   required from here
1>  C:\Users\User\source\repos\Test2\Test2\main.cpp(9,27): message : required from here
1>  /mnt/d/Libraries/poco/Foundation/include/Poco/Dynamic/Var.h:195:41: error: no matching function for call to ‘Poco::Dynamic::VarHolder::convert(Poco::Data::LOB<unsigned char>&)’
1>  D:\Libraries\poco\Foundation\include\Poco\Dynamic\Var.h(195,41): error : no matching function for call to ‘Poco::Dynamic::VarHolder::convert(Poco::Data::LOB<unsigned char>&)’
1>    195 |                         pHolder->convert(result);
1>        |                         ~~~~~~~~~~~~~~~~^~~~~~~~
1>  In file included from /mnt/d/Libraries/poco/Data/include/Poco/Data/LOB.h:23,
1>                   from /mnt/c/Users/User/source/repos/Test2/Test2/main.cpp:1:
1>  /mnt/d/Libraries/poco/Foundation/include/Poco/Dynamic/VarHolder.h:455:13: note: candidate: ‘virtual void Poco::Dynamic::VarHolder::convert(Poco::Int8&) const’
1>  D:\Libraries\poco\Foundation\include\Poco\Dynamic\VarHolder.h(455,13): message : candidate: ‘virtual void Poco::Dynamic::VarHolder::convert(Poco::Int8&) const’
1>    455 | inline void VarHolder::convert(Int8& /*val*/) const
1>        |             ^~~~~~~~~
1>  /mnt/d/Libraries/poco/Foundation/include/Poco/Dynamic/VarHolder.h:455:32: note:   no known conversion for argument 1 from ‘Poco::Data::LOB<unsigned char>’ to ‘Poco::Int8&’ {aka ‘signed char&’}
1>  D:\Libraries\poco\Foundation\include\Poco\Dynamic\VarHolder.h(455,32): message : no known conversion for argument 1 from ‘Poco::Data::LOB<unsigned char>’ to ‘Poco::Int8&’ {aka ‘signed char&’}
1>    455 | inline void VarHolder::convert(Int8& /*val*/) const
1>        |                                ^~~~~
...

As as workaround for now I am using
const Poco::Data::BLOB &blob2 = dvar.extract<Poco::Data::BLOB>();

[andrewauclair]

Looks similar to #4214.

[aleks-f]

Not a bug. The solution is possible but I'm not sure I'm willing to let it in because it requires forward declaring template objects from Poco::Data namespace in Foundation and adding VarHolder::convert(BLOB&) overloads.

poco/Data/include/Poco/Data/DynamicLOB.h

Lines 26 to 33 in 4a9285c

	namespace Poco {
	namespace Data {
	template <typename T> class LOB;
	using BLOB = LOB<unsigned char>;
	using CLOB = LOB<char>;
	} } // namespace Poco::Data

If you really need it, you can send a pull and we'll consider it. Otherwise, it'll stay as is.