Return any type of object from a method

Question

Return any type of object from a method

Flaviu_ 791

I have the following code:

struct SResponse
{
 SResponse(bool state, std::any value) : _state(state), _value(value) {}
 bool        _state = false;
 std::any    _value;
};

CSomeClass
{
public:
 SResponse GetResponse();
};

// implementation
SResponse CSomeClass::GetResponse()
{
 std:string message{"Aloha"};
 SResponse res(true, std::make_any<std::string>(message);

 return res;
}

Using code:

CSomeClass some;
std::any result = some.GetResponse();
std::string message{};
try { message = std::any_cast<std::string>(result.second); }
catch (...) {}

In fact, I have returned std::any. But I want to replace this with something else. But what ? As you can see, in std::any I can put anything, but what can I use instead of std::any ? Because std::any take pretty much memory at 64bit apps. Beside that, std::any is available only from c++17, and this could not be used on old code.

Igor Tandetnik 1,106 Reputation points

2021-02-17T14:39:17.283+00:00

The solution rather depends on how SResponse is expected to be used by the caller. In the example shown, how is the caller supposed to obtain the message? To extract the payload from std::any, one must know what type was packed into it. How does the caller of CSomeClass::GetResponse know that it was a std::string and not something else?
Viorel 106.4K Reputation points

2021-02-17T15:30:03.693+00:00

If you know all the returned types, consider variant instead of any. But since you need a solution for older compilers, maybe define a base abstract class and a series of derived classes for each case. The function will return a pointer to base class, or unique_ptr.

Or implement a template class, which is similar to any or variant from your point of view.

Flaviu_ 791

Using this code:

CSomeClass some;
std::any result = some.GetResponse();
std::string message{};
try { message = std::any_cast<std::string>(result.second); }
catch (...) {}

Jeanine Zhang-MSFT 7,696 Reputation points Microsoft Vendor

2021-02-18T05:57:59.763+00:00

@Flaviu_

std::any is available only from c++17, and this could not be used on old code.

Maybe you could try to use boost::any to instead.
Flaviu_ 791 Reputation points

2021-02-18T07:20:27.127+00:00

Yes, that could be a problem. And how to solve this issue ?

Igor Tandetnik 1,106 Reputation points

2021-02-17T14:39:17.283+00:00

The solution rather depends on how SResponse is expected to be used by the caller. In the example shown, how is the caller supposed to obtain the message? To extract the payload from std::any, one must know what type was packed into it. How does the caller of CSomeClass::GetResponse know that it was a std::string and not something else?
Viorel 106.4K Reputation points

2021-02-17T15:30:03.693+00:00

If you know all the returned types, consider variant instead of any. But since you need a solution for older compilers, maybe define a base abstract class and a series of derived classes for each case. The function will return a pointer to base class, or unique_ptr.

Or implement a template class, which is similar to any or variant from your point of view.
Flaviu_ 791 Reputation points

2021-02-17T15:33:15.87+00:00

Using this code:

CSomeClass some; std::any result = some.GetResponse(); std::string message{}; try { message = std::any_cast<std::string>(result.second); } catch (...) {}
Jeanine Zhang-MSFT 7,696 Reputation points Microsoft Vendor

2021-02-18T05:57:59.763+00:00

@Flaviu_

std::any is available only from c++17, and this could not be used on old code.

Maybe you could try to use boost::any to instead.
Flaviu_ 791 Reputation points

2021-02-18T07:20:27.127+00:00

Yes, that could be a problem. And how to solve this issue ?

Answer 1

Guido Franzke 2,201

Hello,
what is the benefit of this code?
I suggest you should use templates instead. templates

Regards, Guido

Flaviu_ 791 Reputation points

2021-02-17T15:31:24.297+00:00

But maybe using templates would complicate the response code. I'll update the original post.
Flaviu_ 791 Reputation points

2021-02-17T17:35:06.487+00:00

The fact is that SResponse could take care of any kind of "response", int, string, object, anything. That is the request.
Flaviu_ 791 Reputation points

2021-02-18T07:22:31.863+00:00

Maybe sound crazy, but implement this std::any approach with templates, will move all code inside header file (if I am right), and I don't really like that.
Guido Franzke 2,201 Reputation points

2021-02-18T07:33:03.827+00:00

The code of templates is generated at compile time. So all definitions of the class template must be present (in header file). You can separate the class declaration and class function definitions by using a .h (class declaration) and .hpp file (function definitions). But at the the end of the .h file you must include the hpp file (this is easier than always including the h and hpp file in the source code).

Return any type of object from a method

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