Boost how to create a map for types selection?

Question

so i use BOOST.EXTENTION to load modules. I have a special file that describes each module. I read variables from that file.

so such example:

shared_library m("my_module_name");
// Call a function that returns an int and takes a float parameter.
int result = m.get<int, float>("function_name")(5.0f);
m.close();

for me would turn into:

shared_library m("my_module_name");
// Call a function that returns an int and takes a float parameter.
int result = m.get<myMap["TYPE_1_IN_STRING_FORM"], myMap["TYPE_2_IN_STRING_FORM"]>("function_name")(5.0f);
m.close();

How to create such map that would map standard and costume types?

Update:

may be with variant:

  shared_library m("my_module_name");
  int result = m.get<boost::variant< int, float, ... other types we want to support >, boost::variant< int, float, ... other types we want to support > >("function_name")(5.0f);
    m.close();

can halp? so we would not care as long as all types we want are declared in it?

Answer 1

For that, you would need a heterogeneous map - that is, its elements can be of different types. Furthermore you would need the ability to return types from functions, not just variables.

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Now, a heterogeneous map would be possible with Boost.Variant or a simple union, but that binds it to compile time: we need to know every type that is possible to create that variant/union.
Of course a Boost.Any would be possible to store everything and its dog, but the problem strikes again: you need to extract the real type out of that Boost.Any again. The problem repeats itself. And if you know the real type, you can aswell just make a variant/union and save yourself the any_cast trouble.

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Now, for another troublesome thing:

m.get<myMap["TYPE_1_IN_STRING_FORM"], myMap["TYPE_2_IN_STRING_FORM"]>

To make the above line work, you'd need two features that C++ doesn't have: the ability to return types and runtime templates. Lets ignore the first point for a moment.
Templates are compile-time, and the get function is such a template. Now, to use that template, your myMap would need to be able to return types at compile-time, while getting populated at runtime. See the contradiction? That's why runtime templates would be needed.

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Sadly, exactly those three things are not possible (or extremely hard and very very limited) in C++ at runtime: heterogeneous data types (without constant size), returning types and templates.
Everything that involves types needs to be done at compile-time. This blogpost by @Gman somewhat correlates with that problem. It's definitly worth a read if you want to know what C++ just can't do.

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So, to conclude: You'll need to rethink and refactor your problem and solution. :|