如何使用可变参数模板转换每种变体类型

Question

What I want to do is to cast each "VariantType" (union of different types) argument to it's type before calling some function from a shared library. what I'm doing so far now is down below. it's just 3 different types and 2 arguments and it takes a lot lines. but I want to achieve this with variant number of argument with 7 different types. this has to do something with variadic template( another relevant question: Template tuple - calling a function on each element ). or if you know some better way let me know.

template<typename... T>
int call(const char* func_name, T... args) {
    // this will call func_name(args...) somehow from a dll binary.
    // If you need the implementation : https://hastebin.com/ocoyaniruj.cpp
}

int main(int argc, char** argv) {

    const char* func_name = "func_name";
    VariantType arg1 = "hello world!";
    VariantType arg2 = 3.14;

    if (arg1.get_type() == VariantType::INT) {
        if (arg2.get_type() == VariantType::INT) {
            call(func_name, (int)arg1, (int)arg2);
        } else if (arg2.get_type() == VariantType::FLOAT){
            call(func_name, (int)arg1, (float)arg2);
        } else if (arg1.get_type() == VariantType::STRING){
            call(func_name, (int)arg1, arg2.c_str());
        }

    } else if (arg1.get_type() == VariantType::FLOAT){
        if (arg2.get_type() == VariantType::INT) {
            call(func_name, (float)arg1, (int)arg2);
        } else if (arg2.get_type() == VariantType::FLOAT){
            call(func_name, (float)arg1, (float)arg2);
        } else if (arg1.get_type() == VariantType::STRING){
            call(func_name, (float)arg1, arg2.c_str());
        }

    } else if (arg1.get_type() == VariantType::STRING){
        if (arg2.get_type() == VariantType::INT) {
            call(func_name, arg1.c_str(), (int)arg2);
        } else if (arg2.get_type() == VariantType::FLOAT){
            call(func_name, arg1.c_str(), (float)arg2);
        } else if (arg1.get_type() == VariantType::STRING){
            call(func_name, arg1.c_str(), arg2.c_str());
        }
    }
    return 0;
}

Answer 1

You have a messy, and very partial, runtime reflection mechanism. I'm sure you're feeling a lot of pain with this stuff... so the first things to consider would be:

• Do you really need this? If you can avoid it and stick to compile-time reflection, that would make life easier for you; or
• Is C++ the language you want to be using? If you're just getting started with your programming project, and this kind of run-time polymorphism is critical for you, perhaps another language (eg an interpreted language?) might be more appropriate.

Having said that - you could, in many cases, be satisfied with C++'s own variant type: std::variant introduced in C++17, together with std::visit , and a templated visitor (as in this question - but with two templates).

Here's what this would look like:

#include <variant>
#include <iostream>
#include <string>

int main() {
    using variant_type = std::variant<int, float, std::string>;
    variant_type v1{"hello world!"};
    variant_type v2{3.14f};

    std::visit([](auto&& x, auto&& y) { 
        // I used this as a stub:
        //
        //   std::cout << x << " , " << y << '\n';
        //
        // but you want:
        call(func_name, x, y);
    }, v1, v2);
}

GodBolt

There is one caveat, though - this won't extract the c_str() from your strings. If you want to also do that, you can do one of two things:

1. Store a const char* in the variant to begin with.
2. instead of passing x and y , you can have a templated transformer function, which does nothing usually, but applies .c_str() for const string references.

but I don't really like the second approach.

Answer 2

I've finally found the simple way to cast each argument with a variadic template (kinda reinvent the wheel std::visit ).

template<typename... Targs>
void visit(const char* func_name, int visiting_arg, VariantType front, Targs... p_args) {

    if (visiting_arg == 0) {
        call(func_name, p_args...);
        return;
    }

    if (front.get_type() == VariantType::INT) {
        visit(func_name, visiting_arg-1, p_args..., (int)front);

    } else if (front.get_type() == VariantType::FLOAT) {
        visit(func_name, visiting_arg - 1, p_args..., (float)front);

    } else if (front.get_type() == VariantType::STRING) {
        visit(func_name, visiting_arg - 1, p_args..., front.c_str());

    }

}

int main(int argc, char** argv) {

    const char* func_name = "func_name";
    int argcount = 3;

    VariantType s = "Hello world!";
    VariantType f = 3.14;
    VariantType i = 42;

    visit(func_name, argcount, s, f, i, VariantType("--placeholder--"));

    return 0;
}