Obtain integer from collection of possible types at compile time?-CodePudding

The following code does not compile, because I don't know if what I want to do is possible, but it does show what I would like. I build, at compile time (if possible!), a collection of types and integer values; this is then used at compile time in the assignment operator which looks at the type that has been passed and stores that the corresponding integer from the collection in the member variable type_:

struct MyStructure {
  MyStructure(char* d, int size) {}

  std::array<pair<TYPE, int>> map {
    { int, 1 },
    { char, 2 },
    { double, 4 },
    { std::string, 8 }
  };

  template <typename T>
  auto operator=(const T &arg) {
    // Depending on the type of 'arg', I want to write a certain value to the member variable 'type_'
  }
  int type_ = 0;
};

int main() {

  MyStructure myStruct;
  myStruct = 1;             // Should cause 1 to be stored in member 'type_ '
  myStruct = "Hello world"; // Should cause 8 to be stored in member 'type_'
}

I need to solve this in C 17 please; extra respect for anyone who additionally gives a C 20 solution as that would be a learning opportunity!

CodePudding user response：

Here's a basic blueprint that, with some cosmetic tweaks, can be slotted into your MyStructure:

#include <string>
#include <iostream>

template<typename T> struct type_map;

template<>
struct type_map<int> {
    static constexpr int value=1;
};

template<>
struct type_map<char> {
    static constexpr int value=2;
};

template<>
struct type_map<double> {
    static constexpr int value=4;
};

template<>
struct type_map<std::string> {
    static constexpr int value=8;
};

template<typename T>
void some_function(const T &arg)
{
    std::cout << type_map<T>::value << std::endl;
}

int main()
{
    some_function(1);                   // Result: 1
    some_function('a');                 // Result: 2
    some_function(std::string{"42"});   // Result: 8
    return 0;
}