intuitive string-to-type conversion in C++ using user defined conversion; use `int i = val("22");`

_main.cpp

#include "headers.h"

struct Val_t {
    const std::string_view s;

    template<typename T>
    operator T() const { return S<T>::cast(s); }

    // can't have partial function specialization,
    // hence we use an intermediate structure
    template<typename T>
    struct S {
        static T cast(const std::string_view &s);
    };
};

using namespace std;

template<> int Val_t::S<int>::cast(const string_view &s) { return 22; }

template<typename T>
//#define IF_ONLY_I_COULD_WRITE
#ifdef IF_ONLY_I_COULD_WRITE
// error C2244: 'Val_t::S<std::vector<T,std::allocator<_Ty>>>::cast': unable to match function definition to an existing declaration
auto Val_t::S<vector<T>>::cast(const string_view &s) {
        return vector<T>{ 1, 2, 4 };
}
#else
struct Val_t::S<vector<T>> {
    static auto Val_t::S<vector<T>>::cast(const string_view &s) {
        return vector<T>{ 1, 2, 4 };
    }
};
#endif

int main()
{
    int i = Val_t{"22"};
    assert(i == 22);
  
    vector<int> ints = Val_t{"1, 2, 4"};
    assert((ints == vector<int>{ 1, 2, 4 }));
}

headers.h

#include <iostream>
#include <string>
#include <stdexcept>
#include <vector>
#include <iterator>
#include <algorithm>
#include <numeric>
#include <cassert>
#include <string_view>