Can't assign string literal to boxed std::string vector

Question

This is a simplified version of my type system:

#include <string>
#include <vector>

template<typename T>
class Box {
public:
    Box(const T& value) : _value(value) {};
private:
    T _value;
    /* ... */
};

typedef Box<int> Int;
typedef Box<double> Double;
typedef Box<std::string> String;

int main(int argc, char* argv[]) {
    String a("abc");
    std::vector<String> b = { std::string("abc"), std::string("def") };

    // error C2664: 'Box<std::string>::Box(const Box<std::string> &)' : cannot convert argument 1 from 'const char' to 'const std::string &'
    std::vector<String> c = { "abc", "def" };
}

While a and b compile, c does not and the reason seems to be that I try to initialize from const char. This raises two questions:

1. Why is b possible but not c? Is it because of the nested template in std::vector<Box<std::string> >?

2. Can I make c work without destroying the general boxing mechanism (cf. typedef Box<double> Double?

Answer 1

c currently need 2 implicit user conversions (const char [N] -> std::string -> String) whereas only one is allowed.

You may add template constructor to Box

template<typename T>
class Box {
public:
    Box() = default;
    Box(const Box&) = default;
    Box(Box&&) default;
    ~Box() = default;

    Box& operator=(const Box&) = default;
    Box& operator=(Box&&) = default;

    template <typename U0, typename ...Us,
              std::enable_if_t<std::is_constructible<T, U0, Us...>::value
                               && (!std::is_same<Box, std::decay_t<U0>>::value
                                  || sizeof...(Us) != 0)>* = nullptr>
    Box(U0&& u0, Us&&... us) : _value(std::forward<U0>(u0), std::forward<Us>(us)...) {}
private:
    T _value;
    /* ... */
};

Demo Demo2