How to initialize char[] in a struct with constexpr char[] using initializer list?

Question

I have a struct like:

constexpr char defaultName[6] = "Hello";

struct MyStruct{
   char name[6];
   int val;
};

Now I want to initialize the struct using initializer list as following:

MyStruct structObj{defaultName, 0};

However, this doesn't work. How do I initialize the char array in initializer using defaultName?

I used MyStruct structObj{defaultName, 0}; but this doesn't work. I know I can do {"Hello", 0} but I would like to use constexpr. Also, strcpy works but just wanted to know if there is a way to use the defaultName in initializer list.

Note: The struct is defined in a common library C header which is used by the C++ code so I can't update the struct definition.

plain old raw C arrays are not copyable. If you are dealing with strings, you should use `std::string`. — NathanOliver, Jan 25 '23 at 20:12

Mooing Duck · Accepted Answer · 2023-01-25T20:38:37.137

1

Arrays aren't copiable normally, but std::array is

constexpr std::array<char,6> defaultName {'H', 'e', 'l', 'l', 'o', '\0'};

struct MyStruct{
   std::array<char,6> name;
   int val;
};

MyStruct structObj{defaultName, 0};

If you can't update the struct, then use a default struct instead:

struct MyStruct{
   char name[6];
   int val;
};
constexpr MyStruct defaultStruct{"Hello",0};

MyStruct structObj(defaultStruct);

edited Jan 25 '23 at 20:38

answered Jan 25 '23 at 20:23

Mooing Duck

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Unfortunately, I can't update the struct to use modern c++ STL array due to some legacy thing. It has to be C-style array. – Dev G Jan 25 '23 at 20:28
2

@DevG You can use `constexpr` but you can't use `std::array`? The both started in the same version of C++, C++11. – NathanOliver Jan 25 '23 at 20:34

Ted Lyngmo · Answer 2 · 2023-01-25T21:21:39.250

In C++, you could create a helper function to initialize the C struct.

In C++17:

namespace detail {
template <std::size_t... Is>
constexpr MyStruct make_MyStruct_helper(std::index_sequence<Is...>,
                                        const char (&in)[sizeof...(Is)], int v) {
    return {{in[Is]...}, v};
}
}  // namespace detail

template <std::size_t N>
constexpr MyStruct make_MyStruct(const char (&in)[N], int v) {
    return detail::make_MyStruct_helper(std::make_index_sequence<N>(), in, v);
}

In C++20 and later:

template <std::size_t N>
constexpr MyStruct make_MyStruct(const char (&in)[N], int v) {
    return [&]<std::size_t... Is>(std::index_sequence<Is...>) {
        return MyStruct{{in[Is]...}, v};
    }(std::make_index_sequence<N>());
}

Then used like this:

constexpr char defaultName[6] = "Hello";

int main() {
    constexpr auto structObj = make_MyStruct(defaultName, 0);
    constexpr auto another = make_MyStruct("foo", 1);
}

How to initialize char[] in a struct with constexpr char[] using initializer list?

2 Answers2