std :: integral_constant <char *，“kernel32.dll”=“”>不会编译？</char *，>

Question

我想从char *和“kernel32.dll”制作整数常量，但总是失败。以下是我失败的尝试，有人可以告诉我正确的用法吗？

error 1: cout << std::integral_constant<const char*, "kernel32.dll">::value << endl;
error 2: cout << std::integral_constant<char*, "kernel32.dll">::value << endl;
error 3: cout << std::integral_constant<char[], "kernel32.dll">::value << endl;
error 4: cout << cout << std::integral_constant<char*, static_cast<char*>("kernel32.dll")>::value << endl;

以上4个语句具有相同的错误信息。：

Console.cpp(181): error C2762: 'std::integral_constant' : invalid expression as a template argument for '_Val'
1>          D:\Programfiles\Visual Studio 2013\VC\include\xtr1common(35) : see declaration of 'std::integral_constant'
1>Console.cpp(181): error C2955: 'std::integral_constant' : use of class template requires template argument list
1>          D:\Programfiles\Visual Studio 2013\VC\include\xtr1common(35) : see declaration of 'std::integral_constant'
1>Console.cpp(181): warning C4552: '<<' : operator has no effect; expected operator with side-effect

更新

std::integral_constant<std::string, "abc">::value也不会编译。

结束更新

这是我的场景，我做了一个简单的演示来演示我的目的：

#include <iostream>
#include <type_traits>

template< typename R, typename C, typename... Args>
class delegate
{
public:
    template<R(C::*F)(Args...), typename ... Ts>
    struct adapter
    {
        static R invoke_no_fwd(Args... args)
        {
            C t((Ts::value)...);
            return (t.*F)(args...);
        }
    };
};

class Class
{
public:
    Class(const char* psz) {
        std::cout << psz << std::endl;
    }
    void print(int v)
    {
        std::cout << "Class: " << v << std::endl;
    }
};

int main()
{
    typedef void(*function_t)(int);
    function_t ptrFunc = delegate<void, Class, int>::adapter<&Class::print, std::integral_constant<char*, "money"> >::invoke_no_fwd;
    auto type = delegate<void, Class, int>::adapter<&Class::print, std::integral_constant<int, 42>>::invoke_no_fwd;
    ptrFunc(-42); // 0
    type(0); // 42

    return 0;
}

Answer 1

代码中的模板参数类型（目前使用std::integral_constant<>实例化）仅用于访问::value静态成员，因此您可以将其替换为定义value成员的任何其他类型，如下图所示：

#include <iostream>

template <typename T>
void print()
{
    std::cout << (T::value) << std::endl;
}

struct X
{
    static const char* value;
};

const char* X::value = "ABC";

int main()
{
    print<X>();
}

也就是说，只需将X代替std::integral_constant<>。

function_t ptrFunc
    = delegate<void, Class, int>
          ::adapter<&Class::print, X /*here!*/>
          ::invoke_no_fwd;

Live demo link.

---

更新1

如果你想在模板实例化中指定字符串的内联内容，下面的代码就可以了：

template <char... Chars>
struct MyString
{
    static constexpr char value[] = { Chars..., '\0' };
};

template <char... Chars>
constexpr char MyString<Chars...>::value[];

// MyString<'A', 'B', 'C'>::value is same as const char[4] = { "ABC" };

function_t ptrFunc
    = delegate<void, Class, int>
          ::adapter<&Class::print, MyString<'A', 'B', 'C'>>
          ::invoke_no_fwd;

Another live demo link.

---

更新2

如果您厌倦了输入MyString<'k','e','r','n','e','l','3','2','.','d','l','l'>，则可以使用下面的宏（为简单起见，将"kernel32.dll"等原始字符串扩展为符合MyString<char...>模板的逗号分隔字符列表限于32个字符长的字符串）：

#include <iostream>

#define STR_1(S,I) (I < sizeof(S) ? S[I] : '\0')
#define STR_2(S,I) STR_1(S,I), STR_1(S,I+1)
#define STR_4(S,I) STR_2(S,I), STR_2(S,I+2)
#define STR_8(S,I) STR_4(S,I), STR_4(S,I+4)
#define STR_16(S,I) STR_8(S,I), STR_8(S,I+8)
#define STR_32(S,I) STR_16(S,I), STR_16(S,I+16)
#define STR(S) STR_32(S,0)

template <char... Chars>
struct MyString
{
    static constexpr char value[] = { Chars..., '\0' };
};

template <char... Chars>
constexpr char MyString<Chars...>::value[];

int main()
{
    std::cout << MyString<STR("kernel32.dll")>::value << std::endl;
}

Yet another live demo link

Answer 2

您可以声明static const char*并在std::integral_constant中使用它，例如：

static constexpr const char money[] = "money";

int main()
{
    typedef void(*function_t)(int);
    function_t ptrFunc =
        delegate<void, Class, int>
            ::adapter<
                &Class::print,
                std::integral_constant<const char*, money> >::invoke_no_fwd;
    auto type = delegate<void, Class, int>::
        adapter<&Class::print, std::integral_constant<const char*, money>>::invoke_no_fwd;
    ptrFunc(-42); // 0
    type(0); // 42

    return 0;
}

Live example

您可以使用Aligning static string literals中的内容来允许使用宏在std::integral_constant中写入文字字符串。