std::integral_constant<char*, "kernel32.dll"> won't compile? - c++

Question

I want to make integral constant from char* and "kernel32.dll", but failed always. The following are my failed attempts, anyone can show me the correct usage?
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;
the above 4 statements have the same error info.:
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
Update:
std::integral_constant<std::string, "abc">::value won't compile either.
end update
Here is my scenario, I make a simple demo to demonstrate my purpose:
#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

The template parameter type in your code, currently instantiated with std::integral_constant<>, is used only to access ::value static member, so you can replace it with any other type defining value member, just like shown below:
#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>();
}
That is, just put X in place of std::integral_constant<>.
function_t ptrFunc
= delegate<void, Class, int>
::adapter<&Class::print, X /*here!*/>
::invoke_no_fwd;
Live demo link.
UPDATE 1
If you want to specify the string's content inline, within template instantiation, the below code will do the trick:
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.
UPDATE 2
If you are tired of typing MyString<'k','e','r','n','e','l','3','2','.','d','l','l'> you can instead expand a raw string like "kernel32.dll" into comma-separated characters list compliant with MyString<char...> template, using below macro (that for simplicity is limited to 32-character-long strings):
#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

You may declare a static const char* and use it in std::integral_constant, something like:
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
You may use something like in Aligning static string literals to allow to write the literal string in std::integral_constant with a Macro.