Defining a function using a function typedef in C++

Question

I am implementing a service in Windows. VisualStudio 2012 has the following function typedef:

typedef VOID WINAPI SERVICE_MAIN_FUNCTIONW (
    DWORD dwNumServicesArgs,
    LPWSTR *lpServiceArgVectors
);

There is also a function pointer typedef:

typedef VOID (WINAPI *LPSERVICE_MAIN_FUNCTIONW)(
    DWORD   dwNumServicesArgs,
    LPWSTR  *lpServiceArgVectors
);

How do I define a function with this function signature, using the typedef?

Answer 1

Quoting the current C++ Standard (C++11):

[dcl.fct]/10

A typedef of function type may be used to declare a function but shall not be used to define a function (8.4). [Example:

typedef void F();
F fv;             // OK: equivalent to void fv();
F fv { }          // ill-formed
void fv() { }     // OK: definition of fv

—end example ]

That is, you can declare but not define the function using that typedef. You have to specify the signature explicitly, see Alex Farber's answer.

---

To some extent, you can use the typedef to "define" the function, but it involves some template magic. It's a just-for-fun example to show that you can actually use it to define a function.

// extra definitions for SSCCE
  typedef unsigned int DWORD;
  typedef wchar_t* LPWSTR;
  #define VOID void
  #define WINAPI

// function ptr
typedef VOID (WINAPI *LPSERVICE_MAIN_FUNCTIONW)(
    DWORD   dwNumServicesArgs,
    LPWSTR  *lpServiceArgVectors
);

// function typedef
typedef VOID WINAPI SERVICE_MAIN_FUNCTIONW (
    DWORD dwNumServicesArgs,
    LPWSTR *lpServiceArgVectors
);


template < typename... TT >
struct disassemble_funcptr
{};
template < typename Ret, typename... Args >
struct disassemble_funcptr < Ret(Args...) >
{
    typedef Ret return_type;

    static Ret Func(Args...)
    {
        /* your code here */
    }
};

// using the typedef SERVICE_MAIN_FUNCTIONW to define the function
LPSERVICE_MAIN_FUNCTIONW my_func_ptr =
  & disassemble_funcptr < SERVICE_MAIN_FUNCTIONW > :: Func;

int main()
{
    LPWSTR str = nullptr;
    my_func_ptr(42, &str);
}