I realized this is wrong but I could not delete it

Question

Author: This of course can not be done. Learn from the answers below.

In C++ how do we do the following

// fundamental language construct        
   type name = value ; 
// for example 
   int x = y;

with function pointers?

 typedef (char)(*FP)(unsigned);

 // AFAIK not possible in C++
 FP x = y ;

I can use lambdas:

 FP x = []( unsigned k) -> char { return char(k); }

But I do not know how to do this without the lambda. Any ideas?

Answer 1

The code:

typedef char(*FP)(int);
FP x = y;

fails to compile with current C++ compilers if y is a lambda expression capturing a variable.

// Compiles OK
FP x0 = [](int k) -> char { return char(k); };

// Fails to compile
int i = 123;
FP x1 = [=](int k) -> char { return char(k); };
FP x2 = [=](int k) -> char { return char(k+i); };
FP x3 = [&](int k) -> char { return char(k+i); };
FP x4 = [i](int k) -> char { return char(k+i); };
// error: cannot convert ‘main()::<lambda(int)>’ to ‘FP {aka char (*)(int)}’
//        in initialization

The reason why it fails to compile is that the size of the right side of the assignment to x1...x4 is greater than size of FP.

For a C++ compiler to make assignments to x1...x4 be valid it would need to generate code at runtime. Current C++ compilers such as GCC and clang do not support this, mainly because it would cause memory leaks because C++ isn't a garbage collected language. Some garbage collected language implementations, such as earlier versions of the official Go compiler, do support such assignments by performing runtime code generation.

Answer 2

You can use auto:

auto fptr = &f;

It skips the need of a typedef and conserve a nice syntax.

Answer 3

Whenever you can write a typedef, you can also write a variable declaration with no typedef, with almost identical syntax.

Example:

 // typedef
 typedef char(*FP)(unsigned);
 FP x = y ;

 // no typedef
 char(*x)(unsigned) = y;

Remove the typedef keyword, and you have a variable declaration. Slap an initialisation on it if you want.

Answer 4

Well... if you're using lambdas, you can also use auto, so

auto x = foo;

The following is a full compiling example with a static_assert() that verify the obtained type

#include <type_traits>

char foo (unsigned)
 { return ' '; }

int main ()
 {
   auto x = foo;

   static_assert( std::is_same<decltype(x), char(*)(unsigned)>::value, "!" );
 }

Using auto with lambda in the way you used it with FP

auto y = []() ->bool { return true; };

leads to something different: the type of y above is an unnamed class with an operator(), not a function pointer type to that operator().

If you want a pointer to function, you have to convert the lambda to it using the operator +, as you can verify with the following static_assert()

auto y = +[]() ->bool { return true; };

static_assert( std::is_same<decltype(y), bool(*)()>::value, "!" );

Answer 5

Many thanks all for the lively roller-coaster of useful comments. Somebody on Reddit, where I asked the same question, under the user name "TheTiefMaster", dropped this "one liner":

// also works as C
char whatever(unsigned k) { return char(k); } char(*F)(unsigned) = whatever;

Let me clarify: I do understand these are two statements on one line. And no there is no type in here, but one function pointer pointing to the same function. The usage:

    auto x = whatever(65); // 'A'
    auto y = F(66); // 'B'

Then I figured the following will make the function definition and its type declaration:

    // FP is a type of function whoever
    char whoever(unsigned k) { return 'A'; } typedef char(*FP)(unsigned) ;

Calling whoever behaves as expected

   auto w = whoever(42) ; // 'A'

FP is where it starts getting interesting. FP is a type, and as it turns out one can cast to the type.

     // using FP as a type
     // c++ style type cast
     // empty cast returns nullptr
     auto fun = FP();
     // calling fun() above crashes
     // but it is "invocable" as per C++ rules
     static_assert(std::is_invocable_v<P2F()>);

Passing any argument to this cast, works and returns non null address:

      // update: this compiles only on MSVC
      // and is a bug
      auto fun = FP(42); 
      // type of fun is char (*) (unsigned)

Calling the result of this fun crashes, obviously:

     // reading access violation
     fun(123) ;

This cast with an instance from any required function, works:

    auto fun = FP(whatever); 

    // works, obviously
    fun(65) ; // 'A'

To use this knowledge we will use the static_cast to safely cast to what we can call. C++ type cast is too forceful, just like C style type cast is.

     // does not compile
     // fun is the wrong type and can not be called
     auto fun = static_cast<FP>(42); 

     // does compile, fun is safe to call
         auto fun = static_cast<FP>(whatever);

    // works, obviously
    fun(65) ; // 'A'

This investigation is obviously far from over. I shall proceed with it, elsewhere.

Update:

       using FP = char (*)(int) ;
       // must not compile, compiles under MSVC
       auto oops = FP(42) ;

Is the bug in MSVC, I reported it today.

Answer 6

But I do not know how to do this without lambda. Any ideas?

Just dont use a lambda but a function:

typedef char(*FP)(unsigned);   

char foo(unsigned){ return 0;}

int main() {
    FP x = foo;
}

Function pointer typedefs are rather nasty, if you can better use using:

using FP = char(*)(unsigned);

Live Demo

Answer 7

It is almost the same as Lambdas, but hard to read i think:

void my_int_func(int x)
{
    std::cout << "ther param is: " << x << std::endl;
}

//
int main(int argc, char *argv[])
{
    void (*foo)(int) = my_int_func;
    foo(1);