I have a problem understanding function types (they appear e.g. as the Signature template parameter of a std::function):
typedef int Signature(int); // the signature in question
typedef std::function<int(int)> std_fun_1;
typedef std::function<Signature> std_fun_2;
static_assert(std::is_same<std_fun_1, std_fun_2>::value,
"They are the same, cool.");
int square(int x) { return x*x; }
Signature* pf = square; // pf is a function pointer, easy
Signature f; // but what the hell is this?
f(42); // this compiles but doesn't link
The variable f can not be assigned, but can be called. Weird. What is it good for, then?
Now if I const-qualify the typedef, I can still use it to build further types but apparently for nothing else:
typedef int ConstSig(int) const;
typedef std::function<int(int) const> std_fun_3;
typedef std::function<ConstSig> std_fun_4;
static_assert(std::is_same<std_fun_3, std_fun_4>::value,
"Also the same, ok.");
ConstSig* pfc = square; // "Pointer to function type cannot have const qualifier"
ConstSig fc; // "Non-member function cannot have const qualifier"
What remote corner of the language have I hit here? How is this strange type called and what can I use it for outside of template parameters?
Here's the relevant paragraph from the Standard. It pretty much speaks for itself.
8.3.5/10
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
A typedef of a function type whose declarator includes a cv-qualifier-seq shall be used only to declare the function type for a non-static member function, to declare the function type to which a pointer to member refers, or to declare the top-level function type of another function typedef declaration.
Example:
typedef int FIC(int) const;
FIC f; // ill-formed: does not declare a member function
struct S {
FIC f; // OK
};
FIC S::*pm = &S::f; // OK
In your case, std_fun_1 and std_fun_2 are identical objects with identical type signatures. They are both std::function<int(int)>, and can both hold function pointers or callable objects of type int(int).
pf is a pointer to int(int). That is, it serves the same basic purpose as std::function, but without the machinery of that class or support for instances of callable objects.
Similarly, std_fun_3 and std_fun_4 are identical objects with identical type signatures, and can both hold function pointers or callable objects of type int(int) const.
Also similarly, pfc is a function pointer of type int(int) const, and can hold pointers to functions of that type, but not instances of callable objects.
But f and fc are function declarations.
The line:
Signature fc;
Is identically equivalent to:
int fc(int) const;
Which is a declaration for a function named fc of type int(int) const.
There's nothing strange going on here. You've simply happened upon syntax you probably already understand, from a perspective you're not accustomed to.
