C++ function returning function

Question

Where in the standard are functions returning functions disallowed? I understand they are conceptually ridiculous, but it seems to me that the grammar would allow them. Acco

Answer 1

With C++11 (but not previous versions of C++) you can not only return C-like function pointers, but also C++ closures, notably with anonymous functions. See also std::function

The standard disallows (semantically, not syntactically - so it is not a question of grammar ; see Barry's answer for the citation) returning functions (and also disallow sizeof on functions!) but permits to return function pointers.

^{BTW, I don't think that you could return entire functions. What would that mean? How would you implement that? Practically speaking, a function is some code block, and its name is (like for arrays) a pointer to the start of the function's machine code.}

A nice trick might be to build (using mechanisms outside of the C++ standard) a function at runtime (and then handling its function pointer). Some external libraries might permit that: you could use a JIT library (e.g. asmjit, gccjit, LLVM ...) or simply generate C++ code, then compile and dlopen & dlsym it on POSIX systems, etc.

PS. You are probably right in understanding that the C++11 grammar (the EBNF rules in the standard) does not disallow returning functions. It is a semantic rule stated in plain English which disallows that (it is not any grammar rule). I mean that the EBNF alone would allow:

 // semantically wrong... but perhaps not syntactically
 typedef int sigfun_T(std::string);
 sigfun_T foobar(int);

and it is for semantics reasons (not because of EBNF rules) that a compiler is rightly rejecting the above code. Practically speaking, the symbol table matters a lot to the C++ compiler (and it is not syntax or context-free grammar).

The sad fact about C++ is that (for legacy reasons) its grammar (alone) is very ambiguous. Hence C++11 is difficult to read (for humans), difficult to write (for developers), difficult to parse (for compilers), ....