如何存储从 std::bind 返回的可调用对象以获取指向成员函数的指针？

Question

I want to know how to declare an object of the return type of std::function when used with pointer to a member function:

int main(){

    void foo(std::string, int);
    string s = "hi";
    auto fn = std::bind(foo, s, std::placeholders::_1);
    fn(5);// ok

    /*auto fn2 = */std::bind(&std::string::empty, std::placeholders::_1); // error if I uncomment /*auto fn2 = */

    std::vector<std::string> vs{"The", "Quick", "Brown", "Fox", "Jumped", "", "Over", "The", "", "Lazy", "Dog"};
    auto it = std::find_if(vs.cbegin(), vs.cend(),
    std::bind(&std::string::empty, std::placeholders::_1)); // OK
    std::cout << "Empty string found at index: " << (it != vs.cend() ? std::distance(vs.cbegin(), it) : -1) << '\n'; // 5

    std::cout << "\ndone!\n";
}

• If I un-comment auto fn2 = I get an error:

||=== Build: Debug in myproj (compiler: GNU GCC Compiler) ===| /home/Itachi57/Desktop/myproj/main.cxx||In function 'int main()':| /home/Itachi57/Desktop/myproj/main.cxx|845| in 'constexpr' expansion of 'std::bind(_Func&&, _BoundArgs&& ...) [with _Func = bool (std::__cxx11::basic_string<char>::*)() const noexcept; _BoundArgs = {const std::_Placeholder<1>&}; typename std::_Bind_helper<std::__is_socketlike<_Func>::value, _Func, _BoundArgs ...>::type = std::_Bind_helper<false, bool (std::__cxx11::basic_string<char>::*)() const noexcept, const std::_Placeholder<1>&>::type](std::placeholders::_1)'| /home/Itachi57/Desktop/myproj/main.cxx|845|internal compiler error: Segmentation fault| ||=== Build failed: 2 error(s), 0 warning(s) (0 minute(s), 7 second(s)) ===|

• So Why if I call bind without storing its returned value using auto when binding a pointer to a member function is OK but I can't store that value (new callable)?

• In fact this is just for practice purpose because in real I'd prefer to use std::mem_fn and std::function .

• Update:

If I run the program from a terminal:

  g++ main.cxx -std=c++2a -o prog && ./prog

It works just fine.

I've found out that one day I've disabled "Copy-elision optimization" from CodeBlocks:

 -fno-elide-constructors 

Now If I remove this and let the CodeBlocks apply it (Copy-elision opt.) the program build and works just fine!

• So does this error have a relationship with Copy-Elision Optimization? Thank you!

Answer 1

// error if I uncomment /*auto fn2 = */
// OK

Actually, no, it's not okay. The C++ standard doesn't generally permit taking the address of functions defined in namespace std (be they free or members). The exceptions are always marked explicitly as addressable functions, and std::string::empty is not addressable.

Why? Implementation leeway and future proofing. The standard may want to add overloads. The implementation may want to use overloads to implement the standard-mandated behavior, or even templates. For that to be possible, and our program be well-defined by the standard, we cannot do &std::string::empty .

Just use lambdas to wrap standard functions that aren't addressable (as you would anyway with your own overload sets). Actually... just use lambdas and forget about std::bind , they are better in practically every way.

Answer 2

It's perfectly fine:

Live On Coliru

#include <functional>
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>

void foo(std::string, int) {}

int main()
{
    auto fn = std::bind(&std::string::empty, std::placeholders::_1);

    std::vector<std::string> vs{"The",  "Quick", "Brown", "Fox",  "Jumped", "",
                                "Over", "The",   "",      "Lazy", "Dog"};

    auto it = std::find_if(vs.cbegin(), vs.cend(), fn);

    std::cout << "Empty string found at index: "
              << (it != vs.cend() ? std::distance(vs.cbegin(), it) : -1)
              << '\n'; // 5

    std::cout << "\ndone!\n";
}

Prints

Empty string found at index: 5

done!