Using bind1st for a method that takes argument by reference

Question

I have a struct like this:

struct A {
    void i(int i) {}
    void s(string const &s) {}
};

Now when I try this:

bind1st(mem_fun(&A::i), &a)(0);
bind1st(mem_fun(&A::s), &a)("");

The first line compiles OK, but the second generates an error:

c:\program files (x86)\microsoft visual studio 10.0\vc\include\xfunctional(299): error C2535: 'void std::binder1st<_Fn2>::operator ()(const std::basic_string<_Elem,_Traits,_Ax> &) const' : member function already defined or declared
          with
          [
              _Fn2=std::mem_fun1_t<void,A,const std::string &>,
              _Elem=char,
              _Traits=std::char_traits<char>,
              _Ax=std::allocator<char>
          ]
          c:\program files (x86)\microsoft visual studio 10.0\vc\include\xfunctional(293) : see declaration of 'std::binder1st<_Fn2>::operator ()'
          with
          [
              _Fn2=std::mem_fun1_t<void,A,const std::string &>
          ]
          c:\work\sources\exception\test\exception\main.cpp(33) : see reference to class template instantiation 'std::binder1st<_Fn2>' being compiled
          with
          [
              _Fn2=std::mem_fun1_t<void,A,const std::string &>
          ]

What could be the problem? How could I fix it?

Edit:

It seems that any reference argument is a problem. So if I change the i method to void i(int &i) {} I get a similar error.

Given that we are in 2011 and have C++11 you might want to have a look at C++11s `std::bind` or `boost::bind` which make those things orders of magnitude easier to handle. — PlasmaHH, Oct 19 '11 at 14:18
@PlasmaHH: Unfortunatelly, I cannot use C++11 in this project. — Juraj Blaho, Oct 19 '11 at 14:19

score 11 · Accepted Answer · answered Oct 19 '11 at 14:37

std::bind1st and std::bind2nd don't accept functors which take reference arguments, because they themselves form references to these arguments. You can

use pointers for your function inputs instead of references
use boost::bind
accept the performance cost of copying the string

bames53 · Answer 2 · 2011-10-19T15:03:00.113

6

The issue is a defect in the library specification.

Take a look at this bug report against gcc and the resulting discussion: Bug 37811 - bind1st fails on mem_fun with reference argument

C++03 lacked the facilities to build a perfect bind library. This issue is fixed in C++11 with perfect forwarding and std::bind.

edited Oct 19 '11 at 15:03

answered Oct 19 '11 at 14:34

bames53

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score 0 · Answer 3 · edited May 23 '17 at 12:16

0

Look at this post were the requirements of template parameters are explained. As you already assumed the reference to the std::string is the problem. It's not a valid template parameter.

edited May 23 '17 at 12:16

Community

1
1

answered Oct 19 '11 at 14:39

codencandy

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1

While this is true, the argument type of the function is transported from mem_fun to bind1st via the first_argument_type typedef, and a reference is a valid typedef. So std::bind1st *could* theoretically accept references, but is implemented differently (and this implementation is mandated by the standard, IIRC). – thiton Oct 19 '11 at 14:42

score 0 · Answer 4 · answered Oct 19 '11 at 14:41

Change references by pointers

#include <functional>
#include <string>

struct A {
    void i(int i) {}
    void s(const std::string *s) {}
};


int main(void)
{
    A a;
    std::bind1st(std::mem_fun(&A::i), &a)(0);
    const std::string s("");
    std::bind1st(std::mem_fun(&A::s), &a)(&s);
}

Using bind1st for a method that takes argument by reference

4 Answers4

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