Question

I try to compile this function:

#include <algorithm>
#include <cmath>
#include <functional>
#include <vector>

void foo(unsigned n)
{
  std::vector<unsigned> some_vector;
  /* fill vector ... */
  auto le_sqrt_n = std::bind(std::less_equal<unsigned>, 
                             std::placeholders::_1, 
                             unsigned(sqrt(n))
                             );
  auto it = std::find_if(some_vector.rbegin(), some_vector.rend(), le_sqrt_n);
  /* do something with the result ... */
}

with gcc 4.6.3:

g++ -std=c++0x test_bind_02.cc

and get this error:

test_bind_02.cc: In function ‘void foo(unsigned int)’:
test_bind_02.cc:10:55: error: expected primary-expression before ‘,’ token
test_bind_02.cc:13:9: error: unable to deduce ‘auto’ from ‘<expression error>’
test_bind_02.cc:14:77: error: unable to deduce ‘auto’ from ‘<expression error>’

What is my (presumably stupid) mistake?

Answer 1

You don’t call std::less_equal<unsigned> c-tor.
This code works okay.

http://liveworkspace.org/code/7f779d3fd6d521e8d4012a4066f2c40f

std::bind has two forms.

template< class F, class... Args >
/*unspecified*/ bind( F&& f, Args&&... args );
template< class R, class F, class... Args >
/*unspecified*/ bind( F&& f, Args&&... args );

since std::less_equal is struct you should pass to this function as F fully-constructed object. Works code is

#include <algorithm>
#include <cmath>
#include <functional>
#include <vector>

void foo(unsigned n)
{
  std::vector<unsigned> some_vector;
  /* fill vector */
  auto le_sqrt_n = std::bind(std::less_equal<unsigned>(),
                 std::placeholders::_1, 
                 unsigned(sqrt(n))
                 );
  auto it = std::find_if(some_vector.rbegin(), some_vector.rend(), le_sqrt_n);
}