std :: bind with std :: less_equal

Question

我尝试编译这个函数:

#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 ... */
}

与gcc 4.6.3:

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

并得到此错误:

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>’

我的(可能是愚蠢的)错误是什么？

Answer 1

你不打电话std::less_equal<unsigned> c-tor.这段代码工作正常.

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

std::bind 有两种形式.

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

既然std::less_equal是struct,你应该将这个函数作为F完全构造的对象传递给它.工程代码是

#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);
}