c++ - std::bind 与 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 ... */
}

使用 gcc 4.6.3：

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

并得到这个错误：

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。此代码工作正常。

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 );

因为是 struct 你应该作为完全构造的对象std::less_equal传递给这个函数。F作品代码是

#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);
}