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
并收到此错误:
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?
答案 0 :(得分:7)
您不能致电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 );
因为std::less_equal
是struct,所以你应该将这个函数作为F
完全构造的对象传递给它。工作代码是
#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);
}