【STL】bind1st与bind2nd函数解析

// bind1st和bind2nd函数把一个二元函数对象绑定成为一个一元函数对象。
// 但是由于二元函数对象接受两个参数，在绑定成为一元函数对象时需要将原来两个参数中的一个绑定下来。
// 也即通过绑定二元函数对象的一个参数使之成为一元函数对象的。
// bind1st是绑定第一个参数，bind2nd则是绑定第二个参数。
// 我个人喜欢用bind2nd，这样子代码读起来更顺。

class greaterthan5: std::unary_function<int, bool>
{
public:
	result_type operator()(argument_type i)
	{
		return (result_type)(i > 5);
	}
};

void test_func_bind()
{
	std::vector<int> v1;
	std::vector<int>::iterator Iter;

	int i;
	for (i = 0; i <= 5; i++)
	{
		v1.push_back(5 * i);
	}

	std::cout << "The vector v1 = ( " ;
	std::copy(v1.cbegin(), v1.cend(), std::ostream_iterator<int>(std::cout, " "));
	std::cout << ")" << std::endl;

	// Count the number of integers > 10 in the vector
	std::vector<int>::iterator::difference_type result1a;
	result1a = count_if(v1.begin(), v1.end(), bind1st(std::less<int>(), 10));
	std::cout << "The number of elements in v1 greater than 10 is: "
		<< result1a << "." << std::endl;

	// Compare: counting the number of integers > 5 in the vector
	// with a user defined function object
	std::vector<int>::iterator::difference_type result1b;
	result1b = count_if(v1.begin(), v1.end(), greaterthan5());
	std::cout << "The number of elements in v1 greater than 5 is: "
		<< result1b << "." << std::endl;

	// Count the number of integers < 10 in the vector
	std::vector<int>::iterator::difference_type result2;
	result2 = count_if(v1.begin(), v1.end(), bind2nd(std::less<int>(), 10));
	std::cout << "The number of elements in v1 less than 10 is: "
		<< result2 << "." << std::endl;
}