ostream_iterator(cout," ")这句该怎么理解啊

[yewennuan]

ostream_iterator<int>是个类型，后面直接加（cout," ")到底什么意思啊

[码工许师傅]

构造函数的参数，语法上相当于有这样的定义：

template<class T>
struct Foo {
     Foo(int, double);  // 两个，别在意类型
};

你就可以用 Foo<char>(1, 1.5); 调用构造函数特定的构造函数。 但这里是cplusplus.com对 ostream_iterator构造函数参数的说明： ostream_iterator (ostream_type& s, const char_type* delimiter); 含义： s A stream object, which is associated to the iterator. Member type ostream_type is the type of such stream (defined as an alias of basic_ostream<charT,traits>, where charT and trais are the second and third class template parameters). delimiter C-string with a sequence of character to be inserted after every insertion. Note that the iterator keeps a copy of the pointer passed (not the content of the C-string).

[码工许师傅]

实际的用处就是，cout连续几次输出的分隔符； 还依稀记得侯捷老师某本书上的一个例子：

copy(istream_iterator<char>(cin), istream_iterator<char>(), ostream_iterator<char>(cout, ""));   

完全是为了使用迭代器而使用的

[yewennuan]

构造函数没有对象可以直接用吗

[iyomumx]

表达式的结果是一个临时对象

[mymtom]

模板啊，构造函数啊

#include <iostream>
#include <iterator>
#include <algorithm>
int main()
{
    std::ostream_iterator<int> i1(std::cout, ", ");
    std::fill_n(i1, 5, -1);
    std::ostream_iterator<double> i2(std::cout);
    *i2++ = 3.14;
}

http://en.cppreference.com/w/cpp/iterator/ostream_iterator/ostream_iterator

[yewennuan]

比如前面声明了vector<int>s(5)..........然后transform(s.begin(),s.end(),ostream_iterator<int>(cout," "),negate<int>()),书上说这里的ostream_iterator<int>(cout," ")和s.begin(),s.end()都是迭代器，是ostream_iterator<int>(cout," ")生成的临时对象就是迭代器对吗

[mymtom]

引用 7 楼 yewennuan 的回复:

六楼，你有对象i1,i2，这样我理解的，我那个直接ostream_iterator<int>(cout," ") 中间没加对象的，怎么理解啊

ostream_iterator<int>(cout," ") 就是构造一个迭代器嘛，只不过这个迭代器有些特殊, 1. 单次(single-pass) 2. 输出(output) 3. 可选的分隔符(Optional delimiter) 4. ++为空操作(Incrementing the std::ostream_iterator is a no-op) std::ostream_iterator is a single-pass output iterator that writes successive objects of type T into the std::basic_ostream object for which it was constructed, using operator<<. Optional delimiter string is written to the output stream after every write operation. The write operation is performed when the iterator (whether dereferenced or not) is assigned to. Incrementing the std::ostream_iterator is a no-op. std::ostream_iterator::operator++ Does nothing. These operator overloads are provided to satisfy the requirements of OutputIterator. They make it possible for the expressions *iter++=value and *++iter=value to be used to output (insert) a value into the underlying container.

[mymtom]

引用 7 楼 yewennuan 的回复:

六楼，你有对象i1,i2，这样我理解的，我那个直接ostream_iterator<int>(cout," ") 中间没加对象的，怎么理解啊

看看下面的代码，也许能帮助你理解

#include <iostream>
#include <iterator>
#include <vector>
#include <deque>

using namespace std;

template<class InputIt, class OutputIt>
OutputIt my_copy(InputIt first, InputIt last, 
              OutputIt d_first)
{
    while (first != last) {
        *d_first++ = *first++;
    }
    return d_first;
}

int
main(int argc, char *argv[])
{
    vector<int> v1;
    vector<int>::const_iterator i1;
    deque<int> q2;
    front_insert_iterator<deque<int> > i2(q2);
    ostream_iterator<int> i3(cout," ");

    v1.push_back(0);
    v1.push_back(1);
    v1.push_back(2);
    v1.push_back(3);
    v1.push_back(4);
    v1.push_back(5);
    v1.push_back(6);
    v1.push_back(7);
    v1.push_back(8);
    v1.push_back(9);

    my_copy(v1.begin(), v1.end(), ostream_iterator<int>(cout," "));
    cout << endl;

    for (i1 = v1.begin(); i1 != v1.end(); ++i1)
        *i2++ = *i1;

    for (i1 = v1.begin(); i1 != v1.end(); ++i1)
        *i3 = *i1;
    cout << endl;

    for (i1 = v1.begin(); i1 != v1.end(); ++i1)
        *i3++ = *i1;
    cout << endl;

    my_copy(q2.begin(), q2.end(), i3);
    cout << endl;

    return 0;
}

/* output
0 1 2 3 4 5 6 7 8 9 
0 1 2 3 4 5 6 7 8 9 
0 1 2 3 4 5 6 7 8 9 
9 8 7 6 5 4 3 2 1 0 
*/

[yewennuan]

六楼，你有对象i1,i2，这样我理解的，我那个直接ostream_iterator<int>(cout," ") 中间没加对象的，怎么理解啊