二进制输出的ostream_iterator [英] ostream_iterator for Binary Output

问题描述

我想要使用 ostream_iterator 流式传输到二进制文件。但是 ostream_iterator 使用 FormattedOuputFunction 因此它将写入ASCII而不是二进制：

std :: ostream_iterator 是一个单次传递OutputIterator，它将类型 T 的连续对象写入到它所在的 std :: basic_ostream 构造，使用运算符<

除了编写我自己的迭代器一个使用迭代器编写二进制文件的方法？

一个简单的例子，我要做的，但 copy 语句将ASCII写入我的二进制文件：

  ofstream foo（foo.txt，ios_base ::二进制）; 
 vector< int> bar = {13，42}; 
 
 copy（bar.cbegin（），bar.cend（），ostream_iterator< decltype（bar）:: value_type>（foo） 
  

解决方案

它可以工作，但你必须明确使用 ostream_iterator< char> 。

示例（包括忽略不重要）：

  int main（int argc，char ** argv）{
 std :: vector< int> arr; 
 
 std :: ofstream fd（foo.txt，std :: ios :: binary | std :: ios :: out）; 
 
 for（int i = 0; i <256; i ++）arr.push_back（i）; 
 
 std :: ostream_iterator< char> oi（fd）; 
 std :: copy（arr.begin（），arr.end（），oi）; 
 fd.close（）; 
 return 0; 
} 
  

将在foo.txt中写入0到255的256个字节。 p>

---

上面假设您想​​直接将int的值作为char写入文件。从您的注释，您想要以本地主机端字节中的4字节值（假设int32_t）写入int值。我将使用辅助类来完成这项工作：

  class bint {
 private：
 char c [sizeof（int）]; 
 
 public：
 bint（const int i）{//允许bint b = 5; 
 :: memcpy（c，& i，sizeof（c））; 
} 
 operator int（）const {//允许：bint b = 5; int i = b =>给出i = 5 
 int i; 
 :: memcpy（& i，c，sizeof（int））; 
 return i; 
} 
 const char * getBytes（）const {//给予公共只读访问字节
 return c; 
} 
}; 
 
 std :: ostream& operator<<<（std :: ostream& out，const bint& b）{
 out.write（b.getBytes（），sizeof（int））; 
 return out; 
} 
 
 int main（int argc，char ** argv）{
 std :: vector< int> arr; 
 
 std :: ofstream fd（foo.txt，std :: ios :: binary | std :: ios :: out）; 
 
 for（int i = 0; i <256; i ++）arr.push_back（i）; 
 
 std :: ostream_iterator< bint> oi（fd）; 
 std :: copy（arr.begin（），arr.end（），oi）; 
 fd.close（）; 
 return 0; 
} 
  

这一个写入1024字节（大小为4的整数） 256个第一整数。它会自动适应其他大小的int。

I want to be able to use an ostream_iterator to stream to a binary file. But the ostream_iterator uses a FormattedOuputFunction so it will write ASCII, not binary:

std::ostream_iterator is a single-pass OutputIterator that writes successive objects of type T into the std::basic_ostream object for which it was constructed, using operator<<

Beyond writing my own iterator is there a way to use an iterator to write binary?

A simplified example of what I'm trying to do, but the copy statement is going to write ASCII to my binary file:

ofstream foo("foo.txt", ios_base::binary);
vector<int> bar = {13, 42};

copy(bar.cbegin(), bar.cend(), ostream_iterator<decltype(bar)::value_type>(foo));

解决方案

It works, but you will have to explicitely use an ostream_iterator<char>.

Example (includes omitted for brievety):

int main(int argc, char **argv) {
    std::vector<int> arr;

    std::ofstream fd("foo.txt", std::ios::binary | std::ios::out);

    for (int i=0; i<256; i++) arr.push_back(i);

    std::ostream_iterator<char> oi(fd);
    std::copy(arr.begin(), arr.end(), oi);
    fd.close();
    return 0;
}

will write the 256 bytes for 0 to 255 in foo.txt.

---

Above assumed that you wanted to write directly the value of the int as a char to the file. From your comment, you want to write the int value as a 4 bytes value (assuming int32_t) in native host endianness. I would use an auxilliary class to do the job:

class bint {
private:
    char c[sizeof(int)];

public:
    bint(const int i) { // allows bint b = 5;
        ::memcpy(c, &i, sizeof(c));
    }
    operator int() const {  // allows : bint b = 5; int i=b => gives i=5
        int i;
        ::memcpy(&i, c, sizeof(int));
        return i;
    }
    const char *getBytes() const { // gives public read-only access to the bytes
        return c;
    }
};

std::ostream& operator <<(std::ostream& out, const bint& b) {
    out.write(b.getBytes(), sizeof(int));
    return out;
}

int main(int argc, char **argv) {
    std::vector<int> arr;

    std::ofstream fd("foo.txt", std::ios::binary | std::ios::out);

    for (int i=0; i<256; i++) arr.push_back(i);

    std::ostream_iterator<bint> oi(fd);
    std::copy(arr.begin(), arr.end(), oi);
    fd.close();
    return 0;
}

This one writes 1024 bytes (for integer of size 4) containing the representations of the 256 first integers. It would automatically adapts to other sizes of int.

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