从常量内存创建输入流 [英] Creating an input stream from constant memory

问题描述

我在一个 const char * 指针指向的缓冲区中有一些数据。数据只是一个ASCII字符串。我知道它的大小。我想能够读取数据从流中读取相同的方式。我正在寻找一个解决方案，让我写这样的代码：

I have some data in a buffer pointed to by a const char* pointer. The data is just an ASCII string. I know its size. I would like to be able to read it in the same way data is read from streams. I'm looking for a solution that would allow me to write code like this:

// for example, data points to a string "42 3.14 blah"
MemoryStreamWrapper in(data, data_size);
int x;
float y;
std::string w;
in >> x >> y >> w;

重要条件：不得以任何方式复制或更改数据 （否则我只是使用字符串流。据我所知，不可能从一个const char指针创建一个字符串流，而不复制数据。）

Important condition: the data must not be copied or altered in any way (otherwise I'd just use a string stream. To my best knowledge, it isn't possible to create a string stream from a const char pointer without copying the data.)

推荐答案

这样做的方法是创建一个合适的流缓冲区。例如，可以这样做：

The way to do this is to create a suitable stream buffer. This can, e.g., be done like this:

#include <streambuf>
#include <istream>

struct membuf: std::streambuf {
    membuf(char const* base, size_t size) {
        char* p(const_cast<char*>(base));
        this->setg(p, p, p + size);
    }
};
struct imemstream: virtual membuf, std::istream {
    imemstream(char const* base, size_t size)
        : membuf(base, size)
        , std::istream(static_cast<std::streambuf*>(this)) {
    }
};

唯一有点尴尬的是 const_cast< char *> 在流缓冲区中：流缓冲区不会改变数据，但是接口仍然需要使用 char * ，主要是为了更容易以改变正常流缓冲器中的缓冲器。这样，您可以使用 imemstream 作为正常的输入流：

The only somewhat awkward thing is the const_cast<char*>() in the stream buffer: the stream buffer won't change the data but the interface still requires char* to be used, mainly to make it easier to change the buffer in "normal" stream buffers. With this, you can use imemstream as a normal input stream:

imemstream in(data, size);
in >> value;

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