如何将CFile的内容复制到CMemFile？

Question

如何在windows操作系统中将CFile的全部内容复制到CMemFile中。我尝试了下面的代码来复制内容。

CMemFile memFile;
bool b = memFile.Open( L"C:\\test.txt",  CFile::modeReadWrite);
int Length = memFile.GetLength();
unsigned char* buffer = new unsigned char[Length];
memFile.Read((void*)buffer,Length);
memFile.Close();

但是memFile.GetLength返回0。而如果我尝试使用CFile，GetLength()会返回一些有效的长度。

CFile f;
bool b1 = f.Open( L"C:\\test.txt",  CFile::modeReadWrite);
int Lengh = f.GetLength();
f.Close();

谢谢。

Answer 1

我能够实现我的要求使用以下代码。

CFile f;
bool b = f.Open( L"C:\\test.txt",  CFile::modeReadWrite);
int nLength = f.GetLength();
BYTE* buffer = new BYTE[nLength];
f.Read((void*)buffer,nLength);
f.Close();
CMemFile memFile;
memFile.Attach(buffer, nLength);

Answer 2

要将磁盘文件的内容复制到CMemFile实例控制的内存缓冲区中，需要执行以下步骤：

• 打开磁盘上的文件，传递正确的options.
• Allocate内存供CMemFile实例使用。
• 将磁盘内容读取到CMemFile-controlled内存缓冲区。

以下是一种可能的实现：

// Open file on disk
// typeBinary is necessary to read the raw, untransformed contents
// shareDenyWrite is required to prevent the file from changing in between querying
//     its size and reading the contents
// osSequentialScan could be used to provide an access pattern hint to the OS
//     to allow it to better optimize the (potentially slow) disk accesses
CFile File( L"C:\\test.txt", CFile::modeRead | CFile::typeBinary | CFile::shareDenyWrite );
auto Length = File.GetLength();

// Construct CMemFile instance and allocate memory
CMemFile memFile();
auto buffer = memFile.Alloc( Length );
if ( !buffer ) {
    // Depending on your exception handling system, call AfxThrowMemoryException instead
    throw std::bad_alloc();
}
// Transfer ownership of the buffer to the CMemFile instance
memFile.Attach( buffer, Length );

// Read contents into memory buffer
// Note, that this is limited to file sizes smaller than 4 GB
File.Read( buffer, static_cast<UINT>( Length ) );

此时，磁盘文件的所有内容都已被读取到CMemFile实例的缓冲区中，并假定在此过程中没有引发异常。所有资源都由C++对象控制，不需要手动清理。当memFile对象超出作用域时，它的内存将自动释放。当File实例超出作用域时，系统级文件HANDLE将关闭。