问题
I'm looking for a way to write floats/ints/strings to a file and read them as floats/ints/strings. (basically read/write as ios::binary).
回答1:
I subclassed ifstream and ofstream: ibfstream and obfstream. I made a little helper class that would detect the endianness of the machine I was compiling/running on. Then I added a flag for ibfstream and obfstream that indicated whether bytes in primitive types should be flipped. These classes also had methods to read/write primitive types and arrays of such types flipping the byte order as necessary. Finally, I set ios::binary for these classes by default.
I was often working on a little-endian machine and wanting to write big-endian files or vice versa. This was used in a program that did a lot of I/O with 3D graphics files of various formats.
回答2:
I subclassed ifstream and ofstream: ibfstream and obfstream. I made a class that would detect the endianness of the machine I was compiling/running on. Then I added a flag for ibfstream and obfstream that indicated whether bytes in primitive types should be flipped. These classes also had methods to read/write primitive types and arrays of such types flipping the byte order as necessary.
I was often working on a little-endian machine and wanting to write big-endian files or vice versa. This was used in a program tht did a lot of I/O with 3D graphics files of various formats.
回答3:
I ended up writing it myself. Just wanted to share it with others.
It might not be optimized, but I had some difficulties finding C++ code that mimics C#'s BinaryReader & BinaryWriter classes. So I created one class that handles both read and write.
Quick things to note:
1) "BM" is just a prefix for my classes.
2) BMLogging is a helper class that simply does:
cout << "bla bla bla" << endl;
So you can ignore the calls to BMLogging, I kept them to highlight the cases where we could warn the user.
Here's the code:
#include <iostream>
#include <fstream>
using namespace std;
// Create the macro so we don't repeat the code over and over again.
#define BMBINARY_READ(reader,value) reader.read((char *)&value, sizeof(value))
enum BMBinaryIOMode
{
None = 0,
Read,
Write
};
class BMBinaryIO
{
// the output file stream to write onto a file
ofstream writer;
// the input file stream to read from a file
ifstream reader;
// the filepath of the file we're working with
string filePath;
// the current active mode.
BMBinaryIOMode currentMode;
public:
BMBinaryIO()
{
currentMode = BMBinaryIOMode::None;
}
// the destructor will be responsible for checking if we forgot to close
// the file
~BMBinaryIO()
{
if(writer.is_open())
{
BMLogging::error(BMLoggingClass::BinaryIO, "You forgot to call close() after finishing with the file! Closing it...");
writer.close();
}
if(reader.is_open())
{
BMLogging::error(BMLoggingClass::BinaryIO, "You forgot to call close() after finishing with the file! Closing it...");
reader.close();
}
}
// opens a file with either read or write mode. Returns whether
// the open operation was successful
bool open(string fileFullPath, BMBinaryIOMode mode)
{
filePath = fileFullPath;
BMLogging::info(BMLoggingClass::BinaryIO, "Opening file: " + filePath);
// Write mode
if(mode == BMBinaryIOMode::Write)
{
currentMode = mode;
// check if we had a previously opened file to close it
if(writer.is_open())
writer.close();
writer.open(filePath, ios::binary);
if(!writer.is_open())
{
BMLogging::error(BMLoggingClass::BinaryIO, "Could not open file for write: " + filePath);
currentMode = BMBinaryIOMode::None;
}
}
// Read mode
else if(mode == BMBinaryIOMode::Read)
{
currentMode = mode;
// check if we had a previously opened file to close it
if(reader.is_open())
reader.close();
reader.open(filePath, ios::binary);
if(!reader.is_open())
{
BMLogging::error(BMLoggingClass::BinaryIO, "Could not open file for read: " + filePath);
currentMode = BMBinaryIOMode::None;
}
}
// if the mode is still the NONE/initial one -> we failed
return currentMode == BMBinaryIOMode::None ? false : true;
}
// closes the file
void close()
{
if(currentMode == BMBinaryIOMode::Write)
{
writer.close();
}
else if(currentMode == BMBinaryIOMode::Read)
{
reader.close();
}
}
bool checkWritabilityStatus()
{
if(currentMode != BMBinaryIOMode::Write)
{
BMLogging::error(BMLoggingClass::BinaryIO, "Trying to write with a non Writable mode!");
return false;
}
return true;
}
// Generic write method that will write any value to a file (except a string,
// for strings use writeString instead).
void write(void *value, size_t size)
{
if(!checkWritabilityStatus())
return;
// write the value to the file.
writer.write((const char *)value, size);
}
// Writes a string to the file
void writeString(string str)
{
if(!checkWritabilityStatus())
return;
// first add a \0 at the end of the string so we can detect
// the end of string when reading it
str += '\0';
// create char pointer from string.
char* text = (char *)(str.c_str());
// find the length of the string.
unsigned long size = str.size();
// write the whole string including the null.
writer.write((const char *)text, size);
}
// helper to check if we're allowed to read
bool checkReadabilityStatus()
{
if(currentMode != BMBinaryIOMode::Read)
{
BMLogging::error(BMLoggingClass::BinaryIO, "Trying to read with a non Readable mode!");
return false;
}
// check if we hit the end of the file.
if(reader.eof())
{
BMLogging::error(BMLoggingClass::BinaryIO, "Trying to read but reached the end of file!");
reader.close();
currentMode = BMBinaryIOMode::None;
return false;
}
return true;
}
// reads a boolean value
bool readBoolean()
{
if(checkReadabilityStatus())
{
bool value = false;
BMBINARY_READ(reader, value);
return value;
}
return false;
}
// reads a character value
char readChar()
{
if(checkReadabilityStatus())
{
char value = 0;
BMBINARY_READ(reader, value);
return value;
}
return 0;
}
// read an integer value
int readInt()
{
if(checkReadabilityStatus())
{
int value = 0;
BMBINARY_READ(reader, value);
return value;
}
return 0;
}
// read a float value
float readFloat()
{
if(checkReadabilityStatus())
{
float value = 0;
BMBINARY_READ(reader, value);
return value;
}
return 0;
}
// read a double value
double readDouble()
{
if(checkReadabilityStatus())
{
double value = 0;
BMBINARY_READ(reader, value);
return value;
}
return 0;
}
// read a string value
string readString()
{
if(checkReadabilityStatus())
{
char c;
string result = "";
while((c = readChar()) != '\0')
{
result += c;
}
return result;
}
return "";
}
};
EDIT: I replaced all the read/write methods above with these: (updated the usage code as well)
// Generic write method that will write any value to a file (except a string,
// for strings use writeString instead)
template<typename T>
void write(T &value)
{
if(!checkWritabilityStatus())
return;
// write the value to the file.
writer.write((const char *)&value, sizeof(value));
}
// Writes a string to the file
void writeString(string str)
{
if(!checkWritabilityStatus())
return;
// first add a \0 at the end of the string so we can detect
// the end of string when reading it
str += '\0';
// create char pointer from string.
char* text = (char *)(str.c_str());
// find the length of the string.
unsigned long size = str.size();
// write the whole string including the null.
writer.write((const char *)text, size);
}
// reads any type of value except strings.
template<typename T>
T read()
{
checkReadabilityStatus();
T value;
reader.read((char *)&value, sizeof(value));
return value;
}
// reads any type of value except strings.
template<typename T>
void read(T &value)
{
if(checkReadabilityStatus())
{
reader.read((char *)&value, sizeof(value));
}
}
// read a string value
string readString()
{
if(checkReadabilityStatus())
{
char c;
string result = "";
while((c = read<char>()) != '\0')
{
result += c;
}
return result;
}
return "";
}
// read a string value
void readString(string &result)
{
if(checkReadabilityStatus())
{
char c;
result = "";
while((c = read<char>()) != '\0')
{
result += c;
}
}
}
This is how you would use it to WRITE:
string myPath = "somepath to the file";
BMBinaryIO binaryIO;
if(binaryIO.open(myPath, BMBinaryIOMode::Write))
{
float value = 165;
binaryIO.write(value);
char valueC = 'K';
binaryIO.write(valueC);
double valueD = 1231.99;
binaryIO.write(valueD);
string valueStr = "spawnAt(100,200)";
binaryIO.writeString(valueStr);
valueStr = "helpAt(32,3)";
binaryIO.writeString(valueStr);
binaryIO.close();
}
Here's how you would use it to READ:
string myPath = "some path to the same file";
if(binaryIO.open(myPath, BMBinaryIOMode::Read))
{
cout << binaryIO.read<float>() << endl;
cout << binaryIO.read<char>() << endl;
double valueD = 0;
binaryIO.read(valueD); // or you could use read<double()
cout << valueD << endl;
cout << binaryIO.readString() << endl;
cout << binaryIO.readString() << endl;
binaryIO.close();
}
EDIT 2: You could even write/read a whole structure in 1 line:
struct Vertex {
float x, y;
};
Vertex vtx; vtx.x = 2.5f; vtx.y = 10.0f;
// to write it
binaryIO.write(vtx);
// to read it
Vertex vtxRead;
binaryIO.read(vtxRead); // option 1
vtxRead = binaryIO.read<Vertex>(); // option 2
Hope my code is clear enough.
来源：https://stackoverflow.com/questions/14077781/id-like-to-use-ifstream-and-ofstream-in-c-to-mimic-cs-binaryreader-binary