How to pass string object(std::string) using fifo? [duplicate] - c++

Question

I am trying to write a char* to a binary file.
This is what I have now.
void Write(char* fileName, char* pData)
{
ofstream binFile (fileName, ios::out | ios::binary);
if (binFile.open())
{
binFile.write((char*)&pData, sizeof(pData));
binFile.close();
}
}
void Read(char* fileName, char* pData)
{
ifstream binFile(fileName, ios::in | ios::binary);
if(binFile.open())
{
binFile.read(char*)&pData, sizeof(pData));
binFile.close
}
}
int main()
{
char* testData = "ABCdEFG"; // not real data
char* getTestData;
char* file = "C:\\testData.dat";
Write(file, testData);
Read(file, getTestData);
}
Test data will be of unknown length. May not always be the same.
When i run the program once, and write and read. I can get back the test data.
But when i stop the program and run it again, this time without writing. Just reading, i cannot get back the test data.
I don't really understand whats happening here.
Can some one explain it to me?

Answer 1

binFile.write((char*)&pData, sizeof(pData));
is wrong. It just writes the value of the pointer. It does not write the data.
You need to use:
binFile.write(pData, strlen(pData));
However, that won't be adequate to read the data back. To be able to read the data back, you'll need to write the size of the string first.
size_t len = strlen(pData);
binFile.write((char*)&len, sizeof(len));
binFile.write(pData, len);
And when reading the data back, you will need to use:
size_t len = 0;
binFile.read(char*)&len, sizeof(len));
binFile.read(pData, len);
and then, null terminate the string.
pData[len] = '\0';
PS
Make sure getTestData is properly initialized before using it to read the data.
char getTestData[100];
will be adequate for your test case.
Update
You can make your program a bit better by using std::string instead of char*. The size of the saved data can be more easily managed when a std::string is used.
void Write(std::string const& fileName, std::string const& data)
{
std::ofstream binFile(fileName, std::ios::out | std::ios::binary);
if (binFile.is_open())
{
size_t len = data.size();
binFile.write((char*)&len, sizeof(len));
binFile.write((char*)&data[0], len);
// No need. The file will be closed when the function returns.
// binFile.close();
}
}
void Read(std::string const& fileName, std::string& data)
{
std::ifstream binFile(fileName, std::ios::in | std::ios::binary);
if(binFile.is_open())
{
size_t len = 0;
binFile.read((char*)&len, sizeof(len));
data.resize(len);
binFile.read((char*)&data[0], len);
}
}
int main()
{
std::string file = "testData.dat";
std::string testData = "ABCdEFG";
Write(file, testData);
std::string getTestData;
Read(file, getTestData);
std::cout << getTestData << std::endl;
}