C ++中的错误，该怎么办？：找不到与ostream :: write（long *，unsigned int）的匹配项

Question

I am trying to write data stored in a binary file using turbo C++. But it shows me an error

could not find an match for ostream::write(long *, unsigned int)

I want to write a 4 byte long data into that file. When i tries to write data using char pointer. It runs successfully. But i want to store large value ie eg. 2454545454 Which can be stored in long only.

I dont know how to convert 1 byte into bit. I have 1 byte of data as a character. Moreover what im trying to do is to convert 4 chars into long and store data into it. And at the other side i want to reverse this so as to retrieve how many bytes of data i have written.

long *lmem;
lmem=new long;
*lmem=Tsize;
fo.write(lmem,sizeof(long));// error occurs here
delete lmem;

I am implementing steganography and i have successfully stored txt file into image but trying to retrieve that file data now.

EDIT:

I am using Turbo Complier.

Answer 1

Cast it to a char*

long *lmem;
lmem=new long;
*lmem=Tsize;
fo.write(reinterpret_cast<char*>(lmem),sizeof(long));
delete lmem;

Or even better (as allocation on the stack is far faster and less error prone)

long lmem = Tsize;
fo.write(reinterpret_cast<char*>(&lmem),sizeof(long));

If Tsize is addressable and a long you could do this:

fo.write(reinterpret_cast<char*>(&Tsize),sizeof(long));

Answer 2

I believe write takes a char* , not a long*

ostream& write ( const char* s , streamsize n );

http://www.cplusplus.com/reference/iostream/ostream/write/

Answer 3

1)

const long L = Tsize;
fo.write(reinterpret_cast<const char*>(&L),sizeof(L));

Answer 4

There are several problems here.

First, you don't need to create an all new long to store Tsize in. You can make a pointer to the Tsize you already have.

Second, the write call takes a stream of bytes. What you need to do is to cast your pointer to char*.

Like this:

fo.write( static_cast<char*>(&Tsize), sizeof(Tsize) );

Third, writing one long like this is pretty inefficient. If you have a large array of long values that you want to write, use a single write call to get all of them, like:

fo.write( static_cast<char*>(array), sizeof(*array) * array_count );

I am pretty sure static_cast works for pointers here, but if it does not then change the code to use reinterpret_cast .