How do I operate on file using fstream pointers?

Question

I want to operate on streams using some abstraction and thus I want to use fstream* instead of ifstream and ofstream. I tried to do sth like that but is wil cause access violation: ``` char* text= "test"; fstream* o = new fstream(); o = &fstream("name.txt"); o->write(text, 4); o->close(); ``` How can I fix it, or use another idea ? I want to use pointer in this case (You can look here for more general information) How to implement my own IO file API in C++ After the changes it now looks like this: ``` class GIO_Persistent_File_System : public GIO_CORE { public: GIO_Persistent_File_System(void); int open(char*, int); int close(); void write(char* s, int size); void read(char* s, int size); public: ~GIO_Persistent_File_System(void); private: fstream file; }; int GIO_Persistent_File_System::open(char* path, int mode){ file.open(path); return 0; } int GIO_Persistent_File_System::close(){ file.close(); return 0; } void GIO_Persistent_File_System::write(char* s, int size){ file.write(s, siz…

Accepted Answer

This code will give you an error, since you can't take the address of a temporary object, as you are doing with `&fstream("name.txt")`.

```
error: taking address of temporary
```

Also, note that the conversion from a string literal to a `char*` has been deprecated and is invalid in C++11. Use a `const char*` instead:

```
const char* text = "test";
```

Nonetheless, let's look at what you're trying to do. Firstly, you are dynamically allocating an `fstream` and initialising a pointer to that object:

```
fstream* o = new fstream();
```

Then in the next line, you create a temporary object with `fstream("name.txt")` and then take its address and assign it to `o` (which gives an error, as we've seen). Now you would have lost any access to the dynamically allocated `fstream` and instead have `o` pointing at a now destroyed temporary object.

Dereferencing that pointer (with `o->`) will give you undefined behaviour.

You are overcomplicating this. You do not need to dynamically allocate your `fstream` object or use pointers at all. Instead, try:

```
fstream o("name.txt");
o.write(text, 4);
o.close();
```

With your updated code, the problem is that you are writing 0 bytes:

```
plik->write(napis, 0);
```

Perhaps you meant:

```
plik->write(napis, 6);
```

How do I operate on file using fstream pointers?

Solution

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