C++ string to C char array for writing to binary file

Question

I am trying to write and read string to/from binary file, but I can't understand why sizeof(t) returns 4.

//write to file
ofstream f1("example.bin", ios::binary | ios::out);
string s = "Valentin";
char* t = new char[s.length()+1];
strcpy(t, s.c_str());
cout << s.length()+1 << " " << sizeof(t) << endl; // prints 9 4
for(int i = 0; i < sizeof(t); i++)
{
    //t[i] += 100;
}
f1.write(t, sizeof(t));
f1.close();

// read from file
ifstream f2("example.bin", ios::binary | ios::in);
while(f2)
{
    int8_t x;
    f2.read((char*)&x, 1);
    //x -= 100;
    cout << x;  //print Valee
}
cout << endl;
f2.close();

It doesn't matter what size I put in char* array t, code always prints "4" as its size. What must I do to write longer than 4 bytes of data?

sizeof(t) just gives you the size of a pointer, because that's what t is. If you want the size of the memory you allocated for t, why not do s.length()+1 instead? — Blaze
– Blaze, Commented Mar 11, 2019 at 14:36
why all this aerobics with string to char * ? string has a data() and thats probably all you need — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Mar 11, 2019 at 14:38
sizeof operator returns the static size of the type. It gets turned into a constant number at compile time (in C++ anyway, in C there is VLA). Size of pointer is usually 4 for 32 bit binaries, 8 for 64 bit binaries. — hyde
– hyde, Commented Mar 11, 2019 at 14:38
sizeof(t) -> strlen(t). But consider the second comment. — Jabberwocky
– Jabberwocky, Commented Mar 11, 2019 at 15:03

john · Accepted Answer · 2019-03-13 08:28:45Z

3

Here's how to do the writing code the easy way

//write to file
ofstream f1("example.bin", ios::binary | ios::out);
string s = "Valentin";
f1.write(s.c_str(), s.size() + 1);
f1.close();

EDIT the OP actually wants something like this

#include <algorithm> // for transform

string s = "Valentin";
// copy s to t and add 100 to all bytes in t
string t = s;
transform(t.begin(), t.end(), t.begin(), [](char c) { return c + 100; });
// write to file
ofstream f1("example.bin", ios::binary | ios::out);
f1.write(t.c_str(), t.size() + 1);
f1.close();

edited Mar 13, 2019 at 8:28

answered Mar 11, 2019 at 15:15

john

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9 Comments

user3007875 Over a year ago

Thank you, but I am looking to add to every char (which I believe is in ASCII dec number) some number (+100 in for loop in my code), so simple write(s.c_str() ...) won't do for me.

john Over a year ago

@user3007875 OK I missed that part of your code. It is a very odd thing to want to do though are you sure you got that right? You wouldn't be the first newbie to spend ages trying to solve the wrong problem. See edit to my answer for a version that does that as well.

user3007875 Over a year ago

Did you test your code from edited part? I copy pasted it and it throws error in transform line (no matching function call). Anyway, it works with me original code (with for loop), so the problem is solved. I marked your answer as correct, thank you.

john Over a year ago

@user3007875 Did you have #include <algorithm>? If you did then try std::transform instead of transform.

user3007875 Over a year ago

I did. My whole code: ''' #include <fstream> #include <string.h> #include <cstring> #include <algorithm> using namespace std; int main() { string s = "Valentin"; // copy s to t and add 100 to all bytes in t string t = s; ransform(t.begin(), t.end(), [](char c) { return c + 100; }); // write to file ofstream f1("example.bin", ios::binary | ios::out); f1.write(t.c_str(), t.size() + 1); f1.close(); } ''' Throws same error. Anyway, it doesn't really matter now, but thank you for your help.

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Jeffrey · Accepted Answer · 2019-03-11 14:36:45Z

2

sizeof(char*) prints the size used by a pointer to (a) char(s). It's 4 on your platform.

If you need the size of the string, you should use strlen. Or, simply, s.length().

answered Mar 11, 2019 at 14:36

Jeffrey

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1 Comment

Deduplicator Over a year ago

Though those can differ if there are unaccounted for internal NULs.

Govind Parmar · Accepted Answer · 2019-03-11 14:39:29Z

2

char *t is a pointer, not an array, so sizeof will return the size of a pointer on your machine, which is apparently 4 bytes.

The correct way to determine the length of a C-style string is to include <cstring> and use std::strlen.

answered Mar 11, 2019 at 14:39

Govind Parmar

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