fstream to const char *

Question

What I want to do is read a file called "test.txt", and then have the contents of the file be a type const char *. How would one do this?

Are you sure you need a `const char *`? If you need something to pass into a function that takes `const char *`, then you can just pass in a `char *` if that's more convenient for you. — Steve Jessop, Oct 24 '10 at 20:18
possible duplicate of [What is the best way to slurp a file into a std::string in c++?](http://stackoverflow.com/questions/116038/what-is-the-best-way-to-slurp-a-file-into-a-stdstring-in-c) — Konrad Rudolph, Oct 24 '10 at 20:19

score 7 · Accepted Answer · answered Oct 24 '10 at 20:14

7

#include <string>
#include <fstream>

int main()
{
   std::string line,text;
   std::ifstream in("test.txt");
   while(std::getline(in, line))
   {
       text += line + "\n";
   }
   const char* data = text.c_str();
}

Be careful not to explicitly call delete on data

answered Oct 24 '10 at 20:14

Armen Tsirunyan

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would `std::string text; std::getline(in, text, EOF); const char* data = text.c_str();` work faster? – Mooing Duck Jul 05 '12 at 23:11
I was planning to add another solution that uses sstream instead of string but I found the solution by eq-, which I strongly recommend over this one as it is much more efficient as you here allocate and free the memory with each line. – Mostafa Hassan Jul 04 '17 at 16:16

eq- · Answer 2 · 2012-07-05T23:06:55.730

6

It's highly unlikely you really want to do that. The contents of the file (which may be either text, or binary data) are unlikely to represent a (valid) pointer to a char on your architecture, so it is not really meaningful to represent it [the content] as a const char *.

What you may instead want is to load the contents of the file in memory, and then store a pointer (of type const char*) to the beginning of the given block. </pedantry> One way of achieving that:

#include <sstream>
#include <fstream>
// ...
{
    std::ostringstream sstream;
    std::ifstream fs("test.txt");
    sstream << fs.rdbuf();
    const std::string str(sstream.str());
    const char* ptr = str.c_str();
    // ptr is the pointer we wanted - do note that it's only valid
    // while str is valid (i.e. not after str goes out of scope)
}

edited Jul 05 '12 at 23:06

answered Oct 24 '10 at 21:08

eq-

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Yeah. I mess this up every time, which is odd since I usually learn from my mistakes. (I use stringstreams rarely, and "every time" I refer to the wrong header.) Fixed. – eq- Jul 05 '12 at 23:06
`do note that it's only valid while str is valid (i.e. not after str goes out of scope)` -- +1 for this. I had a similar solution but I was returning a `const char* ptr` from a function and couldn't figure out why things wasn't working as expected. – Norman Breau Oct 29 '21 at 00:33

score 1 · Answer 3 · answered Oct 24 '10 at 20:16

You need to:

create a function returning a const char*
open an fstream on the file
seek to its end
determine the file length by looking at the file position (tell)
seek back to the beginning
create a char* to contain the file contents
read the file contents into the char*
return the char* pointer, with the function return adding the const
the file is closed automatically by the fstream going out of scope

fstream to const char *

3 Answers3

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