Convert string to const char* (with binary data)

Question

What I want to do is to convert a string into const char*.

Ideally I could just use "const char* myConstChar = myString.c_str()" but as my example below shows; it doesn´t work well for binary data:

#include <iostream>
#include <fstream>
#include <stdio.h>
#include <string.h>

using namespace std;

int main(){
    string myString;  // This need to be string
    ifstream infile;
const char* myConstChar;  // This need to be const char*

infile.open("binary.bin");

if(infile){
        while(getline(infile, myString)){ 
        std::cout << "Data: " << myString << " string length: "; 
            std::cout << myString.length() << "\n";
            myConstChar = myString.c_str();
            std::cout << "const char Data: " << myConstChar; 
            std::cout << " const char length: "<< strlen(myConstChar) <<"\n";
        }
    }

    infile.close();
    return 0;
}

This returns "string length: 13" and "const char length: 3".

Apparently there are some loss of data when converting string to const char* using myString.c_str()!

How do I convert a string to const char* without losing binary data?!

Why are you using a string to read binary data?

shf301
– shf301

2012-06-17 19:27:42 +00:00
Commented Jun 17, 2012 at 19:27 — shf301
– shf301, Commented Jun 17, 2012 at 19:27
I suggest you switch to std::vector<char>.

jrok
– jrok

2012-06-17 19:28:05 +00:00
Commented Jun 17, 2012 at 19:28 — jrok
– jrok, Commented Jun 17, 2012 at 19:28
How is it c question? o.O

Griwes
– Griwes

2012-06-17 19:28:50 +00:00
Commented Jun 17, 2012 at 19:28 — Griwes
– Griwes, Commented Jun 17, 2012 at 19:28

Oliver Charlesworth · Accepted Answer · 2012-06-17 19:28:09Z

5

This is almost certainly because your binary data contains zero-valued bytes. These are identical to null-terminators, which functions like strlen use to determine the end of a string.

It's arguable that arbitrary binary data shouldn't be treated as a string. So use std::vector<char> instead, and don't use functions like strlen.

answered Jun 17, 2012 at 19:28

Oliver Charlesworth

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7 Comments

Mr Lister Over a year ago

Or, simply keep it in a std::string, and use c_str() combined with length() to access the content!

user1432032 Over a year ago

Okey, there are a few zero-valued bytes in the example: hexdump -C ./binary.bin 00000000 03 01 6a 00 00 07 56 52 03 30 00 01 01. So is it possible to convert it to const char* at all?

user1432032 Over a year ago

The issue with std::vector<char> is that eventually I want to convert the data into readable content. The custom library I use only take const char* as argument. Thus I would end up with needing to convert from vector<char> into const char* instead...

Heisenbug Over a year ago

"I want convert the data into readable content". There is no possible conversion between binary data and strings. All you can do with binary is print them in the hex form, or something like that.

Oliver Charlesworth Over a year ago

@user1432032: Yes; your code already does obtain a valid const char *. The problem is that you cannot treat it like a string, because it isn't one.

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