Passing a string to file.open();

Question

I am used to higher level languages (java, python etc.), where this is dead obvious. I am trying to pass a string the user inputs to cin, the name of a file to open. There appears to be some sort of pointer madness error, and my code will not compile. I deleted some of my code to make it more clear.

#include <iostream>
#include <fstream>
using namespace std;
       
string hash(string filename);
    
int main(){
    cout << "Please input a file name to hash\n";
    string filename;
    cin >> filename;
    cout << hash(filename);
    return 0;
}
        
        
string hash(string filename){
    file.open(filename);
    if(file.is_open()){
        file.close();
    }
    ...
    return returnval;
}

Here is the compile time error.

<code>
$ g++ md5.cpp
md5.cpp: In function ‘std::string hash(std::string)’:
md5.cpp:22: error: no matching function for call to ‘std::basic_ifstream<char, std::char_traits<char> >::open(std::string&)’
/usr/include/c++/4.2.1/fstream:518: note: candidates are: void std::basic_ifstream<_CharT, _Traits>::open(const char*, std::_Ios_Openmode) [with _CharT = char, _Traits = std::char_traits<char>]
</code>

(I know that there are libraries for md5 hashes, but I am trying to learn about how the hash works, and eventually hash collision)

Take your line numbers and extra spacing out before posting. It makes it hard to copy and paste your code for testing. — Martin York, Jun 10 '12 at 05:16
@LokiAstari, I found it out too late, but in most editors you can do Ctrl+Alt+Selection to select the columns with the numbers in and delete them all at once. Saved me a lot of time ever since I found out about it. — chris, Jun 10 '12 at 05:21

chris · Accepted Answer · 2012-06-10T13:14:22.940

21

open() takes a C-style string. Use std::string::c_str() to get this:

file.open (filename.c_str());

In order to use just a string, as pointed out below, you'll need to use a compiler with C++11 support, as the overload was added for C++11.

The reason it's not like Java etc. is that it came from C. Classes didn't exist in C (well, not nearly as well as they do in C++), let alone a String class. In order for C++ to provide a string class and keep compatibility, they need to be different things, and the class provides a conversion constructor for const char * -> std::string, as well as c_str() to go the other way.

Consider passing the argument (and maybe the return too) as const std::string & as well; no unnecessary copies. The optimization would probably catch those, but it's always good to do.

edited Jun 10 '12 at 13:14

answered Jun 10 '12 at 04:54

chris

60,560
13
143
205

Excellent, the first part fixed my problem. But what is the differenace between a C string and a C++ string? Also, where should I put the const std::string & ? I tried the method declaration, the actual method call, but both gave compile time errors. – Muricula Jun 10 '12 at 05:00
@Muricula, A C-style string is more just a plain array of characters. A C++ string is an actual class with functions, state, etc. I'll try to find a good question on here. – chris Jun 10 '12 at 05:01
@Muricula, here's one: http://stackoverflow.com/questions/3454900/whats-the-difference-between-c-strings-and-c-strings – chris Jun 10 '12 at 05:03
7

Another option is to use a newer compiler. The C++11 standard has added an `open(string)` function to the streams. – Bo Persson Jun 10 '12 at 09:22

Passing a string to file.open();

1 Answers1