Errors when putting a string variable into system()

Question

I'm having trouble getting system() to run a command in a string variable.

ostringstream convert;
convert << getSeconds(hours);
string seconds = convert.str();    /* converts the output of 'getSeconds()' into
                                      a string and puts it into 'seconds' */

string cmd = "shutdown /s /t " + seconds;

system(cmd);

getSeconds() just takes an int in hours, converts it into seconds and returns an int in seconds. Everything runs fine, no errors, until it reaches system(cmd);. The compiler then spits out this error:

error: cannot convert 'std::string {aka std::basic_string<char>}' to
'const char*' for argument '1' to 'int system(const char*)'

Here are my includes:

#include <iostream>
#include <string>
#include <cstdlib>
#include <sstream>

system(cmd.c_str());

Borgleader
– Borgleader

2013-07-24 23:39:14 +00:00
Commented Jul 24, 2013 at 23:39 — Borgleader
– Borgleader, Commented Jul 24, 2013 at 23:39
Why don't you put it as an answer when it is the answer?

Neil Kirk
– Neil Kirk

2013-07-24 23:43:45 +00:00
Commented Jul 24, 2013 at 23:43 — Neil Kirk
– Neil Kirk, Commented Jul 24, 2013 at 23:43

Mats Petersson · Accepted Answer · 2013-07-24 23:44:51Z

5

I know this has already been answered by comments, but not really explained:

The system function is a C function. It does not "understand" C++ style strings. For that, you will need to use the c_str() function. In other words, you need system(cmd.c_str());.

This applies to a large number of C style functions that are still available in C++, since one of the main features of C++ is that you can still use traditional C-code (for the most part) in C++. So, the same applies to almost any C style function that takes a string - printf("cmd=%s", cmd.c_str()); would print what your command is.

It would be possibe to write your own wrapper function:

int system(const std::string &cmd)
{
   return system(cmd.c_str());
}

Now, the rest of your code can use system with a regular C++ style string.

answered Jul 24, 2013 at 23:44

Mats Petersson

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Comments

Neil Kirk · Accepted Answer · 2013-07-24 23:44:38Z

2

system takes a C string not a std::string, so you must call the c_str function first.

system(cmd.c_str());

answered Jul 24, 2013 at 23:44

Neil Kirk

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