C++ how to use a variable in a string

Question

The following code doesn't work:

string currency;
currency = "EURUSD";

system("lynx -dump 'http://somesite.com/q?s="+currency+"=X' > file.txt");

How do I use currency inside this line of C++'s system() call?

This is my error:

Error value:
main.cpp: In function ‘int main()’:
main.cpp:22:84: error: cannot convert ‘std::basic_string<char>’ to ‘const char*’ for argument ‘1’ to ‘int system(const char*)’
make: *** [main.o] Error 1
BUILD FAILED (exit value 2, total time: 890ms)

In PHP I use . to join strings, but in C++ I am not sure of the syntax.

perreal · Accepted Answer · 2012-07-25 05:23:07Z

5

Using stringstream:

#include <sstream>
string currency;
currency = "EURUSD";
std::stringstream ss;
ss << "lynx -dump 'http://somesite.com/q?s=" << currency << "=X' > file.txt";
system(ss.str().c_str());

answered Jul 25, 2012 at 5:23

perreal

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1 Comment

johnsyweb Over a year ago

+1: Better than my answer as this will work with "variables" other than std::string (as long as they have the appropriate operator <<()defined. This also allows formatting via manipulators.

Ignacio Vazquez-Abrams · Accepted Answer · 2012-07-25 05:19:59Z

2

std::string::c_str() will give you a char* (const'd somehow) that you can use in C string functions. But you should consider creating a std::string that contains the full command first.

answered Jul 25, 2012 at 5:19

Ignacio Vazquez-Abrams

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4 Comments

Qaz Over a year ago

What do you mean by const'd somehow?

Ignacio Vazquez-Abrams Over a year ago

@chris: There's 3 different ways to const a pointer to a non-void type. I can't recall which way is used here.

Qaz Over a year ago

@IgnacioVazquez-Abrams, It returns const char *.

Ignacio Vazquez-Abrams Over a year ago

@chris: Well there we go then.

johnsyweb · Accepted Answer · 2012-07-25 05:25:29Z

2

You can't + a std::string to a char []. Although you can + a char [] to a std::string.

I suggest using std::strings and getting the const char* just for the call to system():

string const currency = "EURUSD";
string const command_line = "lynx -dump 'http://somesite.com/q?s=" + currency + "=X' > file.txt";

system(command_line.c_str());

answered Jul 25, 2012 at 5:25

johnsyweb

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Comments

steffen · Accepted Answer · 2012-07-25 06:09:40Z

0

That should work.

std::string currency("EURUSD");
std::string command("lynx -dump 'http://somesite.com/q?s="+currency+"=X' > file.txt")
system(command.c_str());

edited Jul 25, 2012 at 6:09

answered Jul 25, 2012 at 5:30

steffen

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3 Comments

steffen Over a year ago

Johnsyweb was faster typing ;-)

Qaz Over a year ago

command.c_street? You've been watching too much Coronation Street.

steffen Over a year ago

Frickin' mobile's auto-completion ;-)

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