String.Format alternative in C++ [duplicate]

Question

I don't have much experience working with C++. Rather I have worked more in C# and so, I wanted to ask my question by relating to what I would have done in there. I have to generate a specific format of the string, which I have to pass to another function. In C#, I would have easily generated the string through the below simple code.

string a = "test"; string b = "text.txt"; string c = "text1.txt";  String.Format("{0} {1} > {2}", a, b, c); 

By generating such an above string, I should be able to pass this in system(). However, system only accepts char*

I am on Win32 C++ (not C++/CLI), and cannot use boost since it would include too much inclusion of all the files for a project which itself is very small. Something like sprintf() looks useful to me, but sprintf does not accept string as the a, b and c parameters. Any suggestions how I can generate these formatted strings to pass to system in my program?

Answer 1

The C++ way would be to use a std::stringstream object as:

std::stringstream fmt; fmt << a << " " << b << " > " << c; 

The C way would be to use sprintf.

The C way is difficult to get right since:

• It is type unsafe
• Requires buffer management

Of course, you may want to fall back on the C way if performance is an issue (imagine you are creating fixed-size million little stringstream objects and then throwing them away).

Answer 2

For the sake of completeness, you may use std::stringstream:

#include <iostream> #include <sstream> #include <string>  int main() {     std::string a = "a", b = "b", c = "c";     // apply formatting     std::stringstream s;     s << a << " " << b << " > " << c;     // assign to std::string     std::string str = s.str();     std::cout << str << "\n"; } 

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Or (in this case) std::string's very own string concatenation capabilities:

#include <iostream> #include <string>  int main() {     std::string a = "a", b = "b", c = "c";     std::string str = a + " " + b + " > " + c;     std::cout << str << "\n"; } 

For reference:

• http://en.cppreference.com/w/cpp/string/basic_string/operator+

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If you really want to go the C way. Here you are:

#include <iostream> #include <string> #include <vector> #include <cstdio>  int main() {     std::string a = "a", b = "b", c = "c";     const char fmt[] = "%s %s > %s";     // use std::vector for memory management (to avoid memory leaks)     std::vector<char>::size_type size = 256;     std::vector<char> buf;     do {         // use snprintf instead of sprintf (to avoid buffer overflows)         // snprintf returns the required size (without terminating null)         // if buffer is too small initially: loop should run at most twice         buf.resize(size+1);         size = std::snprintf(                 &buf[0], buf.size(),                 fmt, a.c_str(), b.c_str(), c.c_str());     } while (size+1 > buf.size());     // assign to std::string     std::string str(buf.begin(), buf.begin()+size);     std::cout << str << "\n"; } 

For reference:

• http://en.cppreference.com/w/cpp/io/c/fprintf

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Then, there's the Boost Format Library. For the sake of your example:

#include <iostream> #include <string> #include <boost/format.hpp>  int main() {     std::string a = "a", b = "b", c = "c";     // apply format     boost::format fmt = boost::format("%s %s > %s") % a % b % c;      // assign to std::string     std::string str = fmt.str();     std::cout << str << "\n"; }