sprintf in c++?

Question

I'm searching for an sprintf in c++.

I want to build a mysql query string but if I do it like (max_limit is an const int)

std::string query = "select * from bla limit " + max_limit;

The query wont work.

Answer 1

In C++11, this has been made too easy. Use std::to_string() as:

std::string query = "select * from bla limit " + std::to_string(max_limit);

Done!

---

OLD SOLUTION, for those who still use C++03.

Use stringbuilder and create std::string on the fly as:

std::string query = stringbuilder() << "select * from bla limit " << max_limit;

where stringbuilder is implemented as:

struct stringbuilder
{
   std::stringstream ss;
   template<typename T>
   stringbuilder & operator << (const T &data)
   {
        ss << data;
        return *this;
   }
   operator std::string() { return ss.str(); }
};

---

You can use stringbuilder in many different ways, such as:

std::string g(int m, int n) 
{
    //create string on the fly and returns it
    if ( m < n )
        return stringbuilder() << m << " is less than " << n ;
    return stringbuilder() << n << " is less than " << m ;
}

void f(const std::string & s );

//call f while creating string on the fly and passing it to the function
f(stringbuilder() << '{' << pc << '}' ); //passed as std::string

//this is my most favorite line
std::string s = stringbuilder() << 23  << " is greater than " << 5 ;

See demo at ideone : http://ideone.com/J995r

And see my blog on this : Create string on the fly just in one line

Answer 2

You don't want sprintf, it doesn't work with strings. Something like this:

#include <sstream>
#include <string>

template <typename T>
std::string Str( const T & t ) {
    std::ostringstream os;
    os << t;
    return os.str();
}

will do the job. You can then say:

std::string query = "select * from bla limit " + Str( max_limit );

Answer 3

Maybe you want to take a look at the boost::format lib. It provides the syntax of sprintf with the convenience of c++.
So your example would be:

std::string str = (boost::format("select * from bla limit %d") % max_limit).str();

Answer 4

Or just use a macro? #define QueryString(msg) ((static_cast<std::ostringstream&>(std::ostringstream().seekp(0, std::ios_base::cur)<<msg)).str())

Usage: std::string query = QueryString("select * from mytable where x="<<30);