Question

Windows XP SP3. Core 2 Duo 2.0 GHz.
I’m finding the boost::lexical_cast performance to be extremely slow. Wanted to find out ways to speed up the code. Using /O2 optimizations on visual c++ 2008 and comparing with java 1.6 and python 2.6.2 I see the following results.

Integer casting:

c++: 
std::string s ;
for(int i = 0; i < 10000000; ++i)
{
    s = boost::lexical_cast<string>(i);
}

java:
String s = new String();
for(int i = 0; i < 10000000; ++i)
{
    s = new Integer(i).toString();
}

python:
for i in xrange(1,10000000):
    s = str(i)

The times I’m seeing are

c++: 6700 milliseconds

java: 1178 milliseconds

python: 6702 milliseconds

c++ is as slow as python and 6 times slower than java.

Double casting:

c++:
std::string s ;
for(int i = 0; i < 10000000; ++i)
{
    s = boost::lexical_cast<string>(d);
}

java:
String s = new String();
for(int i = 0; i < 10000000; ++i)
{
    double d = i*1.0;
    s = new Double(d).toString();
}

python:
for i in xrange(1,10000000):
    d = i*1.0
    s = str(d)

The times I’m seeing are

c++: 56129 milliseconds

java: 2852 milliseconds

python: 30780 milliseconds

So for doubles c++ is actually half the speed of python and 20 times slower than the java solution!!. Any ideas on improving the boost::lexical_cast performance? Does this stem from the poor stringstream implementation or can we expect a general 10x decrease in performance from using the boost libraries.

Answer 1

Edit 2012-04-11

rve quite rightly commented about lexical_cast’s performance, providing a link:

http://www.boost.org/doc/libs/1_49_0/doc/html/boost_lexical_cast/performance.html

I don’t have access right now to boost 1.49, but I do remember making my code faster on an older version. So I guess:

1. the following answer is still valid (if only for learning purposes)
2. there was probably an optimization introduced somewhere between the two versions (I’ll search that)
3. which means that boost is still getting better and better

Original answer

Just to add info on Barry’s and Motti’s excellent answers:

Some background

Please remember Boost is written by the best C++ developers on this planet, and reviewed by the same best developers. If lexical_cast was so wrong, someone would have hacked the library either with criticism or with code.

I guess you missed the point of lexical_cast‘s real value…

Comparing apples and oranges.

In Java, you are casting an integer into a Java String. You’ll note I’m not talking about an array of characters, or a user defined string. You’ll note, too, I’m not talking about your user-defined integer. I’m talking about strict Java Integer and strict Java String.

In Python, you are more or less doing the same.

As said by other posts, you are, in essence, using the Java and Python equivalents of sprintf (or the less standard itoa).

In C++, you are using a very powerful cast. Not powerful in the sense of raw speed performance (if you want speed, perhaps sprintf would be better suited), but powerful in the sense of extensibility.

Comparing apples.

If you want to compare a Java Integer.toString method, then you should compare it with either C sprintf or C++ ostream facilities.

The C++ stream solution would be 6 times faster (on my g++) than lexical_cast, and quite less extensible:

inline void toString(const int value, std::string & output)
{
   // The largest 32-bit integer is 4294967295, that is 10 chars
   // On the safe side, add 1 for sign, and 1 for trailing zero
   char buffer[12] ;
   sprintf(buffer, "%i", value) ;
   output = buffer ;
}

The C sprintf solution would be 8 times faster (on my g++) than lexical_cast but a lot less safe:

inline void toString(const int value, char * output)
{
   sprintf(output, "%i", value) ;
}

Both solutions are either as fast or faster than your Java solution (according to your data).

Comparing oranges.

If you want to compare a C++ lexical_cast, then you should compare it with this Java pseudo code:

Source s ;
Target t = Target.fromString(Source(s).toString()) ;

Source and Target being of whatever type you want, including built-in types like boolean or int, which is possible in C++ because of templates.

Extensibility? Is that a dirty word?

No, but it has a well known cost: When written by the same coder, general solutions to specific problems are usually slower than specific solutions written for their specific problems.

In the current case, in a naive viewpoint, lexical_cast will use the stream facilities to convert from a type A into a string stream, and then from this string stream into a type B.

This means that as long as your object can be output into a stream, and input from a stream, you’ll be able to use lexical_cast on it, without touching any single line of code.

So, what are the uses of lexical_cast?

The main uses of lexical casting are:

1. Ease of use (hey, a C++ cast that works for everything being a value!)
2. Combining it with template heavy code, where your types are parametrized, and as such you don’t want to deal with specifics, and you don’t want to know the types.
3. Still potentially relatively efficient, if you have basic template knowledge, as I will demonstrate below

The point 2 is very very important here, because it means we have one and only one interface/function to cast a value of a type into an equal or similar value of another type.

This is the real point you missed, and this is the point that costs in performance terms.

But it’s so slooooooowwww!

If you want raw speed performance, remember you’re dealing with C++, and that you have a lot of facilities to handle conversion efficiently, and still, keep the lexical_cast ease-of-use feature.

It took me some minutes to look at the lexical_cast source, and come with a viable solution. Add to your C++ code the following code:

#ifdef SPECIALIZE_BOOST_LEXICAL_CAST_FOR_STRING_AND_INT

namespace boost
{
   template<>
   std::string lexical_cast<std::string, int>(const int &arg)
   {
      // The largest 32-bit integer is 4294967295, that is 10 chars
      // On the safe side, add 1 for sign, and 1 for trailing zero
      char buffer[12] ;
      sprintf(buffer, "%i", arg) ;
      return buffer ;
   }
}

#endif

By enabling this specialization of lexical_cast for strings and ints (by defining the macro SPECIALIZE_BOOST_LEXICAL_CAST_FOR_STRING_AND_INT), my code went 5 time faster on my g++ compiler, which means, according to your data, its performance should be similar to Java’s.

And it took me 10 minutes of looking at boost code, and write a remotely efficient and correct 32-bit version. And with some work, it could probably go faster and safer (if we had direct write access to the std::string internal buffer, we could avoid a temporary external buffer, for example).