boost lexical cast string to double

c++ lexical-cast atof
user1400995 picture user1400995 · Aug 6, 2013 · Viewed 8.4k times · Source

I am facing a conversion issue for which I'd like your help. I'm using gcc4 compiler and I am quite restricted to use gcc4.

I want to convert std::string to double.

std::string aQuantity = aRate.getQuantity();
std::string aAmount = aRate.getAmount(); 

// aAmount = "22.05"

double dQuantity = boost::lexical_cast<double>(aQuantity);
double dAmount = boost::lexical_cast<double> (aAmount);

// dAmount =  22.050000000000001

By the way, I also tried atof and I still have the same issue. Is there any way to use istringstream with setprecission(2) to get the correct value shown by aAmount?

Answer

dunc123 picture dunc123 · Aug 6, 2013

Due to the nature of floating point values, 22.050000000000001 is the closest value to 22.05 that can be stored. The same would occure if you simply tried to store 22.05 in a double and then print it.

You should set the precision on the output stream if you want to print 22.05. Alternatively you could investigate a rational number library (for example, Boost.Rational). This would be able to store the value 22.05 precisely, unlike a double (or float).

Related questions

Very poor boost::lexical_cast performance

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 …

Read More
How do I use boost::lexical_cast and std::boolalpha? i.e. boost::lexical_cast< bool >("true")

I've seen some answers to other boost::lexical_cast questions that assert the following is possible: bool b = boost::lexical_cast< bool >("true"); This doesn't work for me with g++ 4.4.3 boost 1.43. (Maybe it's true that it works on …

Read More
How can I extend a lexical cast to support enumerated types?

I have the following function that will convert a string into a numeric data type: template <typename T> bool ConvertString(const std::string& theString, T& theResult) { std::istringstream iss(theString); return !(iss >> theResult).fail(); } …

Read More