Boost :: lexical_cast and double weird behavior

Question

Why does the following code give me different results when I write "2.01" and "2.02" ?

 #include <boost/lexical_cast.hpp> #include <iostream> #include <string> int main() { const std::string str = "2.02"; try { const double b = boost::lexical_cast<double>(str) * 100.0; std::cout << "double: " << b << '\n'; const int a = boost::lexical_cast<int>(b); std::cout << "int: " << a << '\n'; } catch (const std::exception& ex) { std::cerr << ex.what() << '\n'; } } 

Exit

 double: 202 int: 202 

But if I change "2.02" to "2.01" , it gives me the following result:

 double: 201 bad lexical cast: source type value could not be interpreted as target 

Why?

I am using Visual Studio 2013 (msvc-12.0) and increasing 1.57.

Thanks in advance.

Answer 1

Floating point inaccuracy.

There is no exact representation of 2.01 in binary floating point, so multiplying by 100 does not produce an integer.

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 std::cout << "double: " << std::setprecision(18) << std::fixed << b << '\n'; 

Print

 double: 200.999999999999971578 

For this reason, conversion to int ends.