Examples

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## Examples

### Strings to numbers conversion

The following example treats command line arguments as a sequence of numeric data

    #include <boost/lexical_cast.hpp>
    #include <vector>

    int main(int /*argc*/, char * argv[])
    {
        using boost::lexical_cast;
        using boost::bad_lexical_cast;

        std::vector<short> args;

        while (*++argv)
        {
            try
            {
                args.push_back(lexical_cast<short>(*argv));
            }
            catch(const bad_lexical_cast &)
            {
                args.push_back(0);
            }
        }

        // ...
    }

### Numbers to strings conversion

The following example uses numeric data in a string expression:

    void log_message(const std::string &);

    void log_errno(int yoko)
    {
        log_message("Error " + boost::lexical_cast<std::string>(yoko) + ": " + strerror(yoko));
    }

### Converting to string without dynamic memory allocation

The following example converts some number and puts it to file:

    void number_to_file(int number, FILE* file)
    {
        typedef boost::array<char, 50> buf_t; // You can use std::array if your compiler supports it
        buf_t buffer = boost::lexical_cast<buf_t>(number); // No dynamic memory allocation
        std::fputs(buffer.begin(), file);
    }

### Converting part of the string

The following example takes part of the string and converts it to `int`:

    int convert_strings_part(const std::string& s, std::size_t pos, std::size_t n)
    {
        return boost::lexical_cast<int>(s.data() + pos, n);
    }

### Generic programming (Boost.Fusion)

In this example we'll make a `stringize` method that accepts a sequence, converts each element of the sequence into string and appends that string to the result.

Example is based on the example from the Boost C++ Application Development Cookbook by Antony Polukhin, ISBN 9781849514880.

Step 1: Making a functor that converts any type to a string and remembers result:

    #include <boost/lexical_cast.hpp>

    struct stringize_functor {
    private:
        std::string& result;

    public:
        explicit stringize_functor(std::string& res)
            : result(res)
        {}

        template <class T>
        void operator()(const T& v) const {
            result += boost::lexical_cast<std::string>(v);
        }
    };

Step 2: Applying `stringize_functor` to each element in sequence:

    #include <boost/fusion/include/for_each.hpp>
    template <class Sequence>
    std::string stringize(const Sequence& seq) {
        std::string result;
        boost::fusion::for_each(seq, stringize_functor(result));
        return result;
    }

Step 3: Using the `stringize` with different types:

    #include <cassert>
    #include <boost/fusion/adapted/boost_tuple.hpp>
    #include <boost/fusion/adapted/std_pair.hpp>

    int main() {
        boost::tuple<char, int, char, int> decim('-', 10, 'e', 5);
        assert(stringize(decim) == "-10e5");

        std::pair<short, std::string> value_and_type(270, "Kelvin");
        assert(stringize(value_and_type) == "270Kelvin");
    }

### Generic programming (Boost.Variant)

In this example we'll make a `to_long_double` method that converts value of the Boost.Variant to `long double`.

    #include <boost/lexical_cast.hpp>
    #include <boost/variant.hpp>
    #include <cassert>

    struct to_long_double_functor: boost::static_visitor<long double> {
        template <class T>
        long double operator()(const T& v) const {
            // Lexical cast has many optimizations including optimizations for situations that usually
            // occur in generic programming, like std::string to std::string or arithmetic type to arithmetic type conversion.
            return boost::lexical_cast<long double>(v);
        }
    };

    // Throws `boost::bad_lexical_cast` if value of the variant is not convertible to `long double`
    template <class Variant>
    long double to_long_double(const Variant& v) {
        return boost::apply_visitor(to_long_double_functor(), v);
    }

    int main() {
        boost::variant<char, int, std::string> v1('0'), v2("10.0001"), v3(1);

        long double sum = to_long_double(v1) + to_long_double(v2) + to_long_double(v3);
        assert(sum > 11 && sum < 11.1);
    }