Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

PrevUpHomeNext

Synopsis

lexical_cast
bad_lexical_cast
try_lexical_convert

Library features defined in boost/lexical_cast.hpp:

namespace boost
{
    class bad_lexical_cast;

    template<typename Target, typename Source>
      Target lexical_cast(const Source& arg);

    template <typename Target>
      Target lexical_cast(const AnyCharacterType* chars, std::size_t count);

    namespace conversion
    {
        template<typename Target, typename Source>
            bool try_lexical_convert(const Source& arg, Target& result);

        template <typename AnyCharacterType, typename Target>
            bool try_lexical_convert(const AnyCharacterType* chars, std::size_t count, Target& result);

    } // namespace conversion
} // namespace boost

template<typename Target, typename Source>
  Target lexical_cast(const Source& arg);

Returns the result of streaming arg into a standard library string-based stream and then out as a Target object. Where Target is either std::string or std::wstring, stream extraction takes the whole content of the string, including spaces, rather than relying on the default operator>> behavior. If the conversion is unsuccessful, a bad_lexical_cast exception is thrown.

template <typename Target>
  Target lexical_cast(const AnyCharacterType* chars, std::size_t count);

Takes an array of count characters as input parameter and streams them out as a Target object. If the conversion is unsuccessful, a bad_lexical_cast exception is thrown. This call may be useful for processing nonzero terminated array of characters or processing just some part of character array.

The requirements on the argument and result types for both functions are:

  • Source is OutputStreamable, meaning that an operator<< is defined that takes a std::ostream or std::wostream object on the left hand side and an instance of the argument type on the right.
  • Target is InputStreamable, meaning that an operator>> is defined that takes a std::istream or std::wistream object on the left hand side and an instance of the result type on the right.
  • Target is CopyConstructible [20.1.3].
  • Target is DefaultConstructible, meaning that it is possible to default-initialize an object of that type [8.5, 20.1.4].

The character type of the underlying stream is assumed to be char unless either the Source or the Target requires wide-character streaming, in which case the underlying stream uses wchar_t. Following types also can use char16_t or char32_t for wide-character streaming:

  • Single character: char16_t, char32_t
  • Arrays of characters: char16_t *, char32_t *, const char16_t *, const char32_t *
  • Strings: std::basic_string, boost::containers::basic_string
  • boost::iterator_range<WideCharPtr>, where WideCharPtr is a pointer to wide-character or pointer to const wide-character
  • boost::array<CharT, N> and std::array<CharT, N>, boost::array<const CharT, N> and std::array<const CharT, N>
[Important] Important

Many compilers and runtime libraries fail to make conversions using new Unicode characters. Make sure that the following code compiles and outputs nonzero values, before using new types:

std::cout
    << boost::lexical_cast<std::u32string>(1.0).size()
    << "  "
    << boost::lexical_cast<std::u16string>(1.0).size();

Where a higher degree of control is required over conversions, std::stringstream and std::wstringstream offer a more appropriate path. Where non-stream-based conversions are required, lexical_cast is the wrong tool for the job and is not special-cased for such scenarios.

class bad_lexical_cast : public std::bad_cast
{
public:
    ... // same member function interface as std::exception
};

Exception used to indicate runtime lexical_cast failure.

boost::lexical_cast remains the main interface for lexical conversions. It must be used by default in most cases. However some developers wish to make their own conversion functions, reusing all the optimizations of the boost::lexical_cast. That's where the boost::conversion::try_lexical_convert function steps in.

try_lexical_convert returns true if conversion succeeded, otherwise returns false. If conversion failed and false was returned, state of result output variable is undefined.

Actually, boost::lexical_cast is implemented using try_lexical_convert:

template <typename Target, typename Source>
inline Target lexical_cast(const Source &arg)
{
    Target result;

    if (!conversion::try_lexical_convert(arg, result))
        throw bad_lexical_cast();

    return result;
}

try_lexical_convert relaxes the CopyConstructible and DefaultConstructible requirements for Target type. Following requirements for Target and Source remain:

  • Source must be OutputStreamable, meaning that an operator<< is defined that takes a std::ostream or std::wostream object on the left hand side and an instance of the argument type on the right.
  • Target must be InputStreamable, meaning that an operator>> is defined that takes a std::istream or std::wistream object on the left hand side and an instance of the result type on the right.

PrevUpHomeNext