Header
boost/lexical_cast.hppHeader
boost/lexical_cast.hpplexical_castbad_lexical_castint
represented as a string, or vice-versa, when a string
is interpreted as an int. Such examples are common when converting
between data types internal to a program and representation external to a
program, such as windows and configuration files.
The standard C and C++ libraries offer a number of facilities for performing such conversions. However, they vary with their ease of use, extensibility, and safety.
For instance, there are a number of limitations with the family of standard C
functions typified by atoi:
sprintf function, or the loss of
portability associated with non-standard functions such as itoa.
int, long, and double.
complex or rational.
strtol have the same basic
limitations, but offer finer control over the conversion process. However, for
the common case such control is often either not required or not used. The scanf
family of functions offer even greater control, but also lack safety and ease
of use.
The standard C++ library offers stringstream for the kind of
in-core formatting being discussed. It offers a great deal of control over the
formatting and conversion of I/O to and from arbitrary types through text.
However, for simple conversions direct use of stringstream can be
either clumsy (with the introduction of extra local variables and the loss of
infix-expression convenience) or obscure (where stringstream
objects are created as temporary objects in an expression). Facets provide a
comprehensive concept and facility for controlling textual representation, but
their perceived complexity and high entry level requires an extreme degree of
involvement for simple conversions, and excludes all but a few programmers.
The lexical_cast function template offers a convenient and
consistent form for supporting common conversions to and from arbitrary types
when they are represented as text. The simplification it offers is in
expression-level convenience for such conversions. For more involved
conversions, such as where precision or formatting need tighter control than is
offered by the default behavior of lexical_cast, the conventional
stringstream approach is recommended. Where the conversions are
numeric to numeric, numeric_cast
may offer more reasonable behavior than lexical_cast.
For a good discussion of the options and issues involved in string-based
formatting, including comparison of stringstream, lexical_cast,
and others, see Herb Sutter's article,
The String Formatters of Manor Farm. Also, take a look at the Performance section.
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(bad_lexical_cast &)
{
args.push_back(0);
}
}
...
}
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));
}
"boost/lexical_cast.hpp":
namespace boost
{
class bad_lexical_cast;
template<typename Target, typename Source>
Target lexical_cast(const Source& arg);
}
Unit test defined in "lexical_cast_test.cpp".
lexical_castReturns the result of streamingtemplate<typename Target, typename Source> Target lexical_cast(const Source& arg);
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.
The requirements on the argument and result types 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].
char unless
either the Source or the Target requires wide-character
streaming, in which case the underlying stream uses wchar_t.
Source types that require wide-character streaming are wchar_t,
wchar_t *, and std::wstring. Target types that
require wide-character streaming are wchar_t and std::wstring.
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.
bad_lexical_cast
class bad_lexical_cast : public std::bad_cast
{
public:
... // same member function interface as std::exception
};
Exception used to indicate runtime lexical_cast
failure.
| Question: | Why does lexical_cast<int8_t>("127") throw bad_lexical_cast? |
| Answer: | The type int8_t is a typedef to char or signed char.
Lexical conversion to these types is simply reading a byte from source but since the source has
more than one byte, the exception is thrown.
Please use other integer types such as int or short int. If bounds checking
is important, you can also call numeric_cast:
numeric_cast<int8_t>(lexical_cast<int>("127")); |
| Question: | What does lexical_cast<std::string> of an int8_t or uint8_t not do what I expect? |
| Answer: | As above, note that int8_t and uint8_t are actually chars and are formatted as such. To avoid this, cast to an integer type first:
lexical_cast<std::string>(static_cast<int>(n)); |
| Question: | The implementation always resets the ios_base::skipws flag of an underlying stream object. It breaks my operator>> that works only in presence of this flag. Can you remove code that resets the flag? |
| Answer: | May be in a future version. There is no requirement in [N1973] to reset the flag but remember that [N1973] is not yet accepted by the committee. By the way, it's a great opportunity to make your operator>> conform to the standard. Read a good C++ book, study std::sentry and ios_state_saver.
|
| Question: | Why std::cout << boost::lexical_cast<unsigned int>("-1"); does not throw, but outputs 4294967295? |
| Answer: | boost::lexical_cast has the behavior of stringstream, which uses num_get functions of std::locale to convert numbers. If we look at the [22.2.2.1.2] of Programming languages — C++, we'll see, that num_get uses the rules of scanf for conversions. And in the C99 standard for unsigned input value minus sign is optional, so if a negative number is read, no errors will arise and the result will be the two's complement.
|
Target from Source on some conversions (like conversions from string to string, from char array to string, from char to char and others).Source and Target
types. Refer to [Tuning] for more details.
lexical_cast used the default stream
precision for reading and writing floating-point numbers. For numerics that
have a corresponding specialization of std::numeric_limits, the
current version now chooses a precision to match. lexical_cast did not support conversion
to or from any wide-character-based types. For compilers with full language
and library support for wide characters, lexical_cast now supports
conversions from wchar_t, wchar_t *, and std::wstring
and to wchar_t and std::wstring. lexical_cast assumed that the conventional
stream extractor operators were sufficient for reading values. However, string
I/O is asymmetric, with the result that spaces play the role of I/O separators
rather than string content. The current version fixes this error for std::string
and, where supported, std::wstring: lexical_cast<std::string>("Hello,
World") succeeds instead of failing with a bad_lexical_cast
exception. lexical_cast allowed unsafe and meaningless
conversions to pointers. The current version now throws a bad_lexical_cast
for conversions to pointers: lexical_cast<char *>("Goodbye, World")
now throws an exception instead of causing undefined behavior.
| From->To | lexical_cast | std::stringstreamwith construction | std::stringstreamwithout construction | sscanf/sprintf |
| string->char | <1 | 91 | 7 | 10 |
| string->int | 7 | 115 | 23 | 18 |
| string->unsigned int | 7 | 117 | 22 | 17 |
| string->bool | <1 | 104 | 19 | 10 |
| string->float | 85 | 172 | 60 | 33 |
| char->string | 7 | 105 | 16 | 12 |
| int->string | 15 | 131 | 21 | 17 |
| unsigned int->string | 14 | 125 | 21 | 17 |
| bool->string | 7 | 122 | 24 | 12 |
| float->string | 124 | 223 | 115 | 48 |
| char*->string | 9 | 123 | 20 | --- |
| int->int | <1 | 120 | 26 | --- |
| float->float | <1 | 262 | 142 | --- |