问题
I'm working with a library that unfortunately uses boost::lexical_cast to convert from a double to a string.
I need to be able to definitively mirror that behavior on my side, but I was hopping to do so without propagating boost.
Could I be guaranteed identical behavior using to_string, sprintf, or some other function contained within the standard?
回答1:
The boost code ends up here:
bool shl_real_type(double val, char* begin) {
using namespace std;
finish = start +
#if defined(_MSC_VER) && (_MSC_VER >= 1400) && !defined(__SGI_STL_PORT) && !defined(_STLPORT_VERSION)
sprintf_s(begin, CharacterBufferSize,
#else
sprintf(begin,
#endif
"%.*g", static_cast<int>(boost::detail::lcast_get_precision<double>()), val);
return finish > start;
}
You're in luck since the precision is USUALLY compile-time constant (unless boost configures BOOST_LCAST_NO_COMPILE_TIME_PRECISION).
Simplifying a bit and allowing for conforming, modern standard libraries:
Mimicking Boost Lexicalcast
#include <cstdio>
#include <limits>
#include <string>
namespace {
template <class T> struct lcast_precision {
typedef std::numeric_limits<T> limits;
static constexpr bool use_default_precision = !limits::is_specialized || limits::is_exact;
static constexpr bool is_specialized_bin = !use_default_precision && limits::radix == 2 && limits::digits > 0;
static constexpr bool is_specialized_dec = !use_default_precision && limits::radix == 10 && limits::digits10 > 0;
static constexpr unsigned int precision_dec = limits::digits10 + 1U;
static constexpr unsigned long precision_bin = 2UL + limits::digits * 30103UL / 100000UL;
static constexpr unsigned value = is_specialized_bin
? precision_bin
: is_specialized_dec? precision_dec : 6;
};
std::string mimicked(double v) {
constexpr int prec = static_cast<int>(lcast_precision<double>::value);
std::string buf(prec+10, ' ');
buf.resize(sprintf(&buf[0], "%.*g", prec, v));
return buf;
}
}
Regression Tests
To compare the results and check the assumptions:
Live On Coliru
#include <cstdio>
#include <limits>
#include <string>
namespace {
template <class T> struct lcast_precision {
typedef std::numeric_limits<T> limits;
static constexpr bool use_default_precision = !limits::is_specialized || limits::is_exact;
static constexpr bool is_specialized_bin = !use_default_precision && limits::radix == 2 && limits::digits > 0;
static constexpr bool is_specialized_dec = !use_default_precision && limits::radix == 10 && limits::digits10 > 0;
static constexpr unsigned int precision_dec = limits::digits10 + 1U;
static constexpr unsigned long precision_bin = 2UL + limits::digits * 30103UL / 100000UL;
static constexpr unsigned value = is_specialized_bin
? precision_bin
: is_specialized_dec? precision_dec : 6;
};
std::string mimicked(double v) {
constexpr int prec = static_cast<int>(lcast_precision<double>::value);
std::string buf(prec+10, ' ');
buf.resize(sprintf(&buf[0], "%.*g", prec, v));
return buf;
}
}
#include <cmath>
#include <iomanip>
#include <iostream>
#include <string>
#include <boost/lexical_cast.hpp>
#ifdef BOOST_LCAST_NO_COMPILE_TIME_PRECISION
#error BOOM
#endif
#define TEST(x) \
do { \
std::cout << std::setw(45) << #x << ":\t" << (x) << "\n"; \
} while (0)
std::string use_sprintf(double v) {
std::string buf(32, ' ');
buf.resize(std::sprintf(&buf[0], "%f", v));
return buf;
}
void tests() {
for (double v : {
std::numeric_limits<double>::quiet_NaN(),
std::numeric_limits<double>::infinity(),
-std::numeric_limits<double>::infinity(),
0.0,
-0.0,
std::numeric_limits<double>::epsilon(),
M_PI })
{
TEST(v);
TEST(std::to_string(v));
TEST(use_sprintf(v));
TEST(boost::lexical_cast<std::string>(v));
TEST(mimicked(v));
assert(mimicked(v) == boost::lexical_cast<std::string>(v));
}
}
static std::locale DE("de_DE.utf8");
int main() {
tests();
std::cout << "==== imbue std::cout\n";
std::cout.imbue(DE);
tests();
std::cout << "==== override global locale\n";
std::locale::global(DE);
tests();
}
Prints
v: nan
std::to_string(v): nan
use_sprintf(v): nan
boost::lexical_cast<std::string>(v): nan
mimicked(v): nan
v: inf
std::to_string(v): inf
use_sprintf(v): inf
boost::lexical_cast<std::string>(v): inf
mimicked(v): inf
v: -inf
std::to_string(v): -inf
use_sprintf(v): -inf
boost::lexical_cast<std::string>(v): -inf
mimicked(v): -inf
v: 0
std::to_string(v): 0.000000
use_sprintf(v): 0.000000
boost::lexical_cast<std::string>(v): 0
mimicked(v): 0
v: -0
std::to_string(v): -0.000000
use_sprintf(v): -0.000000
boost::lexical_cast<std::string>(v): -0
mimicked(v): -0
v: 2.22045e-16
std::to_string(v): 0.000000
use_sprintf(v): 0.000000
boost::lexical_cast<std::string>(v): 2.2204460492503131e-16
mimicked(v): 2.2204460492503131e-16
v: 3.14159
std::to_string(v): 3.141593
use_sprintf(v): 3.141593
boost::lexical_cast<std::string>(v): 3.1415926535897931
mimicked(v): 3.1415926535897931
==== imbue std::cout
v: nan
std::to_string(v): nan
use_sprintf(v): nan
boost::lexical_cast<std::string>(v): nan
mimicked(v): nan
v: inf
std::to_string(v): inf
use_sprintf(v): inf
boost::lexical_cast<std::string>(v): inf
mimicked(v): inf
v: -inf
std::to_string(v): -inf
use_sprintf(v): -inf
boost::lexical_cast<std::string>(v): -inf
mimicked(v): -inf
v: 0
std::to_string(v): 0.000000
use_sprintf(v): 0.000000
boost::lexical_cast<std::string>(v): 0
mimicked(v): 0
v: -0
std::to_string(v): -0.000000
use_sprintf(v): -0.000000
boost::lexical_cast<std::string>(v): -0
mimicked(v): -0
v: 2,22045e-16
std::to_string(v): 0.000000
use_sprintf(v): 0.000000
boost::lexical_cast<std::string>(v): 2.2204460492503131e-16
mimicked(v): 2.2204460492503131e-16
v: 3,14159
std::to_string(v): 3.141593
use_sprintf(v): 3.141593
boost::lexical_cast<std::string>(v): 3.1415926535897931
mimicked(v): 3.1415926535897931
==== override global locale
v: nan
std::to_string(v): nan
use_sprintf(v): nan
boost::lexical_cast<std::string>(v): nan
mimicked(v): nan
v: inf
std::to_string(v): inf
use_sprintf(v): inf
boost::lexical_cast<std::string>(v): inf
mimicked(v): inf
v: -inf
std::to_string(v): -inf
use_sprintf(v): -inf
boost::lexical_cast<std::string>(v): -inf
mimicked(v): -inf
v: 0
std::to_string(v): 0,000000
use_sprintf(v): 0,000000
boost::lexical_cast<std::string>(v): 0
mimicked(v): 0
v: -0
std::to_string(v): -0,000000
use_sprintf(v): -0,000000
boost::lexical_cast<std::string>(v): -0
mimicked(v): -0
v: 2,22045e-16
std::to_string(v): 0,000000
use_sprintf(v): 0,000000
boost::lexical_cast<std::string>(v): 2,2204460492503131e-16
mimicked(v): 2,2204460492503131e-16
v: 3,14159
std::to_string(v): 3,141593
use_sprintf(v): 3,141593
boost::lexical_cast<std::string>(v): 3,1415926535897931
mimicked(v): 3,1415926535897931
Note that mimicked and boost::lexical_cast<std::string>(double) result in exactly the same output each time.
回答2:
So after a few hours of digging around in Boost templates here's what I've learned:
- The actual call that does the stringifying is:
lexical_cast_do_cast<std::string, double>::lexical_cast_impl - This uses
std::sprintfwithinboost::detail::lexical_stream_limited_src<char, std::char_traits<char>, false> boost::detail::lexical_stream_limited_src<char, std::char_traits<char>, true>::operator<<will be used to insert thedouble, passingbegin, the pointer to an allocatedstd::string's buffer, andval, the inputdouble, yields this call:std::sprintf(begin, "%.*g", static_cast<int>(boost::detail::lcast_get_precision<double>()), val)- So the precision field here comes from
boost::details::lcast_precision<double>::value, which, will usestd::numeric_limits<double>; if it'sis_specializedisfalse,is_exactisfalse,radixis2, anddigitsis greater than0thenboost::details::lcast_precision<double>::valuewill evaluate to:2UL + std::numeric_limits<double>::digits * 30103UL / 100000UL
Thus where begin is the allocated string and val is the input double,
boost::lexical_cast<double> yields a final result the equivalent of:
std::sprintf(begin, "%.*g", 2UL + std::numeric_limits<double>::digits * 30103UL / 100000UL, val)
This is obviously heavily implementation dependent. But on my system this will yield the exact equivalent.
来源：https://stackoverflow.com/questions/48080749/boosts-lexical-cast-from-double-to-string-precision