我正在使用不幸使用boost::lexical_cast从double转换为string的库。
我需要能够明确地反映出我这方面的行为，但我还是在没有传播boost的情况下这样做。
我可以使用to_string，sprintf或标准中包含的其他功能保证相同的行为吗？
答案 0 :(得分：4)
所以经过几个小时的挖掘Boost模板后，这就是我所学到的：
>>> li=list("He'llo")
>>> li[2:4]=[''.join(li[2:4])]
>>> li
['H', 'e', "'l", 'l', 'o']
lexical_cast_do_cast<std::string, double>::lexical_cast_impl std::sprintf
boost::detail::lexical_stream_limited_src<char, std::char_traits<char>, false>将用于插入boost::detail::lexical_stream_limited_src<char, std::char_traits<char>, true>::operator<<，传递double，指向已分配的begin缓冲区的指针，以及std::string，输入val，产生此调用：double std::sprintf(begin, "%.*g", static_cast<int>(boost::detail::lcast_get_precision<double>()), val)，它将使用boost::details::lcast_precision<double>::value;如果std::numeric_limits<double>为is_specialized，则false为is_exact，false为radix，2大于{{1}然后digits将评估为：0 因此boost::details::lcast_precision<double>::value分配的2UL + std::numeric_limits<double>::digits * 30103UL / 100000UL和begin是输入双精度，
string产生的最终结果相当于：
val这显然严重依赖于实现。但是在我的系统上，这将产生完全相同的效果。
答案 1 :(得分：3)
升级代码在此结束：
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;
}
您很幸运，因为精度是USUALLY编译时常量（除非boost配置BOOST_LCAST_NO_COMPILE_TIME_PRECISION）。
简化一点并允许符合标准的现代标准库：
#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;
}
}
比较结果并检查假设：
<强> 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();
}
打印
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
请注意，mimicked和boost::lexical_cast<std::string>(double)每次都会产生完全相同的输出。