[转][C++][STL] numeric_limits的用法

From:http://www.cplusplus.com/reference/std/limits/numeric_limits/

numeric_limits

 

浮点数据精度判断

float x=0.0;

float y=0.0f;

 

if ( abs(x -y) <= numeric_limits<float>::epsilon() )

{

 ...

}

class template

<limits>

Numeric limits type

This class is specialized for each of the fundamental types, with its members returning or set to the different values that define the properties that type has in the specific platform in which it compiles.

For all the other types (non-fundamental types) a specialization of this class should not exist.

The non-specialized class is defined as:

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template <class T> class numeric_limits { public: static const bool is_specialized = false; static T min() throw(); static T max() throw(); static const int digits = 0; static const int digits10 = 0; static const bool is_signed = false; static const bool is_integer = false; static const bool is_exact = false; static const int radix = 0; static T epsilon() throw(); static T round_error() throw(); static const int min_exponent = 0; static const int min_exponent10 = 0; static const int max_exponent = 0; static const int max_exponent10 = 0; static const bool has_infinity = false; static const bool has_quiet_NaN = false; static const bool has_signaling_NaN = false; static const float_denorm_style has_denorm = denorm absent; static const bool has_denorm_loss = false; static T infinity() throw(); static T quiet_NaN() throw(); static T signaling_NaN() throw(); static T denorm_min() throw(); static const bool is_iec559 = false; static const bool is_bounded = false; static const bool is_modulo = false; static const bool traps = false; static const bool tinyness_before = false; static const float_round_style round_style = round_toward_zero; }

A specialization exists for each of the fundamental types: bool, char, signed char, unsigned char, wchar_t, short, unsigned short, int, unsigned int, long, unsigned long, float, double and long double. These specializations define the specific values for the different static const members, and all have is_specialized defined as true.

Members

member type property

is_specialized	bool	true on all specializations of the type, false in the non-specialized version.
min()	T	Minimum finite value. For floating types with denormalization (variable number of exponent bits): minimum positive normalized value. Equivalent to CHAR_MIN, SCHAR_MIN, SHRT_MIN, INT_MIN, LONG_MIN, FLT_MIN, DBL_MIN, LDBL_MIN or 0, depending on type.
max()	T	Maximum finite value. Equivalent to CHAR_MAX, SCHAR_MAX, UCHAR_MAX, SHRT_MAX, USHRT_MAX, INT_MAX, UINT_MAX, LONG_MAX, ULONG_MAX, FLT_MAX, DBL_MAX or LDBL_MAX, depending on type.
digits	int	For integer types: number of non-sign bits (radix base digits) in the representation. For floating types: number of digits (in radix base) in the mantissa (equivalent to FLT_MANT_DIG, DBL_MANT_DIG or LDBL_MANT_DIG).
digits10	int	Number of digits (in decimal base) that can be represented without change. Equivalent to FLT_DIG, DBL_DIG or LDBL_DIG for floating types.
is_signed	bool	true if type is signed.
is_integer	bool	true if type is integer.
is_exact	bool	true if type uses exact representations.
radix	int	For integer types: base of the representation. For floating types: base of the exponent of the representation (equivalent to FLT_RADIX).
epsilon()	T	Machine epsilon (the difference between 1 and the least value greater than 1 that is representable). Equivalent to FLT_EPSILON, DBL_EPSILON or LDBL_EPSILON for floating types.
round_error()	T	Measure of the maximum rounding error.
min_exponent	int	Minimum negative integer value for the exponent that generates a normalized floating-point number. Equivalent to FLT_MIN_EXP, DBL_MIN_EXP or LDBL_MIN_EXP for floating types.
min_exponent10	int	Minimum negative integer value such that 10 raised to that power generates a normalized floating-point number. Equivalent to FLT_MIN_10_EXP, DBL_MIN_10_EXP or LDBL_MIN_10_EXP for floating types.
max_exponent	int	Maximum integer value for the exponent that generates a normalized floating-point number. Equivalent to FLT_MAX_EXP, DBL_MAX_EXP or LDBL_MAX_EXP for floating types.
max_exponent10	int	Maximum integer value such that 10 raised to that power generates a normalized finite floating-point number. Equivalent to FLT_MAX_10_EXP, DBL_MAX_10_EXP or LDBL_MAX_10_EXP for floating types.
has_infinity	bool	true if the type has a representation for positive infinity.
has_quiet_NaN	bool	true if the type has a representation for a quiet (non-signaling) "Not-a-Number".
has_signaling_NaN	bool	true if the type has a representation for a signaling "Not-a-Number".
has_denorm	float_denorm_style	Denormalized values (representations with a variable number of exponent bits). A type may have any of the following enum values: denorm_absent, if it does not allow denormalized values. denorm_present, if it allows denormalized values. denorm_indeterminate, if indeterminate at compile time.
has_denorm_loss	bool	true if a loss of accuracy is detected as a denormalization loss, rather than an inexact result.
infinity()	T	Representation of positive infinity, if available.
quiet_NaN()	T	Representation of quiet (non-signaling) "Not-a-Number", if available.
signaling_NaN()	T	Representation of signaling "Not-a-Number", if available.
denorm_min()	T	Minimum positive denormalized value. For types not allowing denormalized values: same as min().
is_iec559()	T	true if the type adheres to IEC-559 / IEEE-754 standard. An IEC-559 type always has has_infinity, has_quiet_NaN and has_signaling_NaN set to true; And infinity, quiet_NaN and signaling_NaN return some non-zero value.
is_bounded	bool	true if the set of values represented by the type is finite.
is_modulo	bool	true if the type is modulo. A type is modulo if it is possible to add two positive numbers and have a result that wraps around to a third number that is less.
traps	bool	true if trapping is implemented for the type.
tinyness_before	bool	true if tinyness is detected before rounding.
round_style	float_round_style	Rounding style. A type may have any of the following enum values: round_toward_zero, if it rounds toward zero. round_to_nearest, if it rounds to the nearest representable value. round_toward_infinity, if it rounds toward infinity. round_toward_neg_infinity, if it rounds toward negative infinity. round_indeterminate, if the rounding style is indeterminable at compile time.

 

Example

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// numeric_limits example #include <iostream> #include <limits> using namespace std; int main () { cout << boolalpha; cout << "Minimum value for int: " << numeric_limits<int>::min() << endl; cout << "Maximum value for int: " << numeric_limits<int>::max() << endl; cout << "int is signed: " << numeric_limits<int>::is_signed << endl; cout << "Non-sign bits in int: " << numeric_limits<int>::digits << endl; cout << "int has infinity: " << numeric_limits<int>::has_infinity << endl; return 0; }

Possible output:

Minimum value for int: -2147483648
Maximum value for int: 2147483647
int is signed: true
Non-sign bits in int: 31
int has infinity: false

 

See also

climits (limits.h)

Sizes of integral types (header)

cfloat (float.h)

Characteristics of floating-point types (header)

 

 

Test:

#include <iostream>
#include <cstdlib>
#include <limits>
#include <string>
using namespace std;
int main()
{
    //判断各类型有无极值
    cout << boolalpha;
    cout << "specialized(char): " 
      << numeric_limits<char>::is_specialized
        << endl;
    cout << "specialized(wchar_t): " 
      << numeric_limits<wchar_t>::is_specialized
        << endl; 
   cout << "specialized(string): " 
      << numeric_limits<string>::is_specialized
        << endl; 
    cout << noboolalpha << endl;
    
    //各个类型的最大值 
    cout << "max(int): " 
         << numeric_limits<int>::max()
         << endl;
    cout << "max(short): " 
         << numeric_limits<short>::max()
         << endl;
    cout << "max(unsigned int): " 
         << numeric_limits<unsigned int>::max()
         << endl;
    cout << "max(unsigned short): " 
         << numeric_limits<unsigned short>::max()
         << endl;
    cout << "max(unsigned long): " 
         << numeric_limits<unsigned long>::max()
         << endl;
    cout << "max(long): " 
         << numeric_limits<long>::max()
         << endl;
    cout << "max(long long): " 
         << numeric_limits<long long>::max()
         << endl;
    cout << "max(float): " 
         << numeric_limits<float>::max()
         << endl;
    cout << "max(double): " 
         << numeric_limits<double>::max()
         << endl;
    cout << "max(long double): " 
         << numeric_limits<long double>::max()
         << endl;
    cout << endl;
         
     //各个类型的最小值 
    cout << "min(int): " 
         << numeric_limits<int>::min()
         << endl;
    cout << "min(short): " 
         << numeric_limits<short>::min()
         << endl;
    cout << "min(unsigned int): " 
         << numeric_limits<unsigned int>::min()
         << endl;
    cout << "min(unsigned short): " 
         << numeric_limits<unsigned short>::min()
         << endl;
    cout << "min(unsigned long): " 
         << numeric_limits<unsigned long>::min()
         << endl;
    cout << "min(long): " 
         << numeric_limits<long>::min()
         << endl;
    cout << "min(long long): " 
         << numeric_limits<long long>::min()
         << endl;
    cout << "min(float): " 
         << numeric_limits<float>::min()
         << endl;
    cout << "min(double): " 
         << numeric_limits<double>::min()
         << endl;
    cout << "min(long double): " 
         << numeric_limits<long double>::min()
         << endl;
    
    system("pause");     
    return 0;     
}