一般来说，数值类型的极值是一个与平台相关的特性。C++标准程序库通过template numeric_limits提供这些极值，取代传统C语言所采用的预处理常数。你仍然可以使用后者，其中整数常数定义于<climits>和<limits.h>,浮点常数定义于<cfloat>和<float.h>，新的极值概念有两个优点，一是提供了更好的类型安全性，二是程序员可借此写出一些template以核定这些极值。

1. 函数介绍

member	type	property
is_specialized	bool	true for all arithmetic types (i.e., those for which numeric_limits is specialized). false for all other types.
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, LLONG_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, LLONG_MAX, ULLONG_MAX, UINT_LEAST16_MAX, UINT_LEAST32_MAX, FLT_MAX, DBL_MAX or LDBL_MAX, depending on type.
lowest()	T	Minimum finite value. (since C++11) For integral types: the same as min(). For floating-point types: implementation-dependent; generally, the negative of max().
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.
max_digits10	int	Number of digits (in decimal base) required to ensure that values that differ are always differentiated.
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 such that radix raised to (min_exponent-1) 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 such that radix raised to (max_exponent-1) generates a representable finite 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	bool	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.

2. 函数定义

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

3. 示例

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

输出

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

参考文献

• http://www.cplusplus.com/reference/limits/numeric_limits/
• https://blog.csdn.net/yhc166188/article/details/90287807