std::numeric_limits的一个使用注意事项

作者：朱金灿

 

       在C/C++11中，std::numeric_limits为模板类，在库编译平台提供基础算术类型的极值等属性信息，取代传统C语言所采用的预处理常数（具体参考：C++常用数值类型的值范围的宏定义）。其中使用例子如下：

#include <limits>  
#include <iostream>

/* reference: 
    http://www.cplusplus.com/reference/limits/numeric_limits/ 
    https://msdn.microsoft.com/en-us/library/c707ct0t.aspx 
*/  
int test_numeric_limits_1()  
{  
    std::cout << std::boolalpha;  
    std::cout << "Minimum value for int: " << std::numeric_limits<int>::min() << std::endl;  
    std::cout << "Maximum value for int: " << std::numeric_limits<int>::max() << std::endl;  
    std::cout << "int is signed: " << std::numeric_limits<int>::is_signed << std::endl;  
    std::cout << "Non-sign bits in int: " << std::numeric_limits<int>::digits << std::endl;  
    std::cout << "int has infinity: " << std::numeric_limits<int>::has_infinity << std::endl;  

    std::cout << "Minimum value for float: " << std::numeric_limits<float>::min() << std::endl; // min returns the smallest positive value the type can encode, not the lowest  
    std::cout << "Lowest value for float: " << std::numeric_limits<float>::lowest() << std::endl; // the lowest value  
    std::cout << "Maximum value for float: " << std::numeric_limits<float>::max() << std::endl;  
    std::cout << "float is signed: " << std::numeric_limits<float>::is_signed << std::endl;  
    std::cout << "Non-sign bits in float: " << std::numeric_limits<float>::digits << std::endl;  
    std::cout << "float has infinity: " << std::numeric_limits<float>::has_infinity << std::endl;  

    std::cout << "Minimum value for unsigned short: " << std::numeric_limits<unsigned short>::min() << std::endl;  
    std::cout << "Maximum value for unsigned short: " << std::numeric_limits<unsigned short>::max() << std::endl;  

    std::cout << "is_specialized(float): " << std::numeric_limits<float>::is_specialized << std::endl;  
    std::cout << "is_integer(float): " << std::numeric_limits<float>::is_integer << std::endl;  
    std::cout << "is_exact(float): " << std::numeric_limits<float>::is_exact << std::endl;  
    std::cout << "is_bounded(float): " << std::numeric_limits<float>::is_bounded << std::endl;  
    std::cout << "is_modulo(float): " << std::numeric_limits<float>::is_modulo << std::endl;  
    std::cout << "is_iec559(float): " << std::numeric_limits<float>::is_iec559 << std::endl;  
    std::cout << "digits10(float): " << std::numeric_limits<float>::digits10 << std::endl;  
    std::cout << "radix(float): " << std::numeric_limits<float>::radix << std::endl;  
    std::cout << "min_exponent(float): " << std::numeric_limits<float>::min_exponent << std::endl;  
    std::cout << "max_exponent(float): " << std::numeric_limits<float>::max_exponent << std::endl;  
    std::cout << "min_exponent10(float): " << std::numeric_limits<float>::min_exponent10 << std::endl;  
    std::cout << "max_exponent10(float): " << std::numeric_limits<float>::max_exponent10 << std::endl;  
    std::cout << "epsilon(float): " << std::numeric_limits<float>::epsilon() << std::endl;  
    std::cout << "round_style(float): " << std::numeric_limits<float>::round_style << std::endl;  

    std::cout << "The smallest nonzero denormalized value for float: "  
        << std::numeric_limits<float>::denorm_min()<< std::endl;  
    std::cout << "The difference between 1 and the smallest value greater than 1 for float: "  
        << std::numeric_limits<float>::epsilon()<< std::endl;  
    std::cout << "Whether float objects allow denormalized values: "  
        << std::numeric_limits<float>::has_denorm << std::endl;  
    std::cout << "Whether float objects can detect denormalized loss: "  
        << std::numeric_limits<float>::has_denorm_loss << std::endl;  
    std::cout << "Whether float objects have quiet_NaN: "  
        << std::numeric_limits<float>::has_quiet_NaN << std::endl;  
    std::cout << "Whether float objects have a signaling_NaN: "  
        << std::numeric_limits<float>::has_signaling_NaN << std::endl;  
    std::cout << "The base for type float is:  "  
        << std::numeric_limits<float>::radix << std::endl;  
    std::cout << "The maximum rounding error for type float is:  "  
        << std::numeric_limits<float>::round_error() << std::endl;  
    std::cout << "The rounding style for a double type is: "  
        << std::numeric_limits<double>::round_style << std::endl;  
    std::cout << "The signaling NaN for type float is:  "  
        << std::numeric_limits<float>::signaling_NaN() << std::endl;  
    std::cout << "Whether float types can detect tinyness before rounding: "  
        << std::numeric_limits<float>::tinyness_before << std::endl;  
    std::cout << "Whether float types have implemented trapping: "  
        << std::numeric_limits<float>::traps << std::endl;  

    return 0;  
}

         但是在使用std::numeric_limits时经常会出现一个编译错误：

1>main.cpp(473):error C2059: 语法错误:“::”

1>  main.cpp(474): warning C4003: “min”宏的实参不足。

      网上查了一下资料，了解了这个错误的原因：windows.h中定义了min宏，<limits >中也有这个定义，两者冲突了。windows.h中Microsoft定义了min/max两个宏，< limits >中定义的min/max两个宏虽然在自己的命名空间中，可还是被“污染”了。因此要避免冲突，就得先屏蔽windows.h中定义的min宏，具体做法是在所有包含头文件前面（注意是所有）增加下面的预处理语句：

#ifndef NOMINMAX                      
    #define NOMINMAX
#endif