我如何使用numeric_cast策略？

Question

所以我有自己的uint64_t到uint32_t数字转换的政策

struct MyOverflowHandlerPolicy
{
    void operator() ( boost::numeric::range_check_result ) {
         std::cout << "MyOverflowHandlerPolicy called" << std::endl;
         throw boost::numeric::positive_overflow();
    };
} ;

如何让它被boost :: numeric_cast使用？

Answer 1

为了使用numeric_cast,numeric_cast_traits应在每种类型转换上定义专门化.已使用内置数值类型的默认值定义这些特化.可以通过定义BOOST_NUMERIC_CONVERSION_RELAX_BUILT_IN_CAST_TRAITS(细节)来禁用内置类型的特化生成.

这是一个小样本.

#include <iostream>
#include <stdexcept>

#define BOOST_NUMERIC_CONVERSION_RELAX_BUILT_IN_CAST_TRAITS
#include <boost/numeric/conversion/cast.hpp>

using namespace std;

struct YourOverflowHandlerPolicy
{
    void operator() ( boost::numeric::range_check_result r ) { 
         cout << "YourOverflowHandlerPolicy called" << endl;
         if (r != boost::numeric::cInRange) {
            throw logic_error("Not in range!");
         }   
    };  
};

namespace boost { namespace numeric {
template <>
struct numeric_cast_traits<uint32_t, uint64_t>
{
    typedef YourOverflowHandlerPolicy overflow_policy;
    typedef UseInternalRangeChecker   range_checking_policy;
    typedef Trunc<uint64_t>           rounding_policy;
};

template <>
struct numeric_cast_traits<uint64_t, uint32_t>
{
    typedef YourOverflowHandlerPolicy overflow_policy;
    typedef UseInternalRangeChecker   range_checking_policy;
    typedef Trunc<uint32_t>           rounding_policy;
};
}} //namespace boost::numeric;

int main()
{
    try {
        cout << boost::numeric_cast<uint32_t>((uint64_t)1) <<  endl; // OK
        cout << boost::numeric_cast<uint32_t>((uint64_t)1<<31) <<  endl; // OK
        cout << boost::numeric_cast<uint32_t>((uint64_t)1<<32) <<  endl; // Exception
    } catch (...) {
        cout << "exception" << endl;
    }   

    return 0;
}

输出:

YourOverflowHandlerPolicy called
1
YourOverflowHandlerPolicy called
2147483648
YourOverflowHandlerPolicy called
exception

注意:我有1.55.0的升级版本,我不知道编译它的最低版本级别,但它不是用1.46.0编译的.因此,请检查您的增强版本并在必要时进行更新.