在c \ c ++中存储COM的VT_DECIMAL的正确类型是什么? [英] What is the correct type in c\c++ to store a COM's VT_DECIMAL?

问题描述

我正在尝试为ADO写一个包装器.

当 VARIANT 类型为 VT_DECIMAL 时， DECIMAL 是 COM VARIANT 可以是的一种类型./p>

我正在尝试将其放入c本机数据类型，并保留变量值.似乎正确的类型是long double，但出现没有合适的转换错误".

例如:

  _variant_t v;...if(v.vt == VT_DECIMAL){double d =(double)v;//这有效，但恐怕会丢失数据...长双倍ld1 =(长双倍)v;//错误:不止一次应用了从变体到长双精度的转换.长双倍ld2 =(长双倍)v.decVal;//错误:不存在合适的从十进制到long double的转换函数.} 

所以我的问题是:

1. 使用double来存储所有可能的十进制值是否绝对安全?

2. 如果没有，如何将小数转换为长整数?

3. 如何将小数转换为字符串?(使用<<运算符， sprintf 也对我有好处)

解决方案

DECIMAL 的内部表示形式不是双精度浮点值，而是带有符号/比例选项的整数.如果要初始化 DECIMAL 部分，则应初始化这些字段-96位整数值，小数位数，符号，然后获得有效的十进制 VARIANT 值.

MSDN上的

DECIMAL :

• scale-数字的小数位数.有效值为0到28.因此12.345表示为12345，小数位数为3.
• 符号-表示符号；0表示正数，DECIMAL_NEG表示负数.因此-1设置为DECIMAL_NEG时表示为1.
• Hi32-数字的高32位.
• Lo64-数字的低64位.这是_int64.

您的问题:

使用double来存储所有可能的十进制值是否绝对安全?

您不能直接将其初始化为double(例如 VT_R8 )，但可以将其初始化为double变体，并使用变体转换API转换为 VT_DECIMAL .可以对值进行较小的舍入.

如果没有，如何将小数转换为长整数?

如何将小数转换为字符串?(使用<<运算符，sprintf对我也有好处)

VariantChangeType 可以转换十进制变体到另一种类型的变体，包括整数，双精度型，字符串-您提供要转换为的类型.反之亦然，您还可以将其他内容转换为十进制.

I'm trying to write a wrapper to ADO.

A DECIMAL is one type a COM VARIANT can be, when the VARIANT type is VT_DECIMAL.

I'm trying to put it in c native data type, and keep the variable value. it seem that the correct type is long double, but I get "no suitable conversion error".

For example:

_variant_t v;
...

if(v.vt == VT_DECIMAL)
{
  double d = (double)v; //this works but I'm afraid can be loss of data...
  long double ld1 = (long double)v; //error: more then one conversion from variant to long double applied.
  long double ld2 = (long double)v.decVal; //error: no suitable conversion function from decimal to long double exist.  
}

So my questions are:

1. is it totally safe to use double to store all possible decimal values?

2. if not, how can I convert the decimal to a long double?

3. How to convert a decimal to string? (using the << operator, sprintf is also good for me)

解决方案

The internal representation for DECIMAL is not a double precision floating point value, it is integer instead with sign/scale options. If you are going to initialize DECIMAL parts, you should initialize these fields - 96-bit integer value, scale, sign, then you get valid decimal VARIANT value.

DECIMAL on MSDN:

• scale - The number of decimal places for the number. Valid values are from 0 to 28. So 12.345 is represented as 12345 with a scale of 3.
• sign - Indicates the sign; 0 for positive numbers or DECIMAL_NEG for negative numbers. So -1 is represented as 1 with the DECIMAL_NEG bit set.
• Hi32 - The high 32 bits of the number.
• Lo64 - The low 64 bits of the number. This is an _int64.

Your questions:

is it totally safe to use double to store all possible decimal values?

You cannot initialize as double directly (e.g. VT_R8), but you can initialize as double variant and use variant conversion API to convert to VT_DECIMAL. A small rounding can be applied to value.

if not, how can I convert the decimal to a long double?

How to convert a decimal to string? (using the << operator, sprintf is also good for me)

VariantChangeType can convert decimal variant to variant of another type, including integer, double, string - you provide the type to convert to. Vice versa, you can also convert something different to decimal.

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