C++ 单精度和双精度混合计算的控制规则是什么?

问题描述

例如这些变量:

result (double)
a (double)
b (float)
c (float)
d (double)

一个简单的计算:

result = a * (b + c) * d

类型转换的方式和时间，以及如何确定每次计算的精度是多少?

How and when are the types converted and how do I figure out what precision each calculation is performed at?

推荐答案

所有操作都在相同类型的对象上完成(假设正常算术运算).

All operations are done on objects of the same type (assuming normal arithmetic operations).

如果你编写的程序使用不同的类型，那么编译器会自动升级ONE参数，使它们都相同.

If you write a program that uses different types then the compiler will auto upgrade ONE parameter so that they are both the same.

在这种情况下，浮动将升级为双打:

In this situations floats will be upgraded to doubles:

result      = a * (b + c) * d

float  tmp1 = b + c;            // Plus operation done on floats.
                                // So the result is a float

double tmp2 = a * (double)tmp1; // Multiplication done on double (as `a` is double)
                                // so tmp1 will be up converted to a double.

double tmp3 = tmp2 * d;         // Multiplication done on doubles.
                                // So result is a double

result      = tmp3;             // No conversion as tmp3 is same type as result.

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