c++ - C++中缩小和截断之间的区别？

Question

我一直在读一本书(C++ 编程语言 Stroustrup 第 4 版)。
与初始化相关的部分中给出的示例如下:

void f(double d, int i)
{
    int a{ d };     // error : possible truncation
    char b{ i };    // error : possible narrowing
}

截断和缩小之间究竟有什么区别？

Answer 1

缩小转换基本上是任何可能导致信息丢失的转换。严格来说，一个 narrowing conversion是 :

an implicit conversion

• from a floating-point type to an integer type, or
• from long double to double or float, or from double to float, except where the source is a constant expression and the actual value after conversion is within the range of values that can be represented (even if it cannot be represented exactly), or
• from an integer type or unscoped enumeration type to a floating-point type, except where the source is a constant expression and the actual value after conversion will fit into the target type and will produce the original value when converted back to the original type, or
• from an integer type or unscoped enumeration type to an integer type that cannot represent all the values of the original type, except where the source is a constant expression whose value after integral promotions will fit into the target type, or
• from a pointer type or a pointer-to-member type to bool.

请注意，这意味着您发布的两个转换都在缩小转换范围。 int a{ d };是第一种情况和 char b{ i };是第四个。
截断仅在浮点类型和整数类型之间转换时发生。它通常是指丢失的浮点数的小数部分 ( source )。这意味着截断是缩小转换的子集。