C++ 中数字类型转换的最佳实践

Question

在不同数字类型之间进行转换的最佳做法是什么？类型 float、double、int 是我在 C++ 中使用最多的类型。

选项示例，其中 f 是 float，n 是 double 或 int：

float f = static_cast<float>(n);
float f = float(n);
float f = (float)n;

我通常写 static_cast<T>(...) 但想知道 C++ 开发社区是否有任何共识是否有更好的方式。

我明白这可能最终会成为一个基于意见的问题并且可能没有 "standard" 方式，在这种情况下请告诉我没有标准方式所以至少我知道： -)

我知道这个问题 cropped up 与一般转换有关，但是，我对数字特别感兴趣，以及在数字类型的方法中是否有特定的最佳实践。

Answer 1

只需使用static_cast。 C 转换的问题是操作的歧义（即 Explicit type conversion 的第 (1) 点）。

C++ 转换避免了这种情况。此外，在搜索 C++ 强制转换时会更加明显。

用 Stroustrup 的话 (What good is static_cast?):

Even an innocent-looking cast can become a serious problem if, during development or maintenance, one of the types involved is changed. For example, what does this mean?:
  x = (T)y;
We don't know. It depends on the type T and the types of x and y. T could be the name of a class, a typedef, or maybe a template parameter. Maybe x and y are scalar variables and (T) represents a value conversion. Maybe x is of a class derived from y's class and (T) is a downcast. Maybe x and y are unrelated pointer types. Because the C-style cast (T) can be used to express many logically different operations, the compiler has only the barest chance to catch misuses. For the same reason, a programmer may not know exactly what a cast does. This is sometimes considered an advantage by novice programmers and is a source of subtle errors when the novice guessed wrong.

The "new-style casts" were introduced to give programmers a chance to state their intentions more clearly and for the compiler to catch more errors.

[CUT]

A secondary reason for introducing the new-style cast was that C-style casts are very hard to spot in a program. For example, you can't conveniently search for casts using an ordinary editor or word processor.

[CUT]

casts really are mostly avoidable in modern C++

同时考虑 boost::numeric::converter / boost::numeric_cast that are safer alternatives (part of Boost.NumericConversion 库）。

例如

#include <iostream>
#include <boost/numeric/conversion/cast.hpp>

int main()
{
  using boost::numeric_cast;

  using boost::numeric::bad_numeric_cast;
  using boost::numeric::positive_overflow;
  using boost::numeric::negative_overflow;

  try
  {
    int i = 42;
    short s = numeric_cast<short>(i); // This conversion succeeds (is in range)
  }
  catch(negative_overflow &e)  { std::cout << e.what(); }
  catch(positive_overflow &e)  { std::cout << e.what(); }

  return 0;
}

一般来说，对于隐式转换和显式转换（通过 static_cast），缺乏范围保留使得数字类型之间的转换容易出错。

numeric_cast 在转换数字类型时检测范围丢失，如果无法保留范围则抛出异常。

Answer 2

通常，这些转换运算符分为两大类： 特定的转换运算符 和传统的转换运算符。 cplusplus.com 解释如下：

...In order to control these types of conversions between classes, we have four specific casting operators: dynamic_cast, reinterpret_cast, static_cast and const_cast. Their format is to follow the new type enclosed between angle-brackets (<>) and immediately after, the expression to be converted between parentheses.

dynamic_cast <new_type> (expression)

reinterpret_cast <new_type> (expression)

static_cast <new_type> (expression)

const_cast <new_type> (expression)

The traditional type-casting equivalents to these expressions would be:

(new_type) expression

new_type (expression)

but each one with its own special characteristics.

在处理任务时，我们（几乎）都使用特定的转换。在考虑了这些建议之后，这取决于你。

参见resource。

C++ 中数字类型转换的最佳实践

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