为什么printf（）的促进浮到双？ [英] Why does printf() promote a float to a double?

问题描述

从previous <一个href=\"http://stackoverflow.com/questions/4264127/correct-format-specifier-for-double-in-printf?rq=1\">question:

From a previous question:

如果你试图传递一个浮动到的printf ，它会被提升到双
  前的printf 接收它

If you attempt to pass a float to printf, it'll be promoted to double before printf receives it

的printf（）是一个可变参数函数吧？因此，没有一个可变参数函数促进浮动参数传递给双击传递之前？

printf() is a variadic function right? So does a variadic function promote a float argument to a double before passing it?

推荐答案

是的，浮动参数可变参数函数都提升一倍。

Yes, float arguments to variadic function are promoted to double.

借助草案C99标准部分 6.5.2.2 函数调用说：

[...]和论据
  有float类型都提升到两倍。这些被称为默认参数
  促销活动。[...]

[...]and arguments that have type float are promoted to double. These are called the default argument promotions.[...]

从草案C ++标准部分 5.2.2 函数调用：

[...]一个浮点类型，是受浮点
  推广（4.6），该参数的值转换为
  呼叫之前提升的类型。 [...]

[...]a floating point type that is subject to the floating point promotion (4.6), the value of the argument is converted to the promoted type before the call. [...]

和部分 4.6 ：

float类型的prvalue可以转换为double类型的prvalue。的值不变

A prvalue of type float can be converted to a prvalue of type double. The value is unchanged

CP preference 覆盖的默认转换在C可变参数函数的++得好：

  
• 的std :: nullptr_t被转换为void *

  
• 浮动参数转换成兼作浮点推广

  
• 布尔，焦炭，短，无范围的枚举转换成int或更宽的整数类型为整型提升

  

我们可以在C看到并使用C presumably ++这种转换是围绕保持兼容的 K＆放大器; RC 的，从的理由国际标准的编程语言-C （重点煤矿的）：

We can see in C and presumably in C++ this conversion was kept around for compatibility with K&R C, from Rationale for International Standard—Programming Languages—C (emphasis mine):

有关以往的惯例兼容性，所有的参数提升为发生
  在K＆放描述; R在没有原型声明中，包括
  在并不总是可取的推广浮动翻番

For compatibility with past practice, all argument promotions occur as described in K&R in the absence of a prototype declaration, including the not always desirable promotion of float to double.

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