C和C ++编译器何时会隐式将float转换或提升为double？ [英] When do C and C++-compilers convert or promote a float to double, implicitly?

问题描述

对于一个嵌入式项目，我想知道何时符合标准的C编译器（C99）和C ++编译器（C ++ 11）会隐式地将单浮点变量/值提升为两倍。

For an embedded project I'd like to know when does a standard-compliant C-compiler (C99) and C++-compiler (C++11) will most likely implicitly promote a single-float variable/value to double-float.

我知道两种情况：

1. 后缀 f 。例如： 3.14


2. 传递浮点数以使用可变参数列表（ ... ）

1. literals which are not suffixed with f. For example: 3.14
2. passing a float to function with variadic argument-list (...)

还有其他吗？模板呢？

这个问题对我也非常有帮助-在此处包括以供参考。

The answers of this question are very helpful for me as well - including it here for reference.

推荐答案

在C中：

带有的数字文字。，不带后缀，例如 3.14 ，不涉及任何促销。整个生命周期都是翻倍。

A numeric literal with . and no suffix, e.g. 3.14, does not involve any promotion. It is double for its entire lifetime.

浮点数被提升，如果float是函数调用的参数，被调用的函数在范围内没有原型，或者该参数对应于原型中的省略号（ ... ）

A float is promoted to double if the float is an argument to a function call, and the function being called has no prototype in scope, or the argument corresponds to the ellipsis (...) in the prototype in scope.

在以下任何一种情况下，浮点数都会被转换以加倍：

A float is converted to double in any of the following situations:

• 浮点数是函数调用的参数，该函数调用与范围原型中的 double 类型的参数相对应。


• 二进制运算符具有 double 和 float 作为两个参数类型。适用的运算符为： * / +-< > < => = ==！=


• 条件运算符具有 double 和 float 作为第二个和第三个操作数（以任意顺序）


• 浮点型强制转换为 double


• 浮点数已分配给双人间（包括复合分配）

• The float is an argument to a function call corresponding to a parameter of type double in a prototype in scope.
• A binary operator has double and float as the two argument types. The operators this applies to are: * / + - < > <= >= == !=
• The conditional operator has double and float as the second and third operand (in either order)
• The float is cast to double
• The float is assigned to a double (including compound assignment)

在C ++中，除没有原型的情况外，以上所有情况仍然适用（因为C ++要求所有函数调用都必须有一个范围内的原型）。

In C++, all of the above cases still apply, except for the cases about no prototype (since C++ requires all function calls to have a prototype in scope).

有一个新情况：标准转换顺序，它太复杂了，以致无法简单总结。但例如，此C ++代码包含从 float 到 double 的隐式转换：

There is a new case: the standard conversion sequence which is too complicated to summarize briefly. But as an example, this C++ code contains an implicit conversion from float to double :

class T { public: T(double dummy) {} };
void foo(T); 
foo(3.14f); // Conversion sequence: float->double->T

我不确定是否是C ++的详尽清单。

I'm not sure if this is an exhaustive list for C++ though.

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