赋值表达式隐含的转换

Question

标准6.5.16章节有2段：

In simple assignment ( = ), the value of the right operand is converted to the type of the assignment expression and replaces the value stored in the object designated by the left operand.

和

the left operand has atomic, qualified, or unqualified arithmetic type, and the right has arithmetic type;

所以考虑下面的代码：

float f = 1.2f;
int i = f; //error

i和f都是算术类型所以f应该转换成i。虽然编译代码会产生错误：

error: conversion from ‘float’ to ‘int’ may change value [-Werror=float-conversion]
    6 |     int i = f;

但是当添加显式强制转换时它编译得很好：

float f = 1.2f;
int i = (int) f;  //ok

那么在赋值表达式中允许什么样的转换呢？我虽然标准中的转换等同于强制转换 (type).

更新：

GCC 9.3.0

这是我的旗帜：

-Werror
-Wextra
-pedantic
-Wconversion
-g3
-O3
-Wno-unused-result
-Wno-unused-parameter
-Wstrict-prototypes

Answer 1

来自 gcc - wiki（自 v4.3 起）

-Wconversion

Warn for implicit conversions that may alter a value. This includes conversions between real and integer

  14 float  vfloat;
  15 double vdouble;
  16 
  17 void h (void)
  18 {
  19   unsigned int ui = 3;
  20   int   si = 3;
  21   unsigned char uc = 3;
  22   signed char sc = 3;
  23   float  f = 3;
  24   double d = 3;
  25 
  26   fsi (3.1f); /* { dg-warning "conversion" } */
  27   si = 3.1f; /* { dg-warning "conversion" } */
  28   fsi (3.1);  /* { dg-warning "conversion" } */
  29   si = 3.1;  /* { dg-warning "conversion" } */
  30   fsi (d);    /* { dg-warning "conversion" } */
  31   si = d;    /* { dg-warning "conversion" } */
  32   fui (-1.0); /* { dg-warning "overflow" } */
  33   ui = -1.0;   /* { dg-warning "overflow" } */
  34   ffloat (INT_MAX);  /* { dg-warning "conversion" } */
  35   vfloat = INT_MAX;  /* { dg-warning "conversion" } */
  36   ffloat (16777217); /* { dg-warning "conversion" } */
  37   vfloat = 16777217; /* { dg-warning "conversion" } */
  38   ffloat (si); /* { dg-warning "conversion" } */
  39   vfloat = si; /* { dg-warning "conversion" } */
  40   ffloat (ui); /* { dg-warning "conversion" } */
  41   vfloat = ui; /* { dg-warning "conversion" } */

赋值表达式隐含的转换

Conversions implied by assignment expressions

c

language-lawyer

assignment-operator