C++ 11变量缩小,没有GCC编译器警告

Question

缩小的概念似乎很简单.但是,有人可以解释为什么下面的一些代码导致"缩小"编译器错误而其他代码没有？

此代码在预期的位置产生错误:

constexpr int a = 255;
unsigned char b = a;      // OK
unsigned char c = a + 1;  // Error... expected

此代码不会产生错误,但可能没问题:

int d = 256;
unsigned char e = d;  // Maybe OK because 'd' is not constexpr

此代码应该生成错误(除非我遗漏了一些东西):

int f = 42.0;  // Maybe OK because no fractional part
int g = 42.1;  // OK... should fail!!
constexpr float h = 42.7;
int i = h;     // OK... should fail???

我正在使用g ++ 4.6.2.我搜索了GCC错误数据库,没有发现任何相关内容.谢谢!

Answer 1

说实话,你的样品我看错了.

但是,在许多情况下,编译器似乎接受"违反"标准转换规则......:

初始化列表(第8.5.4节)

但是我在标准中发现了这个:

对于初始化列表,不允许以下内容(第8.5.4节,第3节)

int ai[] = { 1, 2.0 }; // error narrowing

在6.下,它继续给出一个示例的一般列表:

[注意:如上所述,列表初始化中顶层不允许进行此类转换.-end note]

int x = 999; // x is not a constant expression
const int y = 999;
const int z = 99;
char c1 = x; // OK, though it might narrow (in this case, it does narrow)
char c2{x}; // error: might narrow
char c3{y}; // error: narrows (assuming char is 8 bits)
char c4{z}; // OK: no narrowing needed
unsigned char uc1 = {5}; // OK: no narrowing needed
unsigned char uc2 = {-1}; // error: narrows
unsigned int ui1 = {-1}; // error: narrows
signed int si1 =
{ (unsigned int)-1 }; // error: narrows
int ii = {2.0}; // error: narrows
float f1 { x }; // error: might narrow
float f2 { 7 }; // OK: 7 can be exactly represented as a float
int f(int);
int a[] = { 2, f(2), f(2.0) }; // OK: the double-to-int conversion is not at the top level

有趣的是,g ++ 4.6.1 只--std=c++0x -Wall -pedantic捕获了其中一个违规行为:

    char c3{y}; // warning: overflow in implicit constant conversion [-Woverflow]

外部初始化列表......

我不认为考虑将float截断为int narrowing.

它只是一个定义明确的转换,就像

int i = 31;
i /= 4;   // well defined loss of precision...   
i /= 4.0; // equally well-defined conversion from floating point to int