c++ - 从文字缩小不会引起警告

Question

int a = 0;短 b{a};短 c{0};

编译器对 short b{a} 发出警告。我能理解这一点，因为 int 被缩小为 short。

但是，它不会对 short c{0} 发出警告，这对我来说很奇怪。我记得对于文字整数，0 的类型至少应该是 int。因此，从 int 缩小到 short 就发生在这里。为什么编译器不给出警告？

Answer 1

对于短c{0};, narrowing conversion不会发生。因为 0 是一个常量表达式，可以精确地存储在一个 short 中。

(强调我的)

list-initialization limits the allowed implicit conversions by prohibiting the following:

• ...

• conversion from integer or unscoped enumeration type to integer type that cannot represent all values of the original, except where source is a constant expression whose value can be stored exactly in the target type

来自标准的相关解释和例子，$8.6.4/7 List-initialization[dcl.init.list] :

(强调我的)

A narrowing conversion is an implicit conversion

• ...

• from an integer type or unscoped enumeration type to an integer type that cannot represent all the values of the original type, except where the source is a constant expression whose value after integral promotions will fit into the target type.

[ Note: As indicated above, such conversions are not allowed at the top level in list-initializations. — end note ] [ Example:

// ...
const int z = 99;
// ...
char c4{z};               // OK: no narrowing needed
unsigned char uc1 = {5};  // OK: no narrowing needed
// ...
float f2 { 7 };           // OK: 7 can be exactly represented as a float
// ...

— end example ]